from the surface, and its place occupied by colder, and therefore heavier air, which glides in, on both sides, along the surface, from the regions beyond the tropics; while the displaced air, thus raised above its due level, and unsustained by any lateral pressure, flows over, as it were, and forms an upper current in the contrary direction, or towards the poles; which, being cooled in its course, and also sucked down to supply the deficiency in the extra-tropical regions, keeps up thus a continual circulation.

(241.) Since the earth revolves about an axis passing through the poles, the equatorial portion of its surface has the greatest velocity of rotation, and all other parts less in the proportion of the radii of the circles of latitude to which they correspond. But as the air, when relatively and apparently at rest on any part of the earth's surface, is only so because in reality it participates in the motion of rotation proper to that part, it follows that when a mass of air near the poles is transferred to the region near the equator by any impulse urging it directly towards that circle, in every point of its progress towards its new situation it must be found deficient in rotatory velocity, and therefore unable to keep up with the speed of the new surface over which it is brought. Hence, the currents of air which set in towards the equator from the north and 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 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 north-easterly and south-easterly winds.

west.

(242.) 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. 35.), 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.

(243.) 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 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.

(244.) 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 south-easterly, wind must prevail, while the winds in the equatorial belt, which separates the two former, should be comparatively calm and

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.

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.

(245.) 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 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.

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

It seems worth inquiry, 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. 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.

(246.) 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.

(247.) 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. 88.) 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. 119.), 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. In 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.

*

(248.) For the purposes of geodesical measurements and trigonometrical surveys, an exceedingly correct determination of the latitudes of the most important stations is required.

• In, the earth; deois (from dew, to bind), a joining or connexion (of parts).

For this purpose, therefore, the zenith sector (an instrument capable of great precision) is most commonly used to observe stars passing the meridian near the zenith, whose declinations have become known by previous long series of observations at fixed observatories, and which are therefore called standard or fundamental stars. Recently a method has been employed

with great success, which consists in the use of an instrument similar in every respect to the transit instrument, but having the plane of motion of the telescope not coincident with the meridian, but with the prime vertical, so that its axis of rotation prolonged passes through the north and south points of the horizon. Let ABCD be the celestial hemisphere projected on the horizon, P the pole, Z the zenith, A B the meridian, CD the prime vertical, QRS part of the diurnal circle of a star passing near the zenith, whose polar distance PR is but little greater than the co-latitude of the place, or the arc PZ, between the zenith and pole (art. 112.). Then the moments of this star's arrival on the prime vertical at Q and S will, if the instrument be correctly adjusted, be those of its crossing the middle wire in the field of view of the

Devised originally by Römer. Revived or re-invented by Bessel. Nachr. No. 40.