16 Things not generally Known. 



bodies, although he did not correctly understand the cause. 

 John Philoponus, the Alexandrian, probably in the sixth cen- 

 tury, was the first who ascribed the movement of the heavenly 

 bodies to a primitive impulse, connecting with this idea that 

 of the fall of bodies, or the tendency of all substances, whether 

 heavy or light, to reach the ground. The idea conceived by 

 Copernicus, and more clearly expressed by Kepler, who even 

 applied it to the ebb and flow of the ocean, received in 1666 

 and 1674 a new impulse from Robert Hooke ; and next New- 

 ton's theory of gravitation presented the grand means of con- 

 verting the whole of physical astronomy into a true mechanism 

 of the heavens. 



The law of gravitation knows no exception ; it accounts ac- 

 curately for the most complex motions of the members of our 

 own system ; nay more, the paths of double stars, far removed 

 from all appreciable effects of our portion of the universe, are 

 in perfect accordance with its theory. * 



HEIGHT OF FALLS. 



The fancy of the Greeks delighted itself in wild visions of the 

 height of falls. In Hesiod's Theogony it is said, speaking of the 

 fall of the Titans into Tartarus, " if a brazen anvil were to fall 

 from heaven nine days and nine nights long, it would reach 

 the earth on the tenth." This descent of the anvil in 777,600 

 seconds of time gives an equivalent in distance of 309,424 geo- 

 graphical miles (allowance being made, according to Galle's 

 calculation, for the considerable diminution in force of attrac- 

 tion at planetary distances) ; therefore 1 times the distance 

 of the moon from the earth. But, according to the Iliad, He- 

 phaestus fell down to Lemnos in one day ; " when but a little 

 breath was still in him." Note to Humboldt's Cosmos, vol. iii. 



KATE OF THE FALL OF BODIES. 



A body falls in gravity precisely 1 6 T ^ feet in a second, and 

 the velocity increases according to the squares of the time, viz. : 



In i (quarter of a second) a body falls . . 1 foot. 

 i (half a second) 4 feet. 



1 second . . . . . . . 16 ,, 



2 ditto 64 



3 ditto 144 



* Lord Bacon proposed that, in order to determine whether the gravity of the 

 earth arises from the gravity of its parts, a clock-pendulum should be swung in 

 a mine, as was recently done at Harton colliery by the Astronomer Royal. 



When, in 1812, Ampere noted the phenomena of the pendulum, and showed 

 that its movement was produced only when the eye of the observer was fixed 

 on the instrument, and endeavoured to prove thereby that the motion was due to 

 a play of the muscles, some members of the French Academy objected to the 

 consideration of a subject connected to such an extent with superstition. 



