Physical Geography of the Sea and its Meteorology/Chapter 11

by Matthew Fontaine Maury

CHAPTER XI.

§ 501-526. THE CL0UD REGION, THE EQUATORIAL CLOUD RING, AND SEA FOGS.

501. Cloud region—highest in the calm belts.—To simplify the discussion of these phenomena, let us consider fogs at sea to be in character like clouds in the sky. So treating them, and confining our attention to them as they appear to the mariner, we discover that the cloud region in the main is highest in the trade-wind and calm belts, lowest in extra-tropical regions.

502. Fogless regions.—At sea, beyond "the offings," fogs are not often seen between the parallels of 30° N. and S. Sea fogs, therefore, may be considered a rare phenomenon over one-half of the surface of the globe. These fogless regions, though certain parts of them are not unfrequently visited by tempests, tornadoes, and hurricanes, are nevertheless much less frequented by gales of wind, as all furious winds are called, than are the regions on the polar side of these two parallels.

503. The most stormy latitudes.—Taking the Atlantic Ocean, north and south, as an index of what takes place on other waters, the abstract logs of the Observatory show, according to the records of 205,304 observing days contained therein, that for every gale of wind that seamen encounter on the equatorial side of these two parallels of 30° N. and S., they encounter 10.4 on the polar side; and that for every fog on the equatorial they encounter 83 on the polar side. As a rule, fogs and gales increase both in numbers and frequency as you recede from the equator. The frequency of these phenomena between the parallels of 5° N. and 5° S., compared with their frequency between the parallels of 45° and 50° N. and S., is as 1 to 103 for gales, and as 1 to 102 for fogs. The observations do not extend beyond the parallels of 60°. It appears from these, however, that both the most stormy and foggy latitudes in the North Atlantic are between the parallels of 45° and 50°; that in the South Atlantic the most stormy latitudes are between the parallels of 55° and 60°, the most foggy between 50° and 55°.

504. Influences of the Gulf Stream and the ice-hearing currents of the south.—How suggestively do these two groups of phenomena remind us, on the one hand, of the Gulf Stream and the ice- bearing currents of the north, and, on the other, of Cape Horn and the Antarctic icebergs which cluster off the Falkland Islands!^[1]

505. Sea fogs rare within 20° of the equator—red fogs.—Though sea fogs within 20° on either side of the equator are so rarely seen, yet within this distance, on the north side, red fogs of " sea-dust" (§ 322) are not infrequently encountered by navigators. These can scarcely be considered as coming within the category of sea fogs. The falling of this dust in the form of fog is no doubt owing to those influences (§ 331), the effects of which are so often observable morning and evening in the settling smoke from neighboring chimneys. The fogs which at early dawn are discovered hovering over our cities or skirting the base of the hills near by are of the same sort. The "black fogs" of London may be taken as the type of them. These particles of dust, like the atoms of smoke, are brought into conditions favourable for radiation on occasions when the air in which they are floating happens to have a high dew-point. Thus each one of these innumerable little atoms of smoke and microscopic particles of sea-dust become loaded with dew, and, being made visible, have the appearance of fog. Red fogs, therefore, do not properly come under our classification of sea fogs.

506. Cloudless regions and height of clouds at sea.—On the polar side of 40° at sea the weather is for the most part cloudy. On the equatorial side, and especially within the trade-wind region, it is for the most part clear until we approach the cloud-ring, where clouds again indicate the normal state of the sky at sea. What is the height of the cloud region at sea? for vapour plane it can scarce be called. As yet our sailor observers have not turned their attention either to the height or the velocity of clouds. It is to be hoped that they will. Observations here are to be made rather under the direction of the commander of a fleet or squadron than of a single ship, and it is hoped that some of the distinguished admirals and brave old commodores who cruise about the world, with willing hearts and ready hands for the cause we advocate, may signalize their flag by contributing, for the advancement of human knowledge touching the physics of the sea and the machinery of the air, a series of well-conducted observations upon the force of the trade-winds,^[2] upon the height and velocity of the clouds, the height and velocity of the waves, etc., in different parts of the ocean.

507. Height and velocity of waves—plan for determining.—Commodore Wüllerstorf, of the Austrian frigate Novara, made an interesting series of observations upon the height and velocity of the waves during his cruise in that vessel upon his last scientific mission. These, no doubt, will be published with the other important results of that admirably conducted expedition. The most simple plan for determining the velocity of waves—and it may be hourly practised on board of every vessel—is the plan which is followed by Captain Ginn, of the American ship John Knox, one of our co-operators. When he heaves the log with the 8eas following, instead of hauling in the line immediately, he leaves the chip to tow, watching till he observes it on the crest of a wave; he then turns the glass, or notes his watch, and marks the time it takes the wave to reach the ship. The usual velocity of the waves in the Atlantic is 22-3 miles an hour, off Cape Horn 26-8.^[3]

508. Determining the height of clouds at sea.—It would afford a pleasant and agreeable diversion for a squadron of men-of-war, as they pursue their voyage at sea, to amuse themselves and instruct their friends at home with observations upon all such phenomena. Those who are willing to undertake the clouds will have no difficulty in devising a plan both for the upper and the lower strata.

509. Cloud region at sea in the shape of a double inclined plane.—Over the land the cloud region is thought to vary from three to five miles in height; there the height of clouds is known to be very variable. At sea it is no doubt less so. Here the cloud region is somewhat in the form of a double inclined plane, stretching north and south from the equatorial cloud-ring as a sort of ridge-pole. In the balloon ascents which have taken place from the New Observatory in England, it has been ascertained that there the cloud region is from 2000 to 6500 feet high, with a thickness varying from 2000 to 3000 feet, and that its temperature at the top is not lower than it is at the bottom of the cloud, notwithstanding its thickness. We are also indebted to Piazzi Smyth for interesting observations on the cloud region in the belt of north-east trades and of the upper counter current there. They were made from the Peak of Teneriife, at the height of 12,200 feet, during the months of August and September, 1856.^[4] The cloud region of the trades was between 3000 and 5000 feet high; of the upper or south-west current, it was above the mountain. Islands only a few hundred feet high are generally cloud-capped in the trade-wind regions at sea; another indication that, with a given amount of moisture in the wind, the cloud region is higher at sea than it is over the land. For most of the time during his sojourn on the Peak, the sea was concealed from view by the cloud stratum below, though the sky was clear overhead. Farther to the north, in the Atlantic, however, as in the fog region about the meeting of the cool and warm currents near the Grand Banks, the look-out at the mast-head often finds himself above the fog or cloud in which the lower parts of the ship are enveloped. Going still farther towards the north and reaching the ice, the cloud region would again, for obvious reasons, mount up until you reached the open sea there, when again it would touch the earth with its smoke.

510. Fogs in the harbour of Callao.—In the harbour of Callao, in Peru, which is filled with the cool waters of Humboldt's current, I have seen the bay covered with a fog only a few inches high. I have seen fogs there so dense, and with outlines so sharp, as to conceal from view the row-boats approaching the ship's side. These fogs, especially early in the morning, will conceal from view not only the boat, but the persons of the crew up to the neck, so as to leave nothing visible but two rows of trunkless heads nodding catenaries at the oars, apparently skimming through the air and dancing on the fog in a manner at once both magical and fantastic. At other times the cloud stratum is thicker and higher. Then may be seen three masts coming into port with topgallant-sails and royals set, but no ship. These sails, nicely trimmed and swelling to the breeze in the sky swim along over the clouds, and seem like things in a fairy scene. However, there are influences exerted in the formation of clouds and fogs over and near the land which appear not to be felt at sea.

511. The cloudy latitudes.—In the extra-tropical north, the cloud region is high over the land, low over the water; and, as a rule, the farther inland, the dryer the air and the higher the cloud region. In the circum-Antarctic regions, where all is sea, the rising vapours form themselves into clouds low down, and keep the face of the sky almost uninterruptedly obscured. The southern eaves of the cloud plane (§ 509), like the calm belts, vary their latitude as the sun does its declination, though their place is generally found between the parallels of 50° and 70° S.—farther or nearer according to the season; but under this edge, wherever it be, the mariner's heart is seldom made glad by the cheering influences of a clear sky. If not wrapped in mist, or covered with snow, or pelted with hail, or drenched with rain, as he sails through these latitudes, he is dispirited under the influences of the gloomy and murky weather which pervades those regions. His hope in the "brave west winds" and trust in the prowess of a noble ship are then his consolation and his comfort

512. Why there should be less atmosphere in the southern than in the northern hemisphere.—Such are the quantities of Vapour rising up from the engirdling ocean about those austral regions, that it keeps permanently expelled thence a large portion of the atmosphere. The specific gravity of dry air being 1, that of aqueous vapour is 0.6 (§ 252). According to the table (§ 362), the mean height of the barometer at sea, between the equator and 78° 37 north, is 30.01; whilst its mean height in lat. 70° S. is 29.0. To explain the great and grand phenomena of nature by illustrations drawn from the puny contrivances of human device is often a feeble resort, but nevertheless we may, in order to explain this expulsion of air from the watery south, where all is sea, be pardoned for the homely reference. We all know, as the steam or vapour begins to form in the tea-kettle, it expels air thence, and itself occupies the space which the air occupied. If still more heat be applied, as to the boiler of a steam-engine, the air will be entirely expelled, and we have nothing but steam above the water in the boiler. Now at the south, over this great waste of circumfluent waters, we do not have as much heat for evaporation as in the boiler or the tea-kettle; but, as far as it goes, it forms vapour which has proportionally precisely the same tendency that the vapour in the tea-kettle has to drive off the air above and occupy the space it held. Nor is this all. This austral vapour, rising up, is cooled and condensed. Thus a vast amount of heat is liberated in the upper regions, which goes to heat the air there, expand it, and thus, by altering the level, causing it to flow off. This unequal distribution of atmosphere between the two halves of the globe is imperfectly represented in barometric profile on Plate I. (§ 215)—the shading around the periphery of the circle being intended to represent the relative height, and the scales standing up in it, the barometric column.

513. Influence of Antarctic icebergs in expelling the air from austral regions.—This part of the southern ocean where the barometer shows diminished pressure is frequented by icebergs, many of them very large and high, and some of them sending up towers, minarets, and steeples, which give them the appearance in the distance of beautiful cities afloat. Each one of them is a centre of condensation. Could an eye from aloft look down upon the scene, the upper side of the cloud stratum would present somewhat the appearance of an immense caldron, boiling, and bubbling, and intumescing in the upper air. These huge bergs condense the vapour, and the liberated heat causes the air above them to swell out, and to stand like so many curiously-shaped fungi above the general cloud level. And thus, where the icebergs are thick, the clouds are formed low down. Icebergs, like islands, facilitate the formation of clouds and promote precipitation.

514. The horse latitudes—the doldrums.—Turn we now to the equatorial cloud-ring. Seafaring people have, as if by common consent, divided the ocean off into regions, and characterized them according to the winds; e.g., there are the "trade-wind regions," the "variables," the "horse latitudes," the "doldrums," .etc. The "horse latitudes" are the belts of calms and light airs (§ 210) which border the polar edge of the north-east trades. They were so called from the circumstance that vessels formerly bound from New England to the West Indies, with a deck-load of horses, were often so delayed in this calm belt of Cancer, that for the want of water for their animals, they were compelled to throw a portion of them overboard. The "equatorial doldrums" is another of these calm places (§ 212). Besides being a region of calms and baffling winds, it is a region noted for its rains and clouds, which make it one of the most oppressive and disagreeable places at sea. The emigrant ships from Europe for Australia have to cross it. They are often baffled in it for two or three weeks; then the children and the passengers who are of delicate health suffer most. It is a frightful graveyard on the wayside to that golden land. A vessel bound into the southern hemisphere from Europe or America, after clearing the region of variable winds and crossing the "horse latitudes," enters the north-east trades. Here the mariner finds the sky sometimes mottled with clouds, but for the most part clear. Here, too, he finds his barometer rising and falling under the ebb and flow of a regular atmospherical tide, which gives a high and low barometer every day with such regularity that the hour within a few minutes may be told by it. The rise and fall of this tide, measured by the barometer, amounts to about one-tenth (0.1) of an inch, audit occurs daily and everywhere between the tropics: the maximum about 10 h. 30 m. a.m., the minimum between 4 h. and 5 h. p.m., with a second maximum and minimum about 10 p.m. and 5 a.m.^[5] The diurnal variation of the needle (§ 344) changes also with the turning of these invisible tides. Continuing his course towards the equinoctial line, and entering the region of equatorial calms and rains, the navigator feels the weather to become singularly close and oppressive; he discovers here that the elasticity of feeling which he breathed from the trade-wind air has forsaken him; he has entered the doldrums, and is under the "cloud-ring."

515. A frigate under the cloud-ring.—I find in the journal of the late Commodore Arthur Sinclair, kept on board the United States frigate Congress during a cruise to South America in 1817-18, a picture of the weather under this cloud-ring that is singularly graphic and striking. He encountered it in the month of January, 1818, between the parallel of 4° north and the equator, and between the meridians of 19° and 23° west. He says of it, "This is certainly one of the most unpleasant regions in our globe. A dense, close atmosphere, except for a few hours after a thunderstorm, during which time torrents of rain fall, when the air becomes a little refreshed; but a hot, glowing sun soon heats it again, and but for your awnings, and the little air put in circulation by the continual flapping of the ship's sails, it would be almost insufferable. No person who has not crossed this region can form an adequate idea of its unpleasant effects. You feel a degree of lassitude unconquerable, which not even the sea-bathing, which everywhere else proves so salutary and renovating, can dispel. Except when in actual danger of ship-wreck, I never spent twelve more disagreeable days in the professional part of my life than in these calm latitudes. I crossed the line on the 17th of January, at eight a.m., in longitude 21° 20′, and soon found I had surmounted all the difficulties consequent to that event; that the breeze continued to freshen and draw-round to the south-south-east, bringing with it a clear sky and most heavenly temperature, renovating and refreshing beyond description. Nothing was now to be seen but cheerful countenances, exchanged as by enchantment from that sleepy sluggishness which had borne us all down for the last two weeks."

516. Subjects which at sea present themselves for contemplation.—One need not go to sea to perceive the grand work which the clouds perform in collecting moisture from the crystal vaults of the sky, in sprinkling it upon the fields, and making the hills glad with showers of rain. Winter and summer, "the clouds drop fatness upon the earth." This part of their office is obvious to all, and I do not propose to consider it now. But the sailor at sea observes phenomena and witnesses operations in the terrestrial economy which tell him that, in the beautiful and exquisite adjustments of the grand machinery of the atmosphere, the clouds have other important offices to perform besides those merely of dispensing showers, of producing the rains, and of weaving mantles of snow for the protection of our fields in winter. As important as are these offices, the philosophical mariner, as he changes his sky, is reminded that the clouds have commandments to fulfil, which, though less obvious, are not therefore the less benign in their influences, or the less worthy of his notice. He beholds them at work in moderating the extremes of heat and cold, and in mitigating climates. At one time they spread themselves out; they cover the earth as with a mantle; they prevent radiation from its crust, and keep it warm. At another time they interpose between it and the sun; they screen it from his scorching rays, and protect the tender plants from his heat, the land from the drought; or, like a garment, they overshadow the sea, defending its waters from the intense forces of evaporation. Having performed these offices for one place, they are evaporated and given up to the sunbeam and the winds again, to be borne on their wings away to other places which stand in need of like offices. Familiar with clouds and sunshine, the storm and the calm, and all the phenomena which find expression in the physical geography of the sea, the right-minded mariner, as he contemplates "the cloud without rain," ceases to regard it as an empty thing; he perceives that it performs many important offices; he regards it as a great moderator of heat and cold—as a "compensation" in the atmospherical mechanism which makes the performance perfect. Marvellous are the offices and wonderful is the constitution of the atmosphere. Indeed, I know of no subject more fit for profitable thought on the part of the truth-loving, knowledge-seeking student, be he seaman or landsman, than that afforded by the atmosphere and its offices. Of all parts of the physical machinery, of all the contrivances in the mechanism of the universe, the atmosphere, with its offices and its adaptations, appears to me to be the most wonderful, sublime, and beautiful. In its construction, the grandeur of knowledge is displayed. The perfect man of Uz, in a moment of inspiration, thus bursts forth in laudation of this part of God's handiwork, demanding of his comforters, "But where shall wisdom be found, and where is the place of understanding? The depth saith, It is not in me; and the sea saith, It is not with me. It cannot be gotten for gold, neither shall silver be weighed for the price thereof. No mention shall be made of coral or of pearls, for the price of wisdom is above rubies. Whence, then, cometh wisdom, and where is the place of understanding? Destruction and Death say, we have heard the fame thereof with our ears. God understandeth the way thereof, and he knoweth the place thereof; for he looketh to the ends of the earth, and seeth under the whole heaven; to make the weight for the winds; and he weigheth the waters by measure. When he made a decree for the rain, and a way for the lightning of the thunder, then did he see it and declare it; he prepared it, yea, and searched it out."^[6] When the pump-maker came to ask Galileo to explain how it was that his pump would not lift water higher than thirty-two feet, the philosopher thought, but was afraid to say, it was owing to "the weight of the winds;" and though the fact that the air has weight is here so distinctly announced, philosophers never recognized the fact until within comparatively a recent period, and then it was proclaimed by them as a great discovery. nevertheles, the fact was set forth as distinctly in the Book of Nature as it is in the Book of Revelation; for the infant, in availing itself of atmospherical pressure to draw milk from its mother's breast, unconsciously proclaimed it.

517. The barometer under the cloud-ring.—The barometer^[7] stands lower under this cloud-ring than on either side of it (§ 362). After having crossed it, the attentive navigator may perceive how this belt of clouds, by screening the parallels over which he may have found it to hang from the sun's rays, not only promotes the precipitation which takes place within these parallels at certain periods, but how, also, the rains are made to change the places upon which they are to fall; and how, by travelling with the calm belt of the equator up and down the earth, this cloud-ring shifts the surface from which the heating rays of the sun are to be excluded; and how, by this operation, tone is given to the atmospherical circulation of the world, and vigour to its vegetation.

518. Its motions.—Having travelled with the calm belt to the north or south, the cloud-ring leaves a clear sky about the equator; the rays of the torrid sun then pour down upon the solid crust of the earth there, and raise its temperature to a scorching heat. The atmosphere dances (§ 356), and the air is seen trembling in ascending and descending columns, with busy eagerness to conduct the heat off and deliver it to the regions aloft, where it is required to give dynamical force to the air in its general channels of circulation. The dry season continues; the sun is vertical; and finally the earth becomes parched and dry; the heat accumulates faster than the air can carry it away; the plants begin to wither, and the animals to perish. Then comes the mitigating cloud-ring. The burning rays of the sun are intercepted by it: the place for the absorption and reflection, and the delivery to the atmosphere of the solar heat, is changed; it is transferred from the upper surface of the earth to the upper surface of the clouds.

519. Meteorological processes.—Radiation from land and sea below the cloud-belt is thus interrupted, and the excess of heat in the earth is delivered to the air, and by absorption carried up to the clouds, and there transferred to their vapours to prevent excess of precipitation. In the mean time, the trade-winds north and south are pouring into this cloud-covered receiver, as the calm and rain belt of the equator may be called, fresh supplies in the shape of ceaseless volumes of heated air, which, loaded to saturation with vapour, has to rise above and get clear of the clouds before it can commence the process of cooling by radiation. In the mean time, also, the vapours which the trade-winds bring from the north and the south, expanding and growing cooler as they ascend, are being condensed on the lower side of the cloud stratum, and their latent heat is set free, to check precipitation and prevent a flood. While this process and these operations are going on upon the nether side of the cloud-ring, one not less important is, we may imagine, going on upon the upper side. There, from sunrise to sunset, the rays of the sun are pouring down without intermission. Every day, and all day long, they play with ceaseless activity upon the upper surface of the cloud stratum. When they become too powerful, and convey more heat to the cloud vapours than the cloud vapours can reflect and give off to the air above them, then, with a beautiful elasticity of character, the clouds absorb the surplus heat. They melt away, become invisible, and retain, in a latent and harmless state, until it is wanted at some other place and on some other occasion, the heat thus imparted. We thus have an insight into the operations which are going on in the equatorial belt of precipitation, and this insight is sufficient to enable us to perceive that exquisite indeed are the arrangements which Nature has provided for supplying this calm belt with heat, and of pushing the snow-line there high up above the clouds, in order that the atmosphere may have room to expand, to rise up, overflow, and course back into its channels of healthful circulation. As the vapour is condensed and formed into drops of rain, a two-fold object is accomplished; coming from the cooler regions of the clouds, the rain-drops are cooler than the air and earth below; they descend, and by absorption take up the heat which has been accumulating in the earth's crust during the dry season, and which cannot now escape by radiation.

520. Snow-line mounts up as it crosses the equatorial calm bell—In the process of condensation, these rain-drops, on the other hand, have set free a vast quantity of latent heat, which has been gathered up with the vapour from the sea by the trade-winds and brought hither. The caloric thus liberated is taken by the air and carried up aloft still farther, to keep, at the proper distance from the earth, the lino of perpetual congelation. Were it possible to trace a thermal curve in the upper regions of the air to represent this line, we should no doubt find it mounting sometimes at the equator, sometimes on this side, and sometimes on that, but always so mounting as to overleap this cloud-ring. This thermal line would not ascend always over the same parallels: it would ascend over those between which this ring happens to be; and the distance of this ring from the equator, north or south, is regulated according to the seasons. If we imagine the atmospherical equator to be always where the calm belt is which separates the north-east from the south-east trade-winds, then the loop in the thermal curve, which should represent the line of perpetual congelation in the air, would be always found to stride this equator; and it may be supposed that a thermometer, kept sliding on the surface of the earth, so as always to be in the middle of this rain-belt, would show very nearly the same temperature all the year round; and so, too, would a barometer the same pressure, though the height of the atmosphere over this calm belt would, in consequence of so much heat and expansion, be very much greater than it is over the trade-winds or tropical calms.

521. Offices of the cloud-ring.—Returning and taking up the train of contemplation as to the office which this belt of clouds, as it encircles the earth, performs in the system of oceanic adaptations, we may see how the cloud-ring and calm zone which it overshadows perform the office both of ventricle and auricle in the immense atmospherical heart, where the heat and the forces which give vitality and power to the system are brought into play—where dynamical strength is gathered, and an impulse given to the air sufficient to send it thence through its long and tortuous channels of circulation.

522. It acts as a regulator.—Thus this ring, or band, or belt of clouds is stretched around our planet to regulate the quantity of precipitation in the rain-belt beneath it; to preserve the due quantum of heat on the face of the earth; to adjust the winds; and send out for distribution to the four corners vapours in proper quantities to make up to each river-basin, climate, and season its quota of sunshine, cloud, and moisture. Like the balance-wheel of an artificial machine, this cloud-ring affords the grand atmospherical machine the most exquisitely-arranged selfcompensation. If the sun fail in his supply of heat to this region more of its vapours are condensed, and heat is discharged from its latent store-houses in quantities just sufficient to keep the machine in the most perfect compensation. If, on the other hand, too much heat be found to accompany the rays of the sun as they impinge upon the upper circumference of this belt, then again on that side the means of self-compensation are ready at hand: so much of the cloud-service as may be requisite is then resolved into invisible vapour—for of invisible vapour are made the vessels wherein the surplus heat of the sun is stored away and held in the latent state until it is called for, when it is instantly set free, and becomes a palpable and an active agent in the grand design.

523. The latent heat liberated in the processes of condensation from and under the cloud-ring, true cause of the trade-winds—Evaporation under this cloud-ring is suspended almost entirely. We know that the trade-winds encircle the earth; that they blow perpetually; that they come from the north and the south, and meet each other near the equator; therefore we infer that this line of meeting extends around the world. By the rainy seasons of the torrid zone, except where it may be broken by the continents, we can trace the declination of this cloud-ring, stretched like a girdle about our planet, up and down the earth ; it travels after the sun up and down the ocean, as from north to south and back. It is broader than the belt of calms out of which it rises. As the air, with its vapours, rises up in this calm belt and ascends, these vapours are condensed into clouds, and this condensation is followed by a turgid intumescence, which causes the clouds to overflow the calm belt, as it were, both to the north and the south. The air flowing off in the same direction assumes the character of winds that form the upper currents that are counter (Plate I.) to the trade-winds. These currents carry the clouds still farther to the north and south, and thus make the cloud-ring broader. At least we infer such to be the case, for the rains are found to extend out into the trade-winds, and often to a considerable distance both to the north and the south of the calm belt.

524. Imagined appearance of the cloud-ring to a distant observer.—Were this cloud-ring luminous, and could it be seen by an observer from one of the planets, it would present to him an appearance not unlike the rings of Saturn do to us. Such an observer would remark that this cloud-ring of the earth has a motion contrary to that of the axis of our planet itself—that while the earth was revolving rapidly from west to east, he would observe the cloud-ring to go slowly, but only relatively, from east to west. As the winds which bring this cloud-vapour to this region of calms rise up with it, the earth is slipping from under them; and thus the cloud-ring, though really moving from west to east with the earth, goes relatively slower than the earth, and would there- fore appear to require a longer time to complete a revolution. But, unlike the rings of Saturn through the telescope, the outer surface, or the upper side to us, of this cloud-ring, would appear exceedingly jagged, rough, and uneven.

525. Thunder.—The rays of the sun, playing upon this peak and then upon that of the upper cloud-surface, melt away one set of elevations and create another set of depressions. The whole stratum is, it may be imagined, in the most turgid state; it is in continued throes when viewed from above; the heat which is liberated from below in the process of condensation, the currents of warm air ascending from the earth, and of cool descending from the sky—all, we may well conceive, tend to keep the upper cloud-surface in a perpetual state of agitation, upheaval, and depression. Imagine in such a cloud-stratum an electrical discharge to take place; the report, being caught up by the cloud-ridges above, is passed from peak to peak, and repeated from valley to valley, until the last echo dies away in the mutterings of the distant thunder. How often do we hear the voice of the loud thunder rumbling and rolling away above the cloud-surface, like the echo of artillery discharged among the hills! Hence we perceive or infer that the clouds intercept the progress of sound, as well as of light and heat, and that this upper surface is often like Alpine regions, which echo back and roll along with rumbling noise the mutterings of the distant thunder.

526. Exceeding interest attached to physical research at sea.—It is by trains of reasoning like this that we are continually reminded of the interest which attaches to the observations which the mariner is called on to make. There is no expression uttered by nature which is unworthy of our most attentive consideration—for no physical fact is too bald for study—and mariners, by registering in their logs the kind of lightning, whether sheet, forked, or streaked, and the kind of thunder, whether rolling, muttering, or sharp, may be furnishing facts which will throw much light on the features and character of the clouds in different latitudes and seasons. Physical facts are the language of Nature, and every expression uttered by her is worthy of our most attentive consideration, for it is the voice of Wisdom.

1. Captain Chadwick reports, by letter of 30th April, 1860, an iceberg, seen first by him 14th September, 1859, in S. lat. 52° 25′, long. 51° 8′ W. : next, on October 10th, in 47° 15′ S., 59° 30′ W., by the Wild Pigeon. Five days later he fell in with it in lat. 45° 40′, long. 58° 40′. It was last seen 7th November, in lat. 43° 44′ S., long. 57° 14′ W., by the British ship "City of Candy." Whether this were the same "berg" or not, it shows that icebergs are not unknown to the north of the Falkland Islands, as, indeed, the aqueous isotherm of 60°, Plate IV., indicates by its sharp curve about those islands.
2. See Maury's Sailing Directions, vol. ii., "Average Force of the Trade-winds."
3. From Captain Ginn's Abstract Log:—"Saturday, September 11th, 1858, doubling Cape Horn. The long regular swell during this part of the day afforded me another opportunity of trying the velocity of the waves. This I did by paying out the log-line enough to be equal to 13 knots with the 14r-second glass; then by watching the chip—to which I had fixed a piece of white rag to render it more distinguishable—as it appeared on the crest of a well-defined wave, and turning the glass at the same time, and then noting where the crest of a wave is at the moment the glass is 'out.' "I have several times before tried the experiment in this way with the same length of line out astern, and have always found about the same rate for the velocity, namely, 22 to 23 miles an hour; but to-day I found it to be considerably more, namely, 26 to 28 miles an hour. Thus the crest of a wave would pass, while the 14-second glass ran out, from the place where the log-chip was towing astern (13 knots) to just ahead of the ship. The length of the ship is equal to about 6½ knots: the ship's speed at the time was 8 knots; thus, 13+6½+8 = 27½. A few days ago I tried the same experiment, and found the velocity to be 22 to 23. What has accelerated the velocity of these waves? Have the soundings anything to do with it?"
4. Teneriffe, An Astronomer's Experiment. London, 1858.
5. See paper on Meteorological Observations in India, by Colonel Sykes, Philosophical Transactions for 1850, part ii., page 297.
6. Job, chap. xxviii.
7. Observations now show that the thermometer stands highest under the cloud-ring. Indeed, the indications ate that it coincides with the thermal equator.