are unduly crowded together; in the stereographic, their projected dimensions are, on the contrary, somewhat enlarged in receding from the centre.

(283.) Both these projections may be considered natural ones, inasmuch as they are really perspective representations of the surface on a plane. Mercator's is entirely an artificial one, representing the sphere as it cannot be seen from any one point, but as it might be seen by an eye carried successively over every part of it. In it, the degrees of longitude, and those of latitude, bear

always to each other their due proportion: the equator is conceived to be extended out into a straight line, and the meridians are straight lines at right angles to it, as in the figure. Altogether, the general character of maps on this projection is not very dissimilar to what would be produced by referring every point in the globe to a circumscribing cylinder, by lines drawn from the centre, and then unrolling the cylinder into a plane. Like the stereographic projection, it gives a true representation, as to form, of every particular small part, but varies greatly in point of scale in its different regions; the polar portions in particular being extravagantly enlarged; and the whole map, even of a single hemisphere, not being comprizable within any finite limits.

(284.) We shall not, of course, enter here into any geographical details; but one result of maritime discovery on the great scale is, so to speak, massive enough to call for mention as an astronomical feature. When the continents and seas are laid down on a globe (and since the discovery of Australia and the recent addition to our antarctic knowledge of Victoria Land by Sir J. C. Ross, we are sure that no very extensive tracts of land remain unknown),

we find that it is possible so to divide the globe into two hemispheres, that one shall contain nearly all the land; the other being almost entirely sea. It is a fact, not a little interesting to Englishmen, and, combined with our insular station in that great highway of nations, the Atlantic, not a little explanatory of our commercial eminence, that London occupies nearly the centre of the terrestrial hemisphere. Astronomically speaking, the fact of this divisibility of the globe into an oceanic and a terrestrial hemisphere is important, as demonstrative of a want of absolute equality in the density of the solid material of the two hemispheres. Considering the whole mass of land and water as in a state of equilibrium, it is evident that the half which protrudes must of necessity be buoyant ; not, of course, that we mean to assert it to be lighter than water, but, as compared with the whole globe, in a less degree heavier than that fluid. We leave to geologists to draw from these premises their own conclusions (and we think them obvious enough) as to the internal constitution of the globe, and the immediate nature of the forces which sustain its continents at their actual elevation; but in any future investigations which may have for their object to explain the local deviations of the intensity of gravity, from what the hypothesis of an exact elliptic figure would require, this, as a general fact, ought not to be lost sight of.

(285) Our knowledge of the surface of our globe is incomplete, unless it include the heights above the sea level of every part of the land, and the depression of the bed of the ocean below the surface over all its extent. The latter object is attainable (with whatever difficulty and howsoever slowly) by direct sounding; the former by two distinct methods: the one consisting in trignometrical measurement of the differences of level of all the stations of a survey; the other, by the use of the barometer, the principle of which is, in fact, identical with that of the sounding line. In both cases we measure the distance of the point whose level we would know from the surface of an equilibrated ocean: only in the one case it is an ocean of water; in the other, of air. In the one case our sounding line is real and tangible; in the other, an imaginary

* More exactly, Falmouth. The central point of the hemisphere which contains the maximum of land falls very nearly indeed upon this port. The land in the opposite hemisphere, with exception of the tapering extremity of South Ame rica and the slender peninsula of Malacca, is wholly insular, and were it not for New Holland would be quite insignificant in amount.

one, measured by the length of the column of quicksilver the superincumbent air is capable of counterbalancing.

(286.) Suppose that instead of air, the earth and ocean were covered with oil, and that human life could subsist under such circumstances. Let ABCDE be a continent, of which the portion A B C projects above the water, but is covered by the oil, Fig. 44.

G

A

B

E

D

Then

which also floats at an uniform depth on the whole ocean. if we would know the depth of any point D below the sea level, we let down a plummet from F. But, if we would know the height of B above the same level, we have only to send up a float from B to the surface of the oil; and having done the same at C, a point at the sea level, the difference of the two float lines gives the height in question.

(287.) Now, though the atmosphere differs from oil in not having a positive surface equally definite, and in not being capable of carrying up any float adequate to such an use, yet it possesses all the properties of a fluid really essential to the purpose in view, and this in particular,-that, over the whole surface of the globe, its strata of equal density supposed in a state of equilibrium, are parallel to the surface of equilibrium, or to what would be the surface of the sea, if prolonged under the continents, and therefore each or any of them has all the characters of a definite surface to measure from, provided it can be ascertained and identified. Now, the height at which, at any station B, the mercury in a barometer is supported, informs us at once how much of the atmosphere is incumbent on B, or, in other words, in what stratum of the general atmosphere (indicated by its density) B is situated: whence we are enabled finally to conclude, by mechanical reasoning,* at what height above the sea-level that degree of density is to be found

* Newton's Princip. ii. Prop. 22.

over the whole surface of the globe. Such is the principle of the application of the barometer to the measurement of heights. For details, the reader is referred to other works.*

(288.) We will content ourselves here with a general caution against an implicit dependence on barometric measurements, except as a differential process, at stations not too remote from each other. They rely in their application on the assumption of a state of equilibrium in the atmospheric strata over the whole globewhich is very far from being their actual state (art. 37.) Winds, especially steady and general currents sweeping over extensive continents, undoubtedly tend to produce some degree of conformity in the curvature of these strata to the general form of the landsurface, and therefore to give an undue elevation to the mercurial column at some points. On the other hand, the existence of localities on the earth's surface where a permanent depression of the barometer prevails to the astonishing extent of nearly an inch, has been clearly proved by the observations of Ermann in Siberia and of Ross in the Antarctic Seas, and is probably a result of the same cause, and may be conceived as complementary to an undue habitual elevation in other regions.

(289.) Possessed of a knowledge of the heights of stations above the sea, we may connect all stations at the same altitude by level lines, the lowest of which will be the outline of the seacoast; and the rest will mark out the successive coast-lines which would take place were the sea to rise by regular and equal accessions of level over the whole world, till the highest mountains were submerged. The bottoms of valleys and the ridge-lines of hills are determined by their property of intersecting all these level lines at right angles, and being, subject to that condition, the shortest and longest, that is to say, the steepest, and the most gently sloping courses respectively which can be pursued from the summit to the sea. The former constitute "the water courses" of a country; the latter its lines of "water-shed" by which it is divided into distinct basins of drainage. Thus originate natural districts of the most ineffaceable character, on which the distribution, limits, and peculiarities of human communities are in great

Biot, Astronomie Physique, vol. iii. For tables, see the work of Biot cited. Also those of Oltmann, annually published by the French board of longitudes in their Annuaire; and Mr. Baily's Collection of Astronomical Tables and Formulæ.

measure dependent. The mean height of the continent of Europe, or that height which its surface would have were all inequalities levelled and the mountains spread equally over the plains, is according to Humboldt 671 English feet; that of Asia, 1137; of North America, 748; and of South America, 1151.