THE FORM _VND THE :MAGNITUDE OF THE EARTH. To a spectator so placed as to have an unob structed view in all directions, the earth appears a circular plain on whose circumferenee the vault of heaven seems to rest. In ancient time., even philosophers looked upon the earth as a fiat disk swimming upon the water. But many ap pearances were soon observed to be at variance with this idea, and even in antiquity the curved form of the earth began to he suspected. It is only by assuming the earth to be curved that we can explain how our circle of vision becomes wider as our position is more elevated. and how the tops of towers, mountains, masts of ships. and the _like. Polite first into view as we ap proach. There are many other proofs that the earth is a globe. Thus, as we advance from the poles toward the equator, new stars, formerly in isible, come gradually into view ; the shadow the earth upon the moon during an eclipse is always round: the same momentary appear anee in the heavens is seen at different hours of the day in different places on the earth's sur face; and lastly: sinee 1519, the earth has been eircumnavigated times. It is not. however. strictly true that the earth is a sphere; it is slightly flattened or compressed at two op. posite points—the poles—as has been proved by actual measurement of degrees of latitude (q.v.) and by observation, of the pendulum. It is found that a degree of a meridian is not everywhere of the same linear length f DEGREE OF it' PE) , which NV Mild he the case if the earth were a perfect sphere. but increases front the equator to the poles, from which it is rightly inferred that the earth is flattened there. A
pendulum. again, of a given length is found to move faster when parried toward the poles. and slowly when carried toward the equator: which shows that the force of gravity is less at the equator than at the poles, or. in other words. that the centre is more distant at the fernier than at the latter. The diminished force of gravity at the equator has, it is true, an ether cause—namely. the centrifugal force aris ing from the rotation of the earth, which acts counter to gravitation, and is necessarily great est at the equator. where the linear velocity of rotation is greatest, and gradually lessens as we move inwthward or southward, till at the pules it is noodling. Itut the diminution of the force of gravity at the equator. arising from the centrifugal force, amounts to only of the whole force; while the diminution indicated by the pendulum difference, or nearly, remains assignable to the greater distance of the surface from the centre at the equator than at the poles. From the most accurate measure ments of degrees that have been made, the flat tenhig or ellipticity of the earth has liven deter mined at nearly; or the equatorial radius is to the polar as 301) to 299. These measurements of degrees determine not only the shape. but also the size of the earth. It is thus found that the equatorial semi-Inane-ter amounts to 3963 miles, while the polar semi-diameter is 3950 miles.