2oo PROGRESS OF SCIENCE. 



time of the moon's rotation ; III. the exact length of the 

 earth's radius (line drawn from the surface to the centre). 

 The last proposition caused a pause of sixteen years until 

 Picard (Jean, 1620 1682) found the exact measurement of 

 the radius (20,922,000 feet). Then came the great law of 

 Newton (1687), " The force of attraction of a body is equal to 

 the mass divided by the square of the distance " in other 

 words : The attraction varies inversely as the square of the 

 distance or again : As much as the square of the distance 

 increases, so much the attraction decreases.* By this law 

 Newton accounted, not only for I, the three laws of Galileo 

 and Kepler, but for 2, all the movements of the celestial 

 bodies, then for 3, the tides of the ocean (spring and neap 

 tides) ; 4, he proved experimentally the rotation of the earth 

 on its axis; 5, determined the figure of the earth to be an 

 oblate spheroid a phenomenon unsuspected, and solved 

 entirely by calculation ; 6, explained the cause of the pre- 

 cession of the equinoxes, and showed it to be due to the 

 flattening of the earth at the poles, and to the additional 

 attraction of the sun and moon exercised upon the extra 

 mass of matter gathered at the equator ; 7, determined the 

 specific gravity of the planets Saturn, for instance, is com- 

 posed of matter about nine times lighter than that of the 

 earth ; 8, accounted for the elliptical motions of the planets ; 

 9, for irregularities of planetary movements ; 10, saw that 

 some comets must move in parabolas ; n, demonstrated the 

 chief inequalities of the moon; 12, laid down the problem 

 of the nutation of the moon afterwards resolved by Euler, 

 d'Alembert, and Laplace. The great principle Newton dis- 

 covered made it possible to account for, and determine, all the 



* If at a given point a planet is at one million miles from the sun, the 

 sun will attract it, say, at the rate of 100 billions of tons per square foot 

 of its surface ; when the planet is as far again that is two million miles 

 away, the square of 2 being 4 (2 x 2=4), the sun will only attract it one- 

 fourth part, or at the rate of 25 billions of tons per square foot ; if the 

 planet is three million miles off, the square of 3 being 9 (3 x 3=9), the sun 

 will attract it one-ninth part only of the rate it drew it at starting. In 

 other words : If the distance be increased from one to two, three, four 

 units, the attraction will be decreased to the fourth, the ninth, the 

 sixteenth part of its former intensity. 



