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south must, as they glide along the surface, at the same time lag, or hang back, and drag upon it in the direction opposite to the earth's rotation, i. e. from east to west. Thus these currents, which but for the rotation would be simply northerly and southerly winds, acquire, from this cause, a relative direction towards the west, and assume the character of permanent northeasterly and south-easterly winds.

(197.) Were any considerable mass of air to be suddenly transferred from beyond the tropics to the equator, the difference of the rotatory velocities proper to the two situations would be so great as to produce not merely a wind, but a tempest of the most destructive violence. But this is not the case: the advance of the air from the north and south is gradual, and all the while the earth is continually acting on, and by the friction of its surface accelerating its rotatory velocity. Supposing its progress towards the equator to cease at any point, this cause would almost immediately communicate to it the deficient motion of rotation, after which it would revolve quietly with the earth, and be at relative rest. We have only to call to mind the comparative thinness of the coating which the atmosphere forms around the globe (art. 34.), and the immense mass of the latter, compared with the former (which it exceeds at least 100,000,000 times), to appreciate fully the absolute command of any extensive territory of the earth over the atmosphere immediately incumbent on it, in point of motion.

(198.) It follows from this, then, that as the winds on both sides approach the equator, their easterly tendency must diminish.* The lengths of the diurnal circles increase very slowly in the immediate vicinity of the equator, and for several degrees on either side of it hardly change at all. Thus the friction of the surface has more time to act in accelerating the velocity of the

See Captain Hall's "Fragments of Voyages and Travels," 2d series, vol. i. p. 162., where this is very distinctly, and, so far as I am aware, for the first time, reasoned out. (Author.)

air, bringing it towards a state of relative rest, and diminishing thereby the relative set of the currents from east to west, which, on the other hand, is feebly, and, at length, not at all reinforced by the cause which originally produced it. Arrived, then, at the equator, the trades must be expected to lose their easterly character altogether. But not only this but the northern and southern currents here meeting and opposing, will mutually destroy each other, leaving only such preponderancy as may be due to a difference of local causes acting in the two hemispheres, which in some regions around the

equator may lie one way, in some another.

(199.) The result, then, must be the production of two great tropical belts, in the northern of which a constant north-easterly and in the southern a southeasterly, wind must prevail, while the winds in the equatorial belt, which separates the two former, should be comparatively calm and free from any steady prevalence of easterly character. All these consequences are agreeable to observed fact, and the system of aërial currents above described constitutes in reality what is understood by the regular trade winds.*

(200.) The constant friction thus produced between the earth and atmosphere in the regions near the equator must (it may be objected) by degrees reduce and at length destroy the rotation of the whole mass. The laws of dynamics, however, render such a consequence, generally, impossible; and it is easy to see, in the present case, where and how the compensation takes place. The heated equatorial air, while it rises and flows over towards the poles, carries with it the rotatory velocity due to its equatorial situation into a higher latitude, where the earth's surface has less motion. Hence, as it travels northward or southward, it will gain continually more and more on the surface of the earth in its diurnal motion, and assume constantly more and more a westerly relative direction; and when at length it returns to the surface, in its circulation, which it must

See the work last cited.

do more or less in all the interval between the tropics and the poles, it will act on it by its friction as a powerful south-west wind in the northern hemisphere, and a north-west in the southern, and restore to it the impulse taken up from it at the equator. We have here the origin of the south-west and westerly gales so prevalent in our latitudes, and of the almost universal westerly winds in the North Atlantic, which are, in fact, nothing else than a part of the general system of the re-action of the trades, and of the process by which the equilibrium of the earth's motion is maintained under their action.*

(201.) In order to construct a map or model of the earth, and obtain a knowledge of the distribution of sea and land over its surface, the forms of the outlines of its continents and islands, the courses of its rivers and mountain chains, and the relative situations, with respect to each other, of those points which chiefly interest us, as centres of human habitation, or from other causes, it is necessary to possess the means of determining correctly the situation of any proposed station on its surface. For this two elements require to be known, the ' latitude and longitude, the former assigning its distance from the poles or the equator, the latter, the meridian on which that distance is to be reckoned. To these, in strictness, should be added, its height above the sea level; but the consideration of this had better be deferred, to avoid complicating the subject.

As it is our object merely to illustrate the mode in which the earth's rotation affects the atmosphere on the great scale, we omit all consideration of local periodical winds, such as monsoons, &c.

It seems worth enquiry, whether hurricanes in tropical climates may not arise from portions of the upper currents prematurely diverted downwards before their relative velocity has been sufficiently reduced by friction on, and gradual mixing with, the lower strata; and so dashing upon the earth with that tremendous velocity which gives them their destructive character, and of which hardly any rational account has yet been given. Their course, generally speaking, is in opposition to the regular trade wind, as it ought to be, in conformity with this idea. (Young's Lectures, i. 704.) But it by no means follows that this must always be the case. In general, a rapid transfer, either way, in latitude, of any mass of air which local or temporary causes might carry above the immediate reach of the friction of the earth's surface, would give a fearful exaggeration to its velocity. Wherever such a mass should strike the earth, a hurricane might arise; and should two such masses encounter in mid-air, a tornado of any degree of intensity on record might easily result from their combination.Author.

(202.) The latitude of a station on a sphere would be merely the length of an arc of the meridian, intercepted between the station and the nearest point of the equator, reduced into degrees. (See art. 86.) But as the earth is elliptic, this mode of conceiving latitudes becomes inapplicable, and we are compelled to resort for our definition of latitude to a generalization of that property, (art. 95.) which affords the readiest means of determining it by observation, and which has the advantage of being independent of the figure of the earth, which, after all, is not exactly an ellipsoid, or any known geometrical solid. The latitude of a station, then, is the altitude of the elevated pole, and is, therefore, astronomically determined by those methods already explained for ascertaining that important element. consequence, it will be remembered that, to make a perfectly correct map of the whole, or any part of the earth's surface, equal differences of latitude are not represented by exactly equal intervals of surface.

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(203.) To determine the latitude of a station, then, is easy. It is otherwise with its longitude, whose exact determination is a matter of more difficulty. The reason is this: -as there are no meridians marked upon the earth, any more than parallels of latitude, we are obliged in this case, as in the case of the latitude, to resort to marks external to the earth, i. e. to the heavenly bodies, for the objects of our measurement; but with this difference in the two cases-to observers situated at stations on the same meridian (i. e. differing in latitude) the heavens present different aspects at all moments. The portions of them which become visible in a complete diurnal rotation are not the same, and stars which are common to both describe circles differently inclined to their horizons, and differently divided by them, and attain different altitudes. On the other hand, to observers situated on the same parallel (i. e. differing only in longitude) the heavens present the same aspects. Their visible portions are the same; and the same stars describe circles equally inclined, and

similarly divided by their horizons, and attain the same altitudes. In the former case there is, in the latter there is not, any thing in the appearance of the heavens, watched through a whole diurnal rotation, which indicates a difference of locality in the observer.

(204.) But no two observers, at different points of the earth's surface, can have at the same instant the same celestial hemisphere visible. Suppose, to fix our ideas, an observer stationed at a given point of the equator, and that at the moment when he noticed some bright star to be in his zenith, and therefore on his meridian, he should be suddenly transported, in an instant of time, round one quarter of the globe in a westerly direction, it is evident that he will no longer have the same star vertically above him: it will now appear to him to be just rising, and he will have to wait six hours before it again comes to his zenith, i. e. before the earth's rotation from west to east carries him back again to the line joining the star and the earth's centre from which he set out.

(205.). The difference of the cases, then, may be thus stated, so as to afford a key to the astronomical solution of the problem of the longitude. In the case of stations differing only in latitude, the same star comes to the meridian at the same time, but at different altitudes. In that of stations differing only in longitude, it comes to the meridian at the same altitude but at different times. Supposing, then, that an observer is in possession of any means by which he can certainly ascertain the time of a known star's transit across his meridian, he knows his longitude; or if he knows the difference between its times of transit across his meridian and across that of any other station, he knows their difference of longitudes. For instance, if the same star pass the meridian of a place A at a certain moment, and that of B exactly one hour of sidereal time, or one twenty-fourth part of the earth's diurnal period, later, then the difference of longitudes between A and B is one hour of time or 15°, and B is so much west of A.

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