axis, if produced northerly, would pass in the Latin lan guage this constellation is called Ursa Miner; and by us the Little Bear. By means of these circles, which are in general called the polar circles, and the tropics, the surface of the globe is di vided into five portions, extending from pole to pole, termned zones, or girdles: that which lies on both sides of the equa tor, and is bounded by the two tropics, is in breadth 23°, 28', 10"X2=46, 56′, 20", and is called the torrid zone, on account of the intense heat which prevails in that middle region of the earth. The spaces comprehended between the poles and the polar circles, each being in breadth 23°, 28', 10", are called the frigid zones, on account of the extreme cold experienced in those tracts of the globe. The intermediate spaces lying between the tropics and the polar circles, each in breadth 43, 03, 40", as extending from latitude 23°, 28', 10", to latitude 66, 31, 50", are termed the temperate zones, on account of the moderate temperature of the atmosphere in those regions, equally ire moved from the heat of the torrid, and the cold of the frigid, zones. Thus the surface of the earth is divided into one torrid zone, begirting its middle, two frigid zones surrounding the poles, and two temperate zones situated between the torrid and the frigid zones. This division of the earth into zones being, however, too general for the purpose of ascertaining the positions of places with respect to each other, or to the equator, the ancients thought it necessary to subdivide its surface into other por tions more minute. The principle on which this subdivision was made, was the respective length of the day and the night, in different situations on the globe, at the summer solstice, or at the longest day of the year. Supposing the day and the night VOL. II. D to to consist each of twelve hours at the equator, it was found that at the distance of 8°, 34', on either side, the day consisted of 12 hours: again, at the distance of 8°, 10′ more, or in latitude 16°, 44' north and south, the day was observed to consist of 13 hours; and, in this manner, by observing the places where the day, exceeded by hour the length of that at a place before observed, a number of concentric circles was supposed to be described round the poles, through those places, and parallel to the equator, as high up as to the polar circles and the narrow bands, or zones, included between these circles, were termed climates, from a Greek word signifying a gradual inclination; so that whatever was the length of the day at the spot through which was drawn the circle nearest to the equator, it was half an hour longer at the spot through which was drawn the adjoining circle nearest to the pole: but this division of the earth's surface is now seldom employed, and at any rate can be of very little service, since the introduction of the use of the parallels of latitude. : It was already observed that the earth not being a perfect sphere, but flattened at the poles, the difference between its axis and the diameter of the equator has by some been calculated at 34 English miles; and that a degree of latitude on either side of the equator, which may be considered as equal to a degree of longitude on the same line, contains about 69,2 English miles. Had the earth been a regular figure produced by the revolution on its shortest axis, of an ellipse whose transverse and conjugate axes differed by 34 miles, we might easily calculate the number of miles contained in a degree of latitude, at any distance from the equator; but from the measurements hitherto made, of degrees in different parts of the globe, so much uncertainty has resulted, that we must conclude either that the earth is not a solid produced by the revolution on its axis of any regular curve whatever, or that errors have occurred in the several measure measurements, for which we are not yet able to account : it is, therefore, impossible to say from analogy what number of English miles, yards, feet, &c. exactly correspond to a degree of latitude at any given spot. As the meridians all terminate in the poles, it is evident that whatever distance may be between any two at the equator, that distance must continually diminish to the poles, where it entirely disappears. If the earth were a perfect sphere, the length of a degree of longitude on any given parallel of latitude would be found by stating this proportion; as radius to the sine-complement of the latitude of the given place, so are the number of English miles in a degree of longitude on the equator, to the number in a degree of longitude at the given latitude: as, for example, if it be required to know how many English miles correspond to a degree of longitude at London, situated in north latitude. 51o, 30, 49′′, by adding together the sine complement of the latitude, and the logarithm of 69,2, the number of English miles in a degree of longitude on the equator, and subtracting radius from the sum, we have the logarithm of 43,06 miles, for the space on the surface of the earth at London, or in its parallel, corresponding to a degree of longitude: and on this principle has been calculated the following table, containing the length in English miles of a degree of longitude at every degree of latitude, from the equator to the poles. Viz. TABLE. J Lat. E. Miles. Lat. Miles. Lat. Milest Lat. Miles. o 69,2000 2363,698 46 48,0705 69 24,7902 10 8250 9,6306 37 55,2659 60 34.6000 83 8,4334 7,2335 6,0315 4,8274 1.5 66,8424 38 54,5303 61 | 33,548984 3,6219 2,4151 1,2075 0,0000 From this table it appears, that a degree of longitude on the equator may be considered as equal to the first degree of latitude on each side, or equal to 69,2 English statute miles, or 691 instead of 691, or even 693, both of which quantities give results greater than the truth. It will also be observed, that at the latitude of 60°, the degree of longitude is reduced to 34,6 English miles, or to one half of a degree on the equator; and that at the poles, or in latitude 90°, longitude disappears, because there the meridians uniting in one point, the intervals between them entirely vanish. In the account laid before the Royal Society of London, in 1787, by the late Major General Roy, of the mode proposed to be followed in the trigonometrical operations for determining the relative situations of the Royal Observatories of Paris and Greenwich, are contained some very important observations on the magnitude and figure of the earth, together with tables of the degrees of latitude and longitude on its surface, calculated agreeably to various hypotheses, in particular to those of the celebrated French mathematician Bouguer. From these tables the following is extracted, exhibiting the lengths of degrees of latitude and longitude in English fathoms (of 6 feet each) calculated for every 5 degrees, from the equator to latitude 40°; from 41° to 60°, both included, for every single degree; and from 60° to the pole for every 5 degrees. This table is founded on the hypothesis, that the earth, instead of being an ellipsoid of any proportionate axes, is a spheroid of such a description, that the lengths of the degrees of the meridian increase as they approach the pole, above that of a degree at the equator, in the proportion of the biquadratic, or fourth power, or squared squares of the sines of the latitudes. By this hypothesis the polar axis of the earth will be to the equatorial as 178,4 to 179,4; and, consequently, their difference will be 38,1 geographical miles, or 43,9 English miles; allowing 69,2 to be equal to a degree at the equator. This difference between the axes is greater than that before mentioned, namely, 34 English miles; but, at the same time, the degrees of latitude, calculated upon this hypothesis, come much nearer to those actually measured on the earth, than such as result from calculations founded on any other supposed figure of the globe. TABLE |