86 GRAVITATION. 



so it cannot be changed or taken away. Heat a mag- 

 netic needle, and it loses its power. Touch an excited 

 electric, and it will attract no longer. But a bullet or a 

 stone can by no human skill be deprived of its weight. 

 The Creator has made this principle the inseparable and 

 immutable property of every material object which he has 

 formed. 



2. It is the same witJi every species of matter. 



This may at the first sight appear untrue. For it will 

 at once occur to the reader, that since gravitation is the 

 cause of weight, and some bodies are much more heavy 

 than others of the same magnitude, there must be a dif- 

 ference in the gravitating power. For example, since a 

 ball of lead falls much more rapidly and with much 

 greater force than a ball of cork, it might be inferred 

 that the gravitation of the former is much greater than 

 that of the latter. 



But there is another way to account for the rapidity and 

 force with which lead descends; i.e. by supposing that there 

 is more lead than cork in balls of equal size. The struc- 

 ture of the cork may be such that the particles are not 

 compact together, so that they may be three times as 

 many particles of lead in the same space. If this were 

 true, the whole mass of lead would fall with three times 

 the force, if every particle, whether of lead or of cork, 

 were attracted alike. Now as the force with which the 

 ball of cork would fall, must be less than that of the lead, 

 its velocity must be less too, for it has the same quantity 

 of air to remove from its path, and less power with which 

 to remove it. It would therefore be more retarded. 



If now the atmosphere is removed, and the two bodies 

 are allowed to fall through empty space, they will fall 

 together, if both kinds of matter are attracted equally. 

 For if each is drawn in proportion to the quantity in 

 each, it is plain that they will be equally affected. This 

 experiment has often been tried ; it is called the guinea 

 and feather, experiment, because a guinea and a feather 

 have been often used as the light and heavy bodies. 

 A tall glass vessel is fitted to the air pump, with the 



