Popular Science Monthly/Volume 3/August 1873/The Weather and the Sun
| THE WEATHER AND THE SUN. |
By RICHARD A. PROCTOR, B. A.
THERE are few scientific questions of greater interest than the inquiry whether it is possible to find a means of predicting the weather for a long time in advance. In former ages many attempts were made to solve this problem by a reference to the motions of the heavenly bodies. Other methods of prediction were, indeed, in vogue; but I am not here considering ordinary weather portents, or mere scientific schemes for anticipating the weather of two or three coming days: and, with a few trifling exceptions, depending on observations of plants and animals, it is the case that the only wide rules for predicting weather were based on the motions of the sun and moon, the planets and the stars. It must be remembered that even astronomers of repute placed faith, until quite recent years, in the seemingly absurd tenets of judicial astrology. We cannot greatly wonder, therefore, if the more reasonable thesis, that the heavenly bodies determine weather-changes, was regarded with favor. Accordingly we find Horrox, more than two centuries ago, drawing the distinction here indicated, where he says that, in anticipating "storm and tempest" from a conjunction of Mercury with the Sun, he coincides "with the opinion of the astrologers, but in other respects despises their more puerile vanities." We find Bacon in like manner remarking that "all the planets have their summer and winter, wherein they dart their rays stronger or weaker, according to their perpendicular or oblique direction." He says, however, that "the commixtures of the rays of the fixed stars with one another are of use in contemplating the fabric of the world and the nature of the subjacent regions, but in no respect for predictions." Bacon remarks again that reasonable astrology (Astrologia sana) "should take into account the apogees and perigees of the planets, with a proper inquiry into what the vigor of planets may perform of itself; for a planet is more brisk in its apogee, but more communicative in its perigee: it should include, also, all the other accidents of the planet's motions, their accelerations, retardations, courses, stations, retrogradations, distances from the sun, increase and diminution of light, eclipses, etc.; for all these things affect the rays of the planets, and cause them to act either weaker or stronger, or in a different manner."
It is a remarkable circumstance that systems of weather prediction based on such considerations were not quickly exploded, owing to their failure when tested by experience. Yet singularly enough it has scarcely ever happened that any wide system of interpretation has been devised, which has not been regarded with favor by its inventor long after it had been in reality disproved by repeated instances of failure. This remark applies to recent systems as well as to those invented in earlier times. Within the last twenty years, for example, methods of prediction based on the moon's movements, on the conjunctions of the planets, and on other relations, have been maintained with astonishing perseverance and constancy, in the face of what outsiders cannot but regard as a most discouraging want of agreement between the predicted weather and the actual progress of events. Here, as in so many cases of prediction, we find the justice of Bacon's aphorism, "Men mark when they hit, and never mark when they miss."
It is noteworthy, indeed, that the very circumstance which appears to present a fatal objection to all schemes of prediction based on the motions of the celestial bodies, supplies the means of imagining that predictions have been fulfilled. The objection I refer to is this: we know that the weather is seldom alike over very wide regions, while, nevertheless, the celestial bodies present the same aspect toward the whole extent of such regions, or an aspect so nearly the same as to suggest that the same conditions of weather should prevail if the weather really depended on the position of the heavenly bodies. It appears, then, that the inventor of a really trustworthy system must have a distinct scheme for each part of every continent—nay, of every country, if not of every county. This objection is not taken into account, however, by the inventors of systems, while the fact on which it depends affords the means of showing that each prediction has been fulfilled. Thus, suppose "bad weather and much wind" have been predicted on a certain day, and that day is particularly fine and calm in London. If this were urged as an objection to the soundness of the system, the answer would run somewhat on this wise: "Unquestionably it was fine in London, but in North Scotland (or in France, or Spain, or Italy, as the case may be) there was very gloomy weather, and in Ireland (suppose) quite strong winds are reported to have prevailed in the afternoon." The readiness with which men satisfy themselves in such cases, corresponds with that mischievous ingenuity wherewith foolish persons satisfy themselves that a fortune-teller had foretold the truth, that a dream had been fulfilled, a superstition justified, and so forth.
The tendency, at present, among those who are desirous of forming a scheme of weather prediction, is to seek the origin of our weather-changes in changes of the sun's condition, and, by determining the laws of the solar changes, to ascertain the laws which regulate changes in the weather.
It may be remarked, in passing, that this new phase of the inquiry does not reject planetary influences altogether. The theory is entertained by many well-known students of science that changes in the condition of the sun are dependent on the varying positions of the planets; so that, if it should be established that our weather-changes are connected with solar changes, we should infer that indirectly the planets in their motions rule the weather on our earth.
I propose now to consider the evidence relating to the sun's influence, and to discuss the question (altogether distinct, be it remarked) whether a means of accurate weather prediction may be obtained if the sun's influence be regarded as demonstrated.
There is one strong point in favor of the new theory, in the fact that the sun is unquestionably the prime cause of all weather changes. To quote the words of Lieutenant-Colonel Strange, an enthusiastic advocate of the theory (and eager to have it tested at this country's charge), "there can hardly be a doubt that almost every natural phenomenon, connected with the climate can be distinctly traced to the sun as the great dominating force, and it is a natural inference" (though not, as he says, an unavoidable one) "that the changes, and what we now call the uncertainties of climate, are connected with the constant fluctuations which we know to he perpetually occurring in the sun itself." I may proceed, indeed, in this place, to quote the following words, in which Colonel Strange enunciates the theory itself which I am about to discuss, and its consequences: "The bearing of climatic changes on a vast array of problems connected with navigation, agriculture, and health, need but be mentioned to show the importance of seeking in the sun, where they doubtless reside, for the causes which govern these changes. It is indeed my conviction that, of all the fields now open for scientific cultivation, there is not one which, quite apart from its transcendent philosophical interest, promises results of such high utilitarian value as the exhaustive systematic study of the sun."
It cannot be doubted, I think, that, if any thing like what is here promised could be hoped for from the study of the sun, it would be a matter of more than national importance to undertake the task indicated by Colonel Strange. The expense of new observatories for this special subject of study would, in that case, be very fully repaid. It would be worth while to employ the most skilful astronomers at salaries comparable with those which are paid to our Government ministers; it would be well to secure, on corresponding terms, the advice of those most competent to decide on the instrumental requirements of the case; and, in fact, the value of the work which is at present accomplished at Greenwich, great though that value is, would sink into utter insignificance, in my judgment, compared with the results flowing in the supposed case from the proposed "exhaustive and systematic study" of the great central luminary of the planetary system.
The subject we are to discuss is manifestly, therefore, of the utmost importance, and cannot be too carefully dealt with. It would be a misfortune on the one hand to be led by careless reasoning to underestimate the chances in favor of the proposed scheme, while, on the other, it would be most mischievous to entertain unfounded expectations where the necessary experiments must be of a costly nature, and where science would be grievously discredited, should it be proved that the whole scheme was illusory.
We note, first, that, besides being "the great dominating force" to which all natural phenomena connected with climate are due, the sun has special influence on all the most noteworthy variations of weather. The seasons are due to solar influence; and here we have an instance of a power of prediction derived from solar study, though belonging to a date so remote that we are apt to forget the fact. It seems so obvious that summer will be on the whole warmer than winter, that we overlook the circumstance that, at some epoch or other, this fact, at least in its relation to the apparent motions of the sun, must have been recognized as a discovery. Men must at one time have learned, or perhaps we should rather say each race of men must at one time have noticed, that the varying warmth on which the processes of vegetation depend, corresponds with the varying diurnal course of the sun. So soon as this was noticed, and so soon as the periodic nature of the sun's varying motions had been ascertained, men had acquired in effect the power of predicting that, at particular times or seasons, the weather on the whole would be warmer than at other seasons. In other words, so soon as men had recognized the period we call the year, they could predict that one half of each year would be warmer than the other half. Simple as this fact may seem, it is important to notice it as the beginning of weather-prediction; for, as will presently appear, it has an important bearing on the more complex questions at present involved in the prognostication of weather-changes.
It became manifest, almost as soon as this discovery had been made, that the weather of particular days, or even of weeks and longer periods, could not, by its means, be predicted. A week in summer may be cold, and a week in winter may be warm; nor, so far as is even yet known, is there a single part of any year the temperature of which can be certainly depended upon, at least within the temperate zone. In certain tropical regions there are tolerably constant weather variations; but, so far is this from being the case in the temperate zones of either hemisphere, that it is impossible to affirm certainly, even that during a week or fortnight at any given summer season there will be one hot day, or that during a corresponding period in winter there will be one day of cold weather.
It became manifest also, at an early epoch, that terrestrial conditions must be intimately involved in all questions of weather, since the year in different countries in the same latitudes presents different features. Such differences are of two kinds—those which have a tendency to be constant, and those which are in their nature variable. For example, the annual weather, in Canadian regions having the same range of latitude as Great Britain, differs always to a very marked degree, though not always to the same degree, from that which prevails in this country; here, then, we have a case of a constant difference due unquestionably to terrestrial relations. Again, when we have a hot or dry summer in this country, warm or damp weather may prevail in other countries in the same latitudes, and vice versa; differences of this kind are ordinarily[1] variable, and in the present position of weather-science are regarded as accidental.
Hitherto, weather-science has depended solely on the study of these terrestrial effects as they vary under varying conditions. Modern meteorological research is confined to the record and study of the actual condition of the weather from day to day at selected stations in different countries. It cannot be denied that the inquiry has not been attended with success. At vast expense, millions of records of heat, rainfall, winds, clouds, barometric pressure, and so on, have been secured; but hitherto no law has been recognized in the variations thus recorded—no law, at least, from which any constant system of prediction for long periods in advance can be deduced.
On this point I shall quote, first, a remarkable saying of Sir W. Herschel's, which appears to me, like many such sayings of his, to be only too applicable to the present state of science. In endeavoring to interpret the laws of weather, "we are in the position," Herschel remarks, "of a man who hears at intervals a few fragments of a long history related in a prosy, unmethodical manner. A host of circumstances omitted or forgotten, and the want of connection between the parts, prevent the hearer from obtaining possession of the entire history. Were he allowed to interrupt the narrator, and ask him to explain the apparent contradictions, or to clear up doubts at obscure points, he might hope to arrive at a general view. The questions that we would address to Nature are the very experiments of which we are deprived in the science of meteorology."
The late Prof. De Morgan, indeed, selected meteorology as the subject on which, above all others, systematic observations had been most completely wasted—as a special instance of the failure of the true Baconian method (which, be it noticed, is not, as is so commonly supposed, the modern scientific method). "There is an attempt at induction going on," says De Morgan, "which has yielded little or no fruit, the observations made in the meteorological observatories. This attempt is carried on in a manner which would have caused Bacon to dance for joy" (query); "for he lived in times when chancellors did dance. Russia, says M. Biot, is covered by an army of meteorographs, with generals, high officers, subalterns, and privates, with fixed and defined duties of observation. Other countries, also, have their systematic observations. And what has come of it? Nothing, says M. Biot, and nothing will ever come of it: the veteran mathematician and experimental philosopher declares, as does Mr. Ellis" (Bacon's biographer), "that no single branch of science has ever been fruitfully explored in this way." A special interest attaches, I may remark, to the opinion of M. Biot, because it was given upon the proposal of the French Government to construct meteorological observatories in Algeria.
It is well known that our Astronomer Royal holds a similar opinion. De Morgan thus quaintly indicates his interpretation of one particular expression of Sir G. Airy's opinion: "In the report to the Greenwich Board of Visitors, for 1867, the Astronomer Royal, speaking of the increase of meteorological observatories, remarks: 'Whether the effect of this movement will be that millions of useless observations will be added to the millions that already exist, or whether something may be expected to result which will lead to a meteorological theory, I cannot hazard a conjecture.' This is a conjecture, and a very obvious one; if Mr. Airy would have given 2¾d. for the chance of a meteorological theory formed by masses of observations, he would never have said what I have quoted."
The simple combination of terrestrial considerations with the effects due to the sun's varying daily path having thus far failed to afford any interpretation of the varying weather from year to year, it is natural to inquire whether the variations in the sun's condition from year to year may not supply the required means of interpreting, and hence of predicting, weather-changes. We know that the sun's condition does vary, because we sometimes see many large spots upon his surface, whereas at others he has no spots, or few and small ones. We can scarcely doubt that these variations affect the supply of heat and light, as well as of chemical action and possibly of other forms of force; and hence we are certainly dealing with a vera causa, though whether this real cause be an efficient cause of weather-changes remains yet to be determined.
It may perhaps be as well to inquire, however, in the first place, whether any peculiarities of weather can be traced to another circumstance which ought to be at least as efficient, one would suppose, as any changes in the sun's action due to the spots. I refer to his varying distance from the earth. It is known doubtless to all my readers that, in June and July, although these are our summer months, the sun is farther away than in December; and this, not by an inconsiderable distance, but by more than 3,000,000 miles. Accordingly, on a summer day in our hemisphere, we receive much less heat than is received on a summer day in the Southern Hemisphere. Or, instead of comparing our summer heat with summer heat in the Southern Hemisphere, we may make comparison between the quantity of heat received by the whole earth on a day in June and on a day in December. Either way of viewing the matter is instructive; and I believe many of my readers will be surprised when they hear what is the actual amount of difference.
We receive in fact, on June 30th, less heat and light than dwellers at our antipodes receive on December 30th, by the amount which would be lost if an opaque disk, having a diameter equal to one-fourth of the sun's,[2] came upon the sun's face as seen on December 30th at our antipodes. It need hardly be said that no spots, whose effects would be comparable with those produced by such a disk of blackness, have ever been seen upon the face of the sun. Spots are not black or nearly black, even in their very nucleus. The largest ever seen has not had an extent approaching that of our imagined black disk, even when the whole dimensions of the spot—nucleus, umbra, and penumbra—have been taken into account. Moreover, all round a spot there is always a region of increased brightness, making up to a great degree, if not altogether, for the darkness of the spot itself. So that unquestionably the summer heat in the Southern Hemisphere exceeds the summer heat in our hemisphere to a much more marked degree than the heat given out by the sun when he is without spots exceeds the heat of a spotted sun.
It is, however, rather difficult to ascertain what effect is to be ascribed to this peculiarity. It is certain that the Australian summer differs in several important respects from the European summer; but it is not easy to say how much of the difference is due to the peculiarity we have been considering, and how much to the characteristic distinction between the northern and southern halves of the earth—the great excess of water-surface over land-surface in the Southern Hemisphere. It is worthy of notice, however, that even in this case, where we cannot doubt that a great difference must exist in the solar action at particular seasons, we find ourselves quite unable to recognize any peculiarities of weather as certainly due to this difference.
I have spoken of a second way of viewing the difference in question, by considering it as it affects the whole earth. The result is sufficiently surprising. It has been shown, by the researches of Sir J. Herschel and Pouillet, that on the average our earth receives each day a supply of heat competent to heat an ocean, 260 yards deep over the whole surface of the earth, from the temperature of melting ice to the boiling-point. Now, on or about June 30th the supply is one-thirtieth less, while on or about December 30th the supply is one-thirtieth greater. Accordingly, on June 30th, the heat received in a single day would be competent only to raise an ocean 251⅓ yards deep from the freezing to the boiling point, whereas on December 30th the heat received from the sun would so heat an ocean 268⅔ yards deep. The mere excess of heat, therefore, on December 30th, as compared with June 30th, would suffice to raise an ocean, more than 17 yards deep and covering the whole earth, from the freezing-point to the temperature of boiling water! It will not be regarded as surprising if terrestrial effects of some importance should follow from so noteworthy an excess, not merely of light and beat, but of gravitating force, of magnetic influence, and of actinic or chemical action, exerted upon the earth as a whole. Accordingly we find that there is a recognizable increase in the activity of the earth's magnetism in December and January as compared with June and July. But, assuredly the effect produced is not of such a character as to suggest that we should find the means of predicting weather, if it were possible for us now to discover any solar law of change resulting in a corresponding variation of solar action upon the earth.
This leads us to consider the first great law of solar change as distinguished from systematic variations like the sun's varying change of distance and his varying daily path on the heavens. This law is that which regulates the increase and decrease of the solar spots within a period of about eleven years. The sun's condition does not, indeed, admit of being certainly predicted by this law, since it not unfrequently happens that the sun shows few spots for several weeks together, in the very height of the time of spot-frequency, while on the other hand it often happens that many and large spots are seen at other times. Nevertheless, this general law holds, that, on the whole, and taking one month with another, there is a variation in spot-frequency, having for its period an interval of rather more than eleven years.
Now, the difference between a year of maximum spot-frequency and one of minimum frequency is very noteworthy, notwithstanding the exceptional features just mentioned, which show themselves but for short periods. This will be manifest on the consideration of a few typical instances. Thus, in the year 1837, the sun was observed on 168 days, during which he was not once seen without spots, while no less than 333 new groups made their appearance. This was a year of maximum spot-frequency. In 1843, the sun was observed on 312 days, and on no less than 149 of these no spots could be seen, while only 34 new groups made their appearance. This was a year of minimum spot-frequency. Passing to the next maximum year, we find that in 1848 the sun was observed on 278 days, during which he was never seen without spots, while 330 new spots made their appearance. In 1855 and 1856 together, he was observed on 634 days, on 239 of which he was without spots, while only 62 new groups made their appearance. The next maximum was not so marked as usual, that is, there was not so definite a summit, if one may so speak, to the wave of increase; but the excess of spot-frequency was none the less decided. Thus, in the four years, 1858-'61, the sun was observed on 335, 343, 333, and 322 days, on not one of which he was spotless, while the numbers of new groups for these four years were, respectively, 202, 205, 211, and 204. The minimum in 1867 was very marked, as 195 days out of 312 were without spots, and only 25 new groups appeared. The increase after 1867 was unusually rapid, since in 1869 there were no spotless days, and 224 new groups were seen, though the sun was only observed on 196 days. The number of spots in 1870, 1871, and 1872, as well as their magnitude and duration, has been above what is usual, even at the period of maximum spot-frequency.
From all this it will be manifest that we have a well-marked peculiarity to deal with, though not one of perfect uniformity. Next to the systematic changes already considered, this alternate waxing and waning of spot-frequency might be expected to be efficient in producing recognizable weather-changes. Assuredly, if this should not appear to be the case, we should have to dismiss all idea that the sun-spots are weather-rulers.
Now, from the first discovery of spots, it was recognized that they must, in all probability, affect our weather to some degree. It was noticed, indeed, that our auroras seemed to be in some way influenced by the condition of the sun's surface, since they were observed to be more numerous when there are many spots than when there are few or none. Singularly enough, the effect of the spots on temperature was not only inquired into much later (for we owe to Cassini and Mairan the observation relating to auroras), but was expected to be of an opposite character from that which is in reality produced. Sir W. Herschel formed the opinion that, when there are most spots, the sun gives out most heat, notwithstanding the diminution of light where the spots are. He sought for evidence on this point in the price of corn in England, and it actually appeared, though by a mere coincidence, that corn had been cheapest in years of spot-frequency, a result regarded by Herschel as implying that the weather had been warmer on the whole in those years. It was well pointed out, however, by Arago, that "in these matters we must be careful how we generalize facts before we have a very considerable number of observations at our disposal." The peculiarities of weather in a single and not extensive country like England are quite insufficient to supply an answer to the wide question dealt with by Herschel. The weather statistics of many countries must be considered and compared. Moreover, very long periods of time must be dealt with.[3]
M. Gautier, of Geneva, and, later, MM. Arago and Barratt, made a series of researches into the tabulated temperature at several stations, and for many successive years. They arrived at the conclusion that, on the whole, the weather is coolest in years of spot-frequency.
But recently the matter has been more closely scrutinized, and it has been found that the effects due to the great solar-spot period, although recognizable, are by no means so obvious as had been anticipated.
These effects may be divided into three classes: those affecting (1) temperature, (2) rainfall, and (3) terrestrial magnetism.
As respects the first, it has been discovered that, when underground temperatures are examined, so that local and temporary causes of change are eliminated, there is a recognizable diminution of temperature in years when spots are most frequent. We owe this discovery to Prof. C. P. Smyth, Astronomer Royal for Scotland. The effect is very slight; indeed, barely recognizable. I have before me, as I write, Prof. Smyth's chart of the quarterly temperatures from 1837 to 1869, at depths of 3, 6, 12, and 24 French feet. Of course, the most remarkable feature, even at the depth of 24 feet, is the alternate rise and fall with the seasons. But it is seen that, while the range of rise and fall remains very nearly constant, the crest and troughs of the waves lie at varying levels. After long and careful scrutiny I find myself compelled to admit that I cannot find the slightest evidence in this of a connection between underground temperatures and the eleven-years period of sun-spots. I turn, therefore, to the chart in which the annual means are given; and, noting in the means at the lesser depths "confusion worse confounded" (this, of course, is no fault of Prof. Smyth's, who here merely records what had actually taken place), I take the temperatures at a depth of 24 French feet. Now, neglecting minor features, I find the waves of temperature thus arranged: They go down to a little more than 46½ degrees of the common thermometer in 1839-'40; rise to about 47¾ degrees in 1847; sink to 47¼ degrees in 1849; mount nearly to 47¾ degrees again in 1852-'53; are at 47 degrees in 1856-'57; are nearly at 48 degrees in 1858-'59; then they touch 47 degrees three times (with short periods of rising between), in 1860, 1864, and 1867; and rise above 47½ degrees in 1869. Now, if we remember that there were maxima of spots in 1837, 1848, 1859-'60, and 1870, while there were minima in 1843, and in 1855-'56, I think it will be found to require a somewhat lively imagination to recognize a very striking association between the underground temperature and the sun's condition with respect to spots. If many spots imply diminution of heat, how does it come that the temperature rises to a maximum in 1859, and again in 1869? if the reverse, how is it that there is a minimum in 1860? I turn, lastly, to the chart in which the sun-spot waves and the temperature-waves are brought into actual comparison, and I find myself utterly unable to recognize the slightest association between them. Nevertheless, I would not urge this with the desire of in any way throwing doubt upon the opinion to which Prof. Smyth has been led, knowing well that the long and careful examination he has given to this subject in all its details may have afforded ample though not obvious evidence for the conclusions at which he has arrived. I note, also, that, as he points out, Mr. Stone, director of the Cape Town Observatory, and Mr. Cleveland Abbe, director of the Cincinnati Observatory, have since, "but it is believed quite independently, published similar deductions touching the earth's temperature in reference to sun-spots." All I would remark is, that the effect is very slight, and very far from being obvious at a first inspection.
Next as to rainfall and wind.
Here, again, we have results which can hardly be regarded as striking, except in the forcible evidence they convey of the insignificance of the effects which are to be imputed to the great eleven-year spot period. We owe to Mr. Baxendell, of Manchester, the most complete series of investigations into this subject. He finds that, at Oxford, during the years when sun-spots were most numerous, the amount of rainfall under west and southwest winds was greater than the amount under south and southeast winds; while the reverse was the case in years when spots were few and small. Applying corresponding processes to the meteorological records for St. Petersburg, he finds that a contrary state of things prevailed there. Next we have the evidence of the Rev. R. Main, director of the Radcliffe Observatory at Oxford, who finds that westerly winds are slightly more common when sun-spots are numerous than at other times. And, lastly, Mr. Meldrum, of Mauritius, notes that years of spot-frequency are characterized, on the whole, by a greater number of storms and hurricanes than years when the sun shows few spots.
The association between the sun-spot period and terrestrial magnetism is of a far more marked character, though I must premise that the Astronomer Royal, after careful analysis of the Greenwich magnetic records, denies the existence of any such association whatever. There is, however, a balance of evidence in its favor. It seems very nearly demonstrated that the daily sway of the magnetic needle is greatest when sun-spots are numerous, that magnetic storms are somewhat more numerous at such times, and that auroras are also more commonly seen. Now, it has been almost demonstrated by M. Marié Davy, chief of the meteorological division in the Paris Observatory, that the weather is affected in a general way by magnetic disturbances. So that we are confirmed in the opinion that, indirectly, if not directly, the weather is affected to some slight degree by the great sun-spot period.
Still, I must point out that not one of these cases of agreement has any thing like the evidence in its favor which had been found for an association between the varying distance of Jupiter and the sun-spot changes. For eight consecutive maxima and minima this association has been strongly marked, and might be viewed as demonstrated—only it chances, unfortunately, that, for two other cases, the relation is precisely reversed; and, in point of fact, whereas the period now assigned to the great sun-spot wave is eleven years and rather less than one month, Jupiter's period of revolution is eleven years and about ten months, a discrepancy of nine months, which would amount to five and a half years (or modify perfect agreement into perfect disagreement) in seven or eight cycles.
But, accepting the association between weather and the sun-spot changes as demonstrated (which is granting a great deal to the believers in solar weather-prediction), have we any reason to believe that by a long-continued study of the sun the great problem of foretelling the weather can be solved? This question, as I have already pointed out, must not be hastily answered. It is one of national, nay, of cosmopolitan importance. If answered in the affirmative, there is scarcely any expense which would be too great for the work suggested; but all the more careful must we be not to answer it in the affirmative, if the true answer should be negative.
But it appears to me that so soon as the considerations dealt with above have been fairly taken into account, there can be no possible doubt or difficulty in replying to the question. The matter has, in effect, though not in intention, been tested experimentally, and the experiments, when carried out under the most favorable conditions, have altogether failed. To show that this is so, I take the position of affairs before Schwabe began that fine series of observations which ended in the discovery of the great spot-period of eleven years. Let us suppose that at that time the question had been mooted whether it might not be possible, by a careful study of the sun, to obtain some means of predicting the weather. The argument would then have run as follows: "The sun is the great source of light and heat; that orb is liable to changes which must in all probability affect the supply of light and heat; those changes may be periodical, and so predictable; and, as our weather must to some extent depend on the supply of light and heat, we may thus find a means of predicting weather-changes." The inquiry might then have been undertaken, and undoubtedly the great spot-period would have been detected, and with this discovery would have come that partial power of predicting the sun's condition which we now possess—that is, the power of saying that in such and such a year, taken as a whole, spots will be numerous or the reverse. Moreover, meteorological observations conducted simultaneously would have shown that, as the original argument supposed, the quantity of heat supplied by the sun varies to a slight degree with the varying condition of the sun. Corresponding magnetic changes would be detected; and also those partial indications of a connection between phenomena of wind and rain and the sun's condition which have been indicated above. All this would be exceedingly interesting to men of science. But, supposing all this had been obtained at the nation's expense, and the promise had been held out that the means of predicting weather would be the reward, the non-scientific tax-paying community might not improbably inquire what was the worth of these discoveries to the nation or to the world at large. Be it understood that I am not here using the cui-bono argument. As a student of science, I utterly repudiate the notion that, before scientific researches are undertaken, it must be shown that they will pay. But it is one thing to adopt this mean and contemptible view of scientific research, and quite another to countenance projects which are based ab initio upon the ground that they will more than repay their cost. Now, I think, if the nation made the inquiry above indicated, and under the circumstances mentioned, it would be very difficult to give a satisfactory reply. The tax-payers would say: "We have supplied so many thousands of pounds to found national observatories for the cultivation of the physics of science, and we have paid so many thousands of pounds yearly to the various students of science who have kindly given their services in the management of these observatories; let us hear what are the utilitarian results of all this outlay. We do not want to hear of scientific discoveries, but of the promised means of predicting the weather." The answer would be: "We have found that storms in the tropics are rather more numerous in some years than others, the variations having a period of eleven years; we can assert pretty confidently that auroras follow a similar law of frequency; southwest winds blow more commonly at Oxford but less commonly elsewhere, when the sun-spots, following the eleven-year period, are at a maximum; and more rain falls with southwesterly winds than with southeasterly winds at Oxford and elsewhere, but less at St. Petersburg and elsewhere, when sun-spots are most numerous, while the reverse holds when the spots are rare." I incline to think that, on being further informed that these results related to averages only, and gave no means of predicting the weather for any given day, week, or month, even as respects the unimportant points here indicated, the British tax-payer would infer that he had thrown away his money. I imagine that the army of observers who had gathered these notable results would be disbanded rather unceremoniously, and that for some considerable time science (as connected, at any rate, with promised "utilitarian" results) would stink in the nostrils of the nation.
But this is very far, indeed, from being all. Nay, we may almost say that this is nothing. Astronomers know the great spot-period; they have even ascertained the existence of longer and shorter periods less marked in character; and they have ascertained the laws according to which other solar features besides the spots vary in their nature. It is certain that whatever remains to be discovered must be of a vastly less-marked character. If, then, the discovery of the most striking law of solar change has led to no results having the slightest value in connection with the problem of weather-prediction, if periodic solar changes of a less marked character have been detected which have no recognizable bearing on weather-changes, what can be hoped from the recognition of solar changes still more recondite in their nature? It is incredible that the complex phenomena involved in meteorological relations regarded as a whole, those phenomena which are but just discernibly affected by the great sun-spot period, should respond to changes altogether insignificant even when compared with the development and decay of a single small sun-spot. It appears to me, therefore, that it is the duty of the true lover of science to indicate the futility of the promises which have been mistakenly held out; for it cannot be to the credit of science, or ultimately to its advantage, if government assistance be obtained on false pretences for any branch of scientific research.
- ↑ I use this qualifying word, because some differences of the kind are more or less regular. Thus, when there is a dry summer in certain regions in the west of Europe, there is commonly a wet summer in easterly regions in the same latitude, and vice versa, the difference simply depending on the height at which the clouds travel which are brought by the southwesterly counter-trade winds. When these clouds travel high, they do not give up their moisture until they have travelled far inland or toward the east; when they travel low, their moisture is condensed so soon as they reach the western land-slopes. It is not uncommonly the case again that, when we in England have dry summers, much rain falls on the Atlantic, and our drought is simply due to the fall of this rain before the clouds from the southwest have reached us. More commonly, however, drought in England is due to the delay of the downfall, in consequence of the clouds from the southwest travelling at a greater height than usual.
- ↑ It is easily shown that such would be the size of the imagined black disk. For the sun's distance varies from about 93,000,000 miles to about 90,000,000, or in the proportion of 31 to 30. Hence the size of Ms disk varies in the proportion of 31 times 31 to 30 times 30, or as 961 to 900. The defect of the latter number 900 amounts to 61, which is about a sixteenth part of the larger number. But a black disk having a diameter equal to a quarter of the sun's would cut off precisely a sixteenth part of his light and heat, which was the fact to be proved.
- ↑ When Herschel made his researches into this subject, the law of spot-frequency had not been discovered. He would probably have found in this law, as some have since done, the explanation of the seven years of plenty and the seven years of famine, typified by the fat kine and lean kine of Joseph's dream. For, if there were a period of eleven years in which corn and other produce of the ground waxed and waned in productiveness, it would be not at all unlikely that, when ever this waxing and waning chanced to be unusually marked, there would result two series of poor and rich years apparently ranging over fourteen instead of eleven years. We have seen, above, that the waves of spot-waxing and spot-waning are not all alike in shape and extent. Whenever, then, a wave more marked than usual came, we should expect to find it borrowing, so to speak, both in trough and crest, from the waves on either side. It would require but a year or so either way to make the wave range over fourteen years; and observed facts, even during the last half-century only, show this to be no unlikely event.