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PHYSICAL SCIENCES. 



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 con- 

 trary, it is drawn out at the poles like an 

 egg, as in fig. 2, it is called a prolate sphe- 

 roid. It is evident that in both these solids 

 the radii C q, C a, C N, &c., are generally 

 unequal ; whereas in the sphere they are 

 all equal. 



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

 point in every body, which if supported, 

 Q 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 par- 

 ticle 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 attrac- 

 tion of the body. 



NOTE 11, pp. 4, 6. Poles and equator. Let fig. 1 or 3 represent the 

 earth, C its centre, N C S 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, exceed- 

 ing the polar diameter, N S, by about 

 26 miles. Lesser circles, 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. 

 Greenwich is in the parallel of 

 51 28' 40". Thus terrestrial lati- 

 tude is the angular distance between 

 the direction of a plumb-line at any 

 place and the plane of the equator. 

 Lines such as N Q S, N G E S, 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 C B, and its longitude, E C 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. 



Fig. 3. 



