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AOTEri. 



of an orange, as m fig. 1 ; it is then called an oblate spheroid, because it 

 is flattened at the poles N and S. Such 

 is the form of the earth and planets. 

 When, on the contrary, it is drawn out 

 of the poles like an egg, as in fig. 2, it is 

 called a prolate spheroid. It is evident 

 that in both these solids the radii C g, C a, 

 CN, &c., are generally unequal ; where- 

 as in the sphere they are all equal. 



NOTE 10, p. 4, Center of gravity. A 

 point in every body, which if supported, 

 the body will remain at rest in what- 2 

 ever position it may be placed. About 

 that point all the parts exactly balance 

 one another. The celestial bodies at- 

 tract each other as if each were con- 

 densed into a single particle situate in 

 the center of gravity, or the particle situ- 

 ate in the center of gravity of each may 

 be regarded as possessing the resultant 

 power of the innumerable oblique forces which constitute the whole 

 attraction of the body. 



NOTE 11, pp. 4, G. Poles and equator. Let fig. 1 or 3 represent the 

 earth, C its center, NCS the axis of rotation, or the imaginary line about 

 which it performs its daily revolution. Then N and S are the north and 

 south poles, and the great circle q E Q, which divides the earth into two 

 equal parts, is the equator. The 

 earth is flattened at the poles fig. 

 1, the equatorial diameter, g Q, 

 exceeding the polar diameter, N S, 

 by about 26 miles. Lesser cir- 

 cles, A B G, which are parallel to 

 the equator, are circles or parallels 

 of latitude, which is estimated in 

 degrees, minutes, and seconds, 

 north and south of the equator, 

 every place in the same parallel 

 having the same latitude : Green- 

 wich is in the parallel of 5128'40". 

 Thus terrestrial latitude is the an- 

 gular distance between the direc- 

 tion of a plumb-line at any place 

 and the plane of the equator. 

 Lines such as NClS, NGES, 

 fig. 3, are called meridians ; all the places in any one of these lines have 

 noon at the same instant. The meridian of Greenwich has been chosen 

 by the British as the origin of terrestrial longitude, which is estimated in 

 degrees, minutes, and seconds, east and west of that line. If N G E S be 

 the meridian of Greenwich, the position of any place, B, is determined, 

 when its latitude, Q,CB, and its longitude, EC Q, are known. 



NOTE 12, p. 4. Mean quantities are such as are intermediate between 

 others that are greater and less. The mean of any number of unequal 

 quantities is equal to their sum divided by their number. For instance, 

 the mean between two unequal quantities'is equal to half their sum. 



NOTE 13, p. 4. Ji certain mean latitude. The attraction of a sphere on 

 an external body is the same as if its mass were collected into one heavy 

 particle in its center of gravity, and the intensity of its attraction dimin- 

 ishes as the square of its distance from the external body increases. But 



