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ders of the hemisphere are unduly crowded together; in the stereographic, their projected dimensions are, on the contrary, somewhat enlarged in receding from the center.

(235.) 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

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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 center, 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.

(236.) 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 we are sure that no very extensive tracts of land remain unknown, except perhaps at the south pole), 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 center 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 ima mediate 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.

(237.) 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 ex. tent. The latter object is attainable (with whatever difficulty and however slowly) by direct sounding; the former by two distinct methods: the one consisting in

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

(238.) 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 ABC projects above the water, but is covered by the oil, which also floats at an uniform depth on the whole ocean. Then 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.

(239.) 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 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 over the whole surface of the globe. Such is the principle of the appli. cation of the barometer to the measurement of heights. For details, the reader is referred to other works. +

(240.) 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 ridgelines 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 courses respectively which can be pursued from the summit to the sea. The former constitute the water. courses of a country; the latter divide it into drainage basins : and thus originate natural districts of the most ineffaceable character, on which the distribution, limits, and peculiarities of human communities are in great measure dependent. * See Cab. Cycl. PNEUMATICS, art. 143.

+ Biot. Astronomie Physique, vol. 3. 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.

CHAP. IV.

OF URANOGRAPHY.

CONSTRUCTION OF CELESTIAL MAPS AND GLOBES BY OBSERV

ATIONS OF RIGHT ASCENSION AND DECLINATION. - CELESTIAL OBJECTS DISTINGUISHED INTO FIXED AND ERRATIC. OF THE CONSTELLATIONS. NATURAL REGIONS IN THE HEAVENS. THE MILKY WAY. —THE ZODIAC. OF THE ECLIPTIC. — CELESTIAL LATITUDES AND LONGITUDES. - PRECESSION OF THE EQUINOXES. NUTATION. ABERRATION. URANOGRAPHICAL PROBLEMS.

(241.) TaE determination of the relative situations of objects in the heavens, and the construction of maps and globes which shall truly represent their mutual configurations, as well as of catalogues which shall preserve a more precise numerical record of the position of each, is a task at once simpler and less laborious than that by which the surface of the earth is mapped and measured. Every star in the great constellation which appears to revolve above us, constitutes, so to speak, a ce. lestial station; and among these stations we may, as upon the earth, triangulate, by measuring with proper instru. ments their angular distances from each other, which, cleared of the effect of refraction, are then in a state for laying down on charts, as we would the towns and villages of a country: and this without moving from our place, at least for all the stars which rise above our horizon.

(242.) Great exactness might, no doubt, be attained by this means, and excellent celestial charts constructed ; but there is a far simpler and easier, and, at the same time, infinitely more accurate course laid open to us, if we take advantage of the earth's rotation on its axis, and by observing each celestial object as it passes our meridian, refer it separately and independently to

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