In the High Heavens/Chapter 3

by Robert Stawell Ball

CHAPTER III.

THE WANDERINGS OF THE NORTH POLE.

N a visit to Cambridge, Professor E. E. Barnard, the discoverer of the fifth satellite of Jupiter, exhibited at the Cavendish Laboratory a most interesting collection of photographs made at the Lick Observatory. These pictures were obtained by a six-inch photographic lens of three-feet focus, attached to an ordinary equatorial, the telescope of which was used as a guider when it was desired to obtain a picture of the stars with a long exposure. Among the advantages of this process may be reckoned the large field that is thereby obtained, many of the plates that he exhibited containing as much as sixteen square degrees. I am, however, not now going to speak of Barnard's marvellous views of the Milky Way, nor of the plate on which a comet was discovered, nor of the vicissitudes of Holme's comet, nor of that wonderful picture in which Swift's comet actually appears to be producing, by a process of gemmation, an offshoot which is evidently adapted for an independent cometary existence. The picture to which I wish specially to refer in connection with our subject was obtained when the instrument was directed towards the North Celestial Pole.

In this particular case the clockwork which is ordinarily employed to keep the stars acting at the same point of the plate was dispensed with. The telescope, in fact, remained fixed while the heavens rotated in obedience to the diurnal motion. Under these circumstances each star, as minute after minute passed by, produced an image on a different part of the plate; the consequence of which was that, when the picture was developed, the record which the star was found to have left was a long trail instead of a sharply defined point. As each star appeared to describe a circle in the sky around the Pole, and as, in the vicinity of the Pole, these circles were small enough to be included in the plate, this polar photograph exhibited a striking spectacle. It displayed a large number of concentric circles, or rather, I should say, of portions of circles, for the exposures having lasted for about four hours, about one-sixth of each circumference was completed during that time. The effect thus produced was that of a number of circular arcs of varying sizes, and of different degrees of brightness. A representation of this photograph is given in Fig. 8.

Most conspicuous was the trail produced by the actual Pole Star itself. It is well known, of course, that though the situation of the Pole is conveniently marked by the fortunate circumstance that a bright star happened during the present century to lie in its immediate vicinity, yet, of course, this star is not actually at the Pole, and consequently, like all the other stars, Polaris itself must be revolving in a circle whereof the centre lies at the true Pole. The brighter the star the brighter is the trail which it produces, so that the circle made by Polaris is much more conspicuous than the circles produced by the other stars of inferior lustre.

Fig. 8.—The Photograph of the Stars in their Diurnal Motion round the Pole.

It is, however, to be noted that some of the faint stars lie much closer to the Pole than Polaris itself. There is, indeed, one very minute object so close to the Pole that the circle in which its movements are performed seems very little more than a point when represented on the screen on which the slide was projected.

The interesting circumstance was noted that there appeared to be occasional interruptions to the continuity of the circular arcs. This was due to the fact that clouds had interposed during the intervals represented by the interruptions. A practical application is thus suggested, which has been made to render useful service at Harvard College Observatory. Every night, and all night long, a plate is there exposed to this particular part of the sky, and the degree in which the Pole Star leaves a more or less complete trail affords an indication of the clearness or cloudiness of the sky throughout the course of the night. From the positions of the parts where the trail has been interrupted it is possible not only to learn the amount of cloudiness that has prevailed, but the particular hours during which it has lasted.

This interesting system of concentric polar circles affords us perhaps the most striking visual representation that could possibly be obtained of the existence of that point in the heavens which we know as the Pole. The picture thus exhibited was a graphic illustration of the Copernican doctrine that the diurnal stellar movement was indeed only apparent, being, of course, due to the rotation of the earth on its axis.

Suppose that a photograph, like that which I have been describing, were to be taken at intervals of a century, it would be found that the centre of the system of circles, that is to say, the veritable Pole itself, was gradually changing on the heavens. I do not by this mean that the stars themselves would be found to have shifted their places relatively to each other. No doubt there is some effect of this kind, but it is an insignificant one, and need not at present concern us. The essential point to be noticed is, that the stars which happen to lie in the vicinity of the Pole would have a changed relation to the Pole in consequence of the fact that this latter point is itself in incessant movement. At the present time the Pole is advancing in such a direction that it is getting nearer to the Pole Star, so that the actual circle which the Pole Star is describing is becoming less and less. The time will come when the circle which this star performs will have reached its lowest dimensions, but the Pole will still be moving on its way, and then, of course, the dimensions of the circle traversed by the Pole Star will undergo a corresponding increase. As hundreds of years, and thousands of years roll by the Pole will retreat further and further from the Pole Star, so that in the course of a period as far in the future as the foundation of Rome was far in the past, the Pole will be no longer sufficiently near the Pole Star to enable the latter to render to astronomers the peculiar services which it does at present.

Looking still further ahead, we find that in the course of about twelve thousand years the Pole will have gained a position as remote as it possibly can from that position which it now occupies. This most critical point in the heavens will then lie not far from the star Vega, the brightest point in the northern sky, and then it will commence to return, so that after the lapse of about twenty-five thousand years the Pole will be found again in the same celestial neighbourhood in which it is to-night, having, in the meantime, traversed a mighty circle through the constellations. In all this there is no novelty; these movements of the Pole are so conspicuous that they were detected long before the introduction of accurate instruments. They were discovered so far back as the time of Hipparchus, and the exposition of their cause by Newton was one of the triumphs of his doctrine of universal gravitation.

In giving the title of "The Wanderings of the North Pole" to this chapter I did not, however, intend to discuss the movements to which I have hitherto referred. They are so familiar that every astronomer has to attend to them practically in the reduction of almost every observation of the place of a celestial body. It was, however, necessary to make the reference which I have done to this subject in order that the argument on which we are presently to enter should be made sufficiently clear. It must be noted that the expression, "the North Pole," is ambiguous. It may mean either of two things, which are quite distinct. In the case we have already spoken of, I understand by the North Pole that point on the celestial sphere which is the centre of the system of concentric circles described by the circumpolar stars. The other sense in which the North Pole is used is the terrestrial one; it denotes that point on this earth which has been the goal of so many expeditions, and to reach which has been the ambition of so many illustrious navigators.

We have a general notion that the terrestrial North Pole lies in a desolate region of eternal ice, somewhat relieved by the circumstance that, for six months of the year, the frozen prospect is brightened by perpetual day, though on the other hand, during the remaining six months of the year this region is the abode of perpetual night. The North Pole is that hitherto unattainable point on our globe on which, if an observer could take his station, he would find that the phenomena of the rising and the setting of the stars, so familiar elsewhere, was non-existent. Each star viewed from the coign of vantage offered by the North Pole would move round and round in a horizontal circle; and the system of concentric circles would be directly overhead. In midsummer the sun would seem to revolve around, remaining practically at the same elevation above the horizon for a few days, until it slowly began to wend its way downwards in a spiral. In a couple of months it would draw near the horizon, and as day after day passed by the luminary would descend lower and lower until its edge grazed the horizon all round. The setting of the sun for the long winter would then be about to commence, and gradually less and less of the disc would remain perceptible. Finally the sun would disappear altogether, though for many days afterwards a twilight glow would travel round the whole hemisphere, ever getting less and less, until at last all indications of the sun had vanished.

The utter darkness of winter would then ensue for months, mitigated only so far as celestial luminaries were concerned by starlight or occasional moonlight. Doubtless, however, the fitful gleams of the aurora would often suffice to render the surrounding desolation visible. Then as the spring drew near, if, indeed, such a word as spring be at all applicable to an abode of utter dreariness, a faint twilight would be just discernible. The illuminated portion of the sky would move round and round the horizon each twenty-four hours, gradually becoming more and more conspicuous, until at last the edge of the sun appeared. Then, by a spiral movement inverse to that with which its descent was accomplished, the great luminary would steal above the horizon, there to continue for a period of six months until the commencement of the ensuing winter. Indeed, the actual duration of apparent summer would be somewhat protracted in consequence of the effect of refraction in raising the sun visually above the horizon when in reality it was still below. The result would be to lengthen the summer at one end and to anticipate it at the other. Such would be the astronomical conditions at the North Pole; that anomalous point, from which every other locality on the globe lies due south, that mysterious point which up to the present never seems to have been approached by man within a distance less than 400 miles, unless, indeed, as is not improbably the case, the Pre-glacial Man who lived in the last genial period found a temperate climate and enjoyable conditions even at the latitude of 90°.

For our present purpose it will be necessary to get a very clear idea as to the precise point on the earth which we mean when we speak of the North Pole. As our knowledge of it is almost entirely derived from astronomical phenomena it is necessary to assign the exact locality of the Pole by a strict definition depending on astronomical facts. Supposing that Nansen does succeed in his expedition, as every one hopes that he will, and does penetrate within that circle of 400 miles' radius where the foot of civilized man has never yet trod, how is he to identify that particular spot on this globe which is to be defined as the North Pole? It was for this purpose that at the commencement of this paper I referred to that photograph of the concentric circles which illustrated so forcibly the position of the Pole in the heavens.

Imagine that your eye was placed at the centre of the earth, and that you had a long slender tube from that centre to the surface through which you could look out at the celestial sphere; if that tube be placed in such a way that, when looking from the centre of the earth through this tube your vision was directed exactly to that particular point of the heavens which is the centre of the circle now described by the Pole Star and the other circumpolar stars, then that spot in which the end of the tube passes out through the surface of the earth is the North Pole. Imagine a stake to be driven into the earth at the place named, then the position of that stake is the critical spot on our globe which has been the object of so much scientific investigation and of so much maritime enterprise.

The reader must not think that I am attempting to be hyper-accurate in this definition of the North Pole; no doubt, in our ordinary language we often think of the Pole as something synonymous with the polar regions, an ill-defined and vaguely known wilderness of ice. For scientific purposes it is, however, essential to understand that the Pole is a very definitely marked point, and we must assign its position accurately, not merely within miles, but even within feet. Indeed, it is a truly extraordinary circumstance that, considering that no one, with the possible exception just referred to, has ever yet been within so many hundreds of miles of the Pole, we should be able to locate it so precisely that we are absolutely certain of its position to within an area not larger than that covered by a good-sized church.

We have seen that the North Pole in the sky is in incessant movement, and that the journeys which it accomplishes in the course of many centuries extend over a wide sweep of the heavens; this naturally suggests the question, Does the Pole in the earth move about in the body of the earth in any similar manner, and if so, what is the nature and extent of its variation? Here is the point about which those researches have been made which it is my object to discuss. Let us first see clearly the issue that is raised. At the time of the building of the Pyramids the Pole in the heavens was in quite a different place from its present position; the present Pole Star had not at that time the slightest title to be so called; in fact, the point around which the heavens revolved lay in a wholly different constellation. It was certainly not far from the star Alpha Draconis about 3000 b.c., and we could indicate its position quite definitely if we had any exact knowledge as to the date of the erection of the Pyramids. It is, however, plain that the difference was so patent between the celestial Pole at the time of the Pyramids and the celestial Pole of later centuries, that it could not be overlooked in attentive observation of the heavens. As the North Pole in the sky was, therefore, so different in the time of the Pharaohs from the North Pole in the time of Victoria, it is proper to ask whether there was a like difference, or any difference at all, between the terrestrial Pole at the time of the building of the Pyramids and that terrestrial Pole, in quest of which Nansen has just set off. If Pharaoh had despatched a successful expedition to the North Pole and driven a post in there to mark it, and if Nansen were now successful, would he find that the North Pole in the earth which he was to mark occupied the same position or a different position from that which had been discovered thousands of years previously?

Fig. 9.—The Movement of the Celestial Pole.

At first one might hastily say that there must be such a difference, for it will be remembered that I have defined the North Pole in the earth as that point through which the tube passes which would permit an eye placed at the centre of the earth to view the North Pole in the sky. If, therefore, the North Pole in the sky had undergone a great change in its position, it might seem obvious that the tube from the earth's centre to its surface which would now conduct the vision from that centre to the north celestial Pole would emerge at a different point of the earth's crust from that which it formerly occupied. We have here to deal with the case that arises not unfrequently in astronomy, in which a fact of broad general truth requires a minute degree of qualification; indeed, it is not too much to say that it is in this qualification of broad general truths that many of the greatest discoveries in physical science have consisted. And such is the case in the present instance. There is a broad general truth and there is the qualification of it. It is the qualification that constitutes the essential discovery which it is my object here to set forth.

Before doing so it will be necessary for me to lay down the broad general truth that the North Pole of the earth as it existed in the time of the Pharaohs appears to be practically the same as the North Pole of the earth now. It seems perfectly certain that at any time within the last 10,000 years the North Pole might have been found within a region on the earth's surface not larger than Hyde Park. Indeed, the limits might be drawn much more closely. It is quite possible that many an edifice in London occupies an area sufficiently great to cover the holes that would be made by all the posts that might be driven to mark the precise sites of the North Pole on the earth not only for the last 5,000 or 10,000 years, but probably for much longer periods. It is very likely that the North Pole at the time of the Glacial Epoch was practically indistinguishable from the North Pole now; in fact, the constancy, or I should, perhaps, rather say, the sensible constancy of the situation of this most critical point in our globe is one of the most astonishing facts in terrestrial physics.

Let us, then, assume this broad general fact of the permanency in the position of the North Pole, and deduce the obvious consequence it implies with regard to the earth's movement. At this point we find the convenience of the time-honoured illustration in our geography books which likens the earth to an orange. Let us thrust a knitting-needle through the orange along its shortest diameter to represent the axis about which the earth rotates. Not only does the earth perform one revolution about this axis in the space of each sidereal day, but the axis itself has a movement. If the earth's axis always remained fixed, or never had any motion except in a direction parallel to itself, then the point on the sky to which it was directed would never change. We have, however, seen that the Pole in the sky is incessantly altering its position; we are therefore taught that the direction of the earth's axis of rotation is constantly changing. To simulate the movement by the orange and knitting-needle we must imagine the orange to rotate around its axis once in that period of twenty-three hours and fifty-six minutes which is well known as the length of the sidereal day; while at the same time the knitting-needle itself, bearing, of course, the orange with it, performs a conical movement with such extreme slowness that not less than 25,000 years is occupied in making the circuit. The movement, as has often been pointed out, is like that of a peg-top which rotates rapidly on its axis while at the same time the axis itself has a slow revolving motion. Thus the phenomena which are presented in the rotation of the earth demonstrate that the axis about which the earth rotates occupies what is, at all events, approximately a fixed position in the earth, though not a 6xed position in space. We can hardly be surprised at this result; it merely implies that the earth acts like a rigid body on the whole, and does not permit the axis about which it is turning to change its position.

It will now be easily understood how it comes to pass that the position of the North Pole upon the earth has not appreciably changed in the course of thousands of years. The axis around which the earth rotates has retained a permanent position relative to the earth itself; but with regard to its direction in space it has continuously changed, it is at this moment changing, and will continue to change.

Fig. 10.—Rotation of the Pole.

So far our knowledge extended up to within the last few years, but quite recently a closer inquiry has been made into this, as into so many other physical subjects, and the result has been to disclose the important fact that, though the phenomena as just described are very nearly true, they must receive a certain minute qualification. Complete examination of this subject is desirable, not only on account of its natural importance, but also because it illustrates the refinements of which modern astronomical processes are susceptible.

I have stated that the position of the terrestrial Pole undergoes no large or considerable fluctuation. But while we admit that no large fluctuation is possible, it is proper to consider whether there may not be a small fluctuation. It is certain that the position of the Pole as it would be marked by a post driven into the earth to-day cannot differ by a mile from the position in which the same point would be marked last year or next year. But does it differ at all? Is it absolutely exactly the same? Would there be a difference not indeed of miles but of yards or of feet between the precise position of the Pole on the earth determined at successive intervals of time? Would it be the same if we carried out our comparisons not merely between one year and another, but day after day, week after week, month after month? No doubt the more obvious phenomena proclaim in the most unmistakable manner that the position of the Pole is substantially invariable. If, therefore, there be any fluctuations in its position, these can only be disclosed by careful scrutiny of minute phenomena which are too delicate to be detected by the coarser methods of observation. There is indeed a certain presumption in favour of the notion that absolute constancy in the position of the Pole need not be expected. Almost every statement of astronomical doctrine requires its qualification, and it would seem indeed unlikely that when sufficient refinement was introduced into the measurements the position of the Pole in the earth should appear to be absolutely unalterable.

Until a very recent period the evidence on the subject was almost altogether negative; it was no doubt recognised that there might be some fluctuations in the position of the Pole, but it was known that they could only be extremely small, and it was believed that in all probability those fluctuations must be comprised within those slender limits which are too much affected by inevitable errors of observation to afford any reliable result. Perseverance in this interesting inquiry has at last been rewarded, and as in so many similar cases we are indebted to the labours of many independent workers for the recent extension of our knowledge. We are, however, at present most interested by the labours of Mr. Chandler, a distinguished American astronomer, who has made an exhaustive examination into the subject. The result has been to prove that the Pole does undergo movement in the body of the earth. Mr. Chandler has been so successful as to have determined the law of those polar movements, and he has found that when they are taken into consideration an important improvement in certain delicate astronomical inquiries is the result. These valuable investigations merit, in the highest degree, the attention, not only of those who are specially devoted to astronomical and mathematical researches, but of that large and ever-increasing class who are anxious for general knowledge with regard to the physical phenomena of our globe.

At first sight it might seem difficult indeed to conduct the investigation of this question. Here is a point on the earth's surface, this wonderful North Pole, which, so far as we certainly know, has never yet been approached within 400 miles, and yet we are so solicitous about the position of this Pole and about its movement that we demand a knowledge of its whereabouts with an accuracy which at first appears wholly unattainable. It sounds almost incredible when we are told that a shift in the position of the North Pole to the extent of twenty yards, or even of twenty feet, is appreciable, notwithstanding that we have never been able to get nearer to it than from one end of England to the other. Indeed, as a matter of fact, our knowledge of the movements of the Pole are derived from observations made not alone hundreds but even many thousands of miles distant. It is in such observatories as those at Greenwich or Berlin, Pulkova or Washington, that the determinations have been made by which changes in the position of the Pole can be ascertained with a delicacy and precision for which those who were not aware of the refinement of modern astronomical methods would hardly be prepared. I do not, however, imply that the observations conducting to the discoveries now about to be considered have been exclusively obtained at the observatories I have named. There is a large number of similar institutions over the globe which have been made to bear their testimony. Tens of thousands of different observations have been brought together, and by discussing them it has been found possible to remove a large part of the errors by which such work is necessarily affected, and to elicit from the vast mass those grains of truth which could not have been discovered had it not been for the enormous amount of material that was available. Mr. Chandler has discussed these matters in a remarkable series of papers, and it will be necessary for me now to enter into some little detail, both as regards the kind of observations that have been made, and the results to which astronomers have been thereby conducted.

Greenwich Observatory lies more than 2,000 miles from the North Pole, and yet if the Pole were to shift by as much as the width of Regent Street, the fact that it had done so would be quite perceptible at Greenwich. Let me endeavour to explain how such a measurement could be achieved. In finding the latitude at any locality we desire, of course, to know the distance between the locality and the Equator, expressed in angular magnitude. But though this is distinctly the definition of latitude, it does not at once convey the idea as to how this element can -be ascertained. How, for instance, would an astronomer at Greenwich be able to learn the angular distance of the observatory from the Equator? The Equator is not marked on the sky, and it is obvious that the observer must employ a somewhat indirect process to ascertain what he wants. Here, again, we have to invoke the aid of that celestial Pole to which I have so often referred.

Think of that point on the sky which is the common centre of the circles exhibited on Professor Barnard's photograph. That point is not indeed marked by any special star, but it is completely defined by the circumstance that it is the centre of the track performed by the circumpolar stars. We thus obtain a clear idea of this definite point in the sky, and the horizon is a perfectly definite circle, at all events from any station where the sea is visible. It is not difficult to imagine that by suitable measurements we can ascertain the altitude of this point in the heavens above the horizon. That altitude is the latitude of the place; it is, in fact, the very angle which lies between the locality on the earth and the Equator. It is quite true that as the Pole is implied by these circles rather than directly marked by them, the measurement of the altitude cannot be effected quite directly. The actual process is to take the Polar Star, or some one of the other circumpolar stars, and to measure the greatest height to which it ascends above the horizon and the lowest altitude to which it declines about twelve hours later. The former of these is as much above the Pole as the latter is below it, so between them we are able to ascertain the altitude of the Pole with a high degree of accuracy.

It is true that in a fixed observatory such as Greenwich there is no visible sea horizon, and even if there were it would not provide so excellent a method as is offered by the equivalent process of first observing the star directly and then observing its reflection in a dish of mercury. In this way the altitude of the star above the horizon is determined with the utmost precision. The practical astronomer will, however, remember that, of course, he has to attend to the effects of atmospheric refraction, which invariably shows a star higher up than it ought to be. This can be allowed for, and in this way the latitude of the observatory is ascertained with all needful accuracy. When the highest degree of precision is sought for, and it is only observations with a very high degree of precision which are available for our present purpose, a considerable number of stars have to be employed, and very many observations have to be taken at different seasons of the year, so as to eliminate as far as possible all sources of casual error. When, however, due attention has been paid to those precautions which the experience of astronomers suggests, the result that is obtained is characterized by extraordinary precision.

How great that precision may be I must endeavour to explain. The latitude of every important observatory is obtained from a large number of observations, and it would be unlikely that it was more than one or two-tenth's of a second different from the actual mean value. Now a tenth of a second on the surface of the earth corresponds to a distance of about ten feet, and this means that the latitude of the observatory or, as we must now speak very precisely, the latitude of the centre of the meridian circle in the observatory, is known to a degree of precision represented by a few paces. It will thus be seen that, with the accuracy attainable in our modern observation, it would often be an appreciable blunder to mistake the latitude of one wall of the observatory for that of the opposite wall; in other words, we know accurately to within the tenth of a second, or within not much more than the tenth of a second, the distance from the centre of the transit circle at Greenwich down to the earth's Equator. But, of course, the distance from the Pole to the Equator is 90, and this being so it follows that the distance from the North Pole of the earth to the centre of the transit circle at Greenwich Observatory has been accurately ascertained within one or two tenths of a second. If any change took place in the distance between the Pole and the meridian circle at Greenwich, then it must be manifested by the changes of latitude. We shall now be able to understand how any movement of the Pole, or rather of the position which it occupies in the earth, would be indicated at Greenwich.

Suppose, for instance, that the Pole actually advanced towards Great Britain, and that it moved a distance of, let us say, thirty feet, the effect of this would be to produce a diminution of the distance between the Pole and Greenwich, that is to say, there must be an increase in the distance from Greenwich to the Equator. This corresponds to a change in the latitude of Greenwich; in fact it would diminish by three-tenths of a second, which is a magnitude quite large enough to be recognisable by the observations I have already indicated as proper for the determination of latitude. A shift of the Pole to a distance of sixty feet would be a conspicuous alteration announced in every observatory in Europe provided with instruments of good modern construction.

Until the last few years there was not much reason to think that the Pole exhibited any unequivocal indications of movement. No doubt, displacements resembling those which have now been definitely ascertained have existed for many years, but they were too small to produce any appreciable effect, except on instruments of a more refined description than those with which the earlier observatories were equipped. It was obvious that the Pole did not make movements of anything like a hundred yards in extent; had it done so the resulting variations in latitude would have been conspicuous enough to have obtained notice many years ago. The actual movements which the Pole does make are of that small character which require very minute discussion of the observations to establish them beyond reach of cavil. There is, however, one striking method of confirming such observations as have been made which leaves no doubt of the accuracy of the results to which they point.

If the observatories in Europe indicated at a certain time that their latitudes had all increased; this would imply that the Equator had receded from them, and that, therefore, the North Pole had approached Europe. If, however, the North Pole had approached Europe it must have retreated from those regions on the opposite side of the world—say, for instance, the Sandwich Islands. Observations in the Sandwich Islands should, therefore, indicate, if our reasoning has been correct, that the Pole had retreated from them, and that the Equator had, therefore, advanced in such a way that the latitudes of localities in the Sandwich Islands had diminished. The various observations which have been brought together by the diligence of Mr. Chandler, including those which he has himself made with an ingenious apparatus of his own design, have been submitted to this test, and they have borne it well. The result has been that it is now possible to follow the movements of the Pole with a considerable degree of completeness. Professor Chandler has tracked the Pole month after month, year after year, through a period of more than a century of exact observations, and he has succeeded in determining the movements which this point undergoes. Let me here endeavour to describe the result at which he has arrived.

Situated in that palæocrystic region which Arctic travellers have so long essayed to enter, but hitherto without success, is that interesting North Pole which is the object of so much speculation. With a particular centre in this region let a circle be supposed to be drawn the radius of which is about thirty feet. In the circumference of this circle the Pole of the earth is constantly to be found. In fact, if at different epochs, month after month and year after year, the position of the Pole was ascertained as the extremity of that tube from which an eye placed at the centre of the earth would be able to see the Pole of the heavens, and if the successive positions of this Pole were marked by pegs driven into the ground, then the several positions in which the Pole would be found must necessarily trace out the circumference of the circle that has been thus described. The period in which each revolution of the Pole around the circle takes place is about 427 days; the result, therefore, of these investigations is to show that the North Pole of the earth is not, as has been so long supposed, a fixed point, but that it revolves around in the earth, accomplishing each revolution in about two months more than the period that the earth requires for the performance of each revolution around the sun.

The detection of the movement of the Pole which I have here described must be regarded as a noteworthy achievement in astronomy, nor is the result to which it leads solely of interest in consequence of the lesson it teaches with regard to the circumstances of the earth's rotation. It has a higher utility, which the practical astronomer will not be slow to appreciate, and of which he has, indeed, already experienced the benefit. There are several astronomical investigations in which the latitude of the observatory enters as a significant element. Latitude is, in fact, at every moment employed as an important factor in many astronomical determinations: to take one of the most simple cases, suppose that we are finding the place of a planet, we deduce its position by measuring its zenith distance, and then to obtain the declination the latitude of the observatory has, of course, to be considered. Now, astronomers have hitherto been in the habit of accepting the determination of their latitude which has been established by a protracted series of observations, and treating it as if it were a constant. This method will be no longer admissible in astronomical work of the highest class. No doubt, from the sailor's point of view, an alteration in latitude which at most amounts to a shift of sixty feet, not a quarter, perhaps, of the length of his vessel, is immaterial. But in the more refined parts of astronomical work these discoveries can no longer be overlooked; indeed, Mr. Chandler has shown that many discrepancies by which astronomers have been baffled, can be removed when note is taken of the circumstance that the latitude of the observatory is in incessant alteration in accordance with the law which his labours have expounded. It will ere long be necessary, in every observatory where important work is being done, to apply each day the correction to the mean value of the latitude, in order to obtain the value appropriate for that day.

There are also other grounds of a somewhat profounder character on which the discoveries now made are eminently instructive. Those who are interested in the physics of our globe often discuss the question as to whether the internal heat, which the earth certainly possesses, is sufficiently intense to render the deep-seated portions of our globe more or less fluid. On the other hand, the effects of pressure, especially of such pressures as are experienced in the depths hundreds and thousands of miles below the surface, must go far to consolidate the materials to form what must resemble a rigid body. The question, therefore, arises, Is the earth to be regarded as a rigid mass, or is it not? The phenomena of the tides had already to some extent afforded information on this subject, and now Mr. Chandler's investigation adds much further light, for it is certain from his result that the earth cannot be a rigid body. It is quite true that, even though the earth were rigid, the Pole might revolve in a circle, and that circle might have a thirty-feet radius, but in such a case the period would be only about three-quarters of the 427 days which he has found. In the interest, therefore, of the theoretical astronomer, as well as on the other grounds which I have set forth, Mr. Chandler's investigations must be regarded as a most important contribution to modern astronomy.