16 Experimental Philosophy. [Lecture 2. 



to their quantities of matter, and inversely pro- 

 portional to the squares of their distances. This 

 is the first grand deduction of the Newtonian 

 philosophy, established upon indubitable prin- 

 ciples, and on which all the momentous facts of 

 physical astronomy depend. The tides, the pre- 

 cession of the equinoxes, the irregularities of the 

 moon's motion, the mutual perturbations of the 

 planets, and many other interesting phaenomena, 

 all receive a satisfactory explication upon the 

 principle of mutual and universal attraction. 



But to proceed : we know by experience that 

 the weight or gravity of a body or thing is not 

 in proportion to its bulk. A bullet of lead, of 

 the same size as one of wood or of cork, will 

 weigh considerably heavier, and one of gold 

 would be heavier still. It is reasonable, there- 

 fore, to suppose that the ball of gold or of lead 

 contains a greater number of solid particles, which 

 are united or pressed closer together than those 

 of the wood or cork; the latter being more porous, 

 and its particles lying less closely compressed or 

 compacted together. One body containing more 

 solid particles within a certain compass, size, 

 bulk, or space, than another, gives origin to the 

 terms specific gravity and density, which are 

 greater or less in proportion as there are more or 

 fewer constituent particles comprised within a 

 given apparent bulk. 



II. The attraction of cohesion is observable in 

 almost every natural object, since in reality it is 



