HYDROSTATICS. 



139 



When ia & 

 floating body 

 in its most sta- 

 ble position ? 



When -will a 

 Bolid float, and 

 when sink ? 



Fig. 126. 



What is the For this reason, all vessels which are light in proportion to 



i"n veMels?*'*^' *^®''" ^^^^ require to be ballasted by depositing in the lowest 

 portions of the vessel, immediately above the keel, a quantity 



of heavy matter, usually iron or stone. The center of gravity may thus be 



brought so low that no force of the wind striking the vessel sideways can 



capsize it. By raising the center of gravity, as when men in a boat stand 



upright, the equdibriura is rendered unstable. 



A body floating is most stable when it floats upon its great- 

 est surface : thus a plank floats with the greatest stability 

 when placed flat upon the water; and its position is unstable 

 when it is made to float edgewise. 



A soUd can never float that is heavier, bulk for bulk, than 

 the liquid in which it is immersed. 



If the weight of a solid bo exactly equal to the weight of 



an equal bulk of liquid, it will sink in it until it is entirely immersed ; but 



when once it is entirely immersed, then, the upward and downward pressure 



being equal, the soUd will neither sink or rise, but will remain suspended 



at any depth at which it may be placed. 



Let A B, Fig. 126, be a cube of wood floating in 



water; then the weight of the water displaced, or 



the weight of a volume of water equal to A B, is 



equal to the whole weight of the wood ; since the 



upward pressure on the bottom of A B is the same 



as that which would support a portion of water 



equal in bulk to the displaced water, or to the cube 



A B ; and as the downward pressure of the body 



is equal to the upward pressure of the liquid, it fol- 

 lows that the weight of the cube is equal to the 



weight of the water displaced. Hence A B will 



neither sink or rise. 



A mass of stone, or any other heavy substance 



beneath the surface of water is more easily moved -1 



than upon the land because, when immersed in the 



water, it is hghter by the weight of its own bulk of 



water than it would be on laud. A boy will often wonder why he can lift a 



stone of a certain weight to the surface of water, but can carry it no farther. 



The least force will lifl; a bucket immersed in water to the surface ; but if it 



be lifl;ed farther, its weight is felt just in proportion to the part of it which is 



above the surface. 



The weight of the human body does not diflermuch from the weight of its 



own bulk of water; consequently, ^\'hcn bathers walk in water chin-decji, 



their feet scarcely press upon the bottom, and they have not sufficient hold 



upon the ground to give them stability ; a current, therefore, will easily take 



them off tlieir feet 



The facility with which different persons are able to float or swim, depends 



upon the physical constitution of the body. Corpulent people are lighter. 



