OF THE PROPERTIES OF MATTER. 11 



the bottom of it. So, if a piece of wood be immersed iJ 

 in a jar of water, the wood will rise to the top of the 

 water, because it has a less degree of weight than its 

 bulk of water has : but if the jar be emptied of water, 

 the wood falls to the bottom. 9 



As every particle of matter has its proper gravity, 

 effect of the whole must be in proportion to the number ted to be as 

 of the attracting particles ; that is, as the quantity of | 



matter in the whole body. This is demonstrable by ex- ter in bo- 

 periments on pendulums ; for, if they are of equal 

 lengths, whatever their weights be, they vibrate in equal 

 times. Now it is plain, that if one be double or triple 

 the weight of another, it must require a double or triple 

 power of gravity to make it move with the same 

 celerity : just as it would require a double or triple 

 force to project a bullet of twenty or thirty pounds weight 

 with the same degree of swiftness that a bullet of ten 

 pounds would require. Hence it is evident, that the 

 power or force of gravity is always proportional to the 

 quantity of matter in bodies, whatever their bulks or 

 figures are. 



Gravity also, like all other virtues or emanations it de- 

 which proceed or issue from a center, decreases as the 



distance multiplied by itself increases : that is, a body f the 

 at twice the distance of another, attracts with only a increases. 

 fourth part of the force ; at thrice the distance, with a 

 ninth part ; at four times the distance, with a sixteenth 

 part ; and so on. This too is confirmed by comparing 

 the distance which the moon falls in a minute, from a 

 right line touching her orbit, and the distance through 

 which heavy bodies near the earth fall in that time. 



Note 9. The floating of wood in water is here shewn to be analo- 

 gous to that of smoke in the air, but a still more striking illustration 

 of the buoyant power of air will be found in the ascent of a balloon, 

 the weight of which frequently exceeds from three to five hundred 

 pounds in weight ; and we shall presently find, that there are other 

 bodies much lighter than the gas or air with which balloons are 

 usually inflated. 



