OF FLOATATION AND THE SPECIFIC GRAVITY OF BODIES. 223 



Apparent or relative gravity, is the excess of the gravity 

 of the body above that of the fluid in which it is placed. 



By this sort of gravity, fluids do not gravitate in their proper places ; 

 that is, they do not preponderate ; but opposing one another's descent, 

 they retain their positions as if they were possessed of no weight. 



249. INF. 5. If a heavy irregular heterogeneous body descends in 

 a fluid, or if it moves in any direction, and a straight line be drawn, 

 connecting the centres of magnitude and gravity of the body : 



It will so dispose itself as Jo move in that line, the centre 

 of gravity preceding the centre of magnitude . 



This is a manifest and a beautiful fact ; for the centre of gravity 

 being surrounded by more matter and less surface than the centre of 

 magnitude, it will meet with less resistance from the fluid ; conse- 

 quently, the body will so arrange itself, as to move in the line of 

 direction with its centre of gravity foremost. 



250. What has been here adverted to, in regard to bodies of greater 

 density or specific gravity sinking in a fluid, must only be understood 

 to apply to such as are solid ; for if a body be hollow, it may swim in 

 a fluid of less specific gravity than that which is due to the substance 

 of which the body is composed ; but if the hollows or cavities are 

 filled with the fluid, the body will then descend to the bottom. 



Again, if bodies of greater specific gravity than the fluid in which 

 they are placed, be reduced to extremely small particles, they may 

 also be suspended in the fluid ; but the principle or force by which 

 this is effected, does not belong to hydrodynamics. 



PROPOSITION IV. 



251. If a solid homogeneous body, be placed in a fluid of 

 greater or less specific gravity than itself: 



It will ascend or descend with a force, which is equivalent 

 to the difference between its own weight, and that of an equal 

 bulk of the fluid. 



The principle announced in this proposition is almost self-evident, 

 yet nevertheless, it may be demonstrated in the following manner. 



Put m the common magnitude of the body and the fluid, 

 w' the weight of the solid body, 

 s' its specific gravity. 



