458 



NOTES. 



NOTE 9, p. 5. Spheroid. A solid body, which sometimes has the shape of an 

 orange, as in fig. 1 ; it is then called an oblate spheroid, because it is flattened 



Fig. 1. 



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 Cq, Ca, CN, &c., are gene- 

 rally unequal; whereas in the sphere they 

 are all equal. 



NOTE 10, p. 5. Centre of gravity. A point 

 in every body, which if supported, the body 

 will remain at rest in whatever position it 

 may be placed. About that point all the 

 parts exactly balance one another. The 

 celestial bodies attract each other as if each 

 were condensed into a single particle situate 

 in the centre of gravity, or the particle 

 situate in the centre 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. 5, 8. Poles and equator. Let fig. 1 or 3 represent the earth, 

 C its centre, 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, q Q, exceeding 

 the polar diameter, NS, by about 26J miles. Lesser circles, ABG, which are 

 parallel to the equator, are circles or parallels of latitude, which is esti- 



