140 WELLS'S NATURAL PHILOSOPHY. 



bulk for bulk, than those of sparer habits : and as fat possesses a less specific 

 gravity than water, a fat person will swim or float easier than a thin one. 



315. It is not, however, necessary, in order that a body should float upon a 

 liquid, that the materials of wliich it is composed should be specifically lighter 

 than the hquid. If the entire mass of a solid is fighter than an equal volume 

 of the liquid, it wiU float. 



A thick piece of iron, weighing half an ounce, loses in water nearly one 

 eightli of its weight ; but if it is hammered into a plate or vessel, of such a 

 fjrm that it occupies eight times as much space as before, it will then weigh 

 Joss than an equal bulk of water, and will consequently float, sinking just to 

 the brim. If made twice as large, it will displace one ounce of water, conse- 

 quently, twice its own weight; it will then sink to the middle, and can be 

 loaded with half an ounce weight before sinking entirely. 



How can a 316. A body composed of any material, how- 

 t'han anT^ti ^ver heavy, can be made to float on any liquid, 

 be'^'mldl^'to liowever light, by giving it such a shape as 

 floa" will render its bulk or volume lighter than an 



equal bulk of water. 



Iron ships and boats are Ulustrations of this principle. A ship carrying a 

 thousand tons' weight will displace just as much water, or float to the same 

 depth, whether her cargo be feathers, cotton, or iron. A ship made of iron 

 floats just as high out of water as a ship of similar form and size made of 

 wood, provided that the iron be proportionally thinner than the wood, and 

 therefore not heavier on the whole. 



The buoyancy of hollow solids is frequently used for lifting or supporting 

 heavy weights in water. Life-preservers, which are inflated bags of India- 

 rubber, are an example. Hollow boxes, or tanks, are used for the purpose 

 of raising sunken vessels. These boxes are sunk, fiUed with water, and 

 attached to the side of the vessel to be raised. The water, by a connection 

 of pipes, is then pumped out of them, when the upward pressure of the liquid 

 becoming greater than the gravity or weight of the entire mass, the whole 

 will rise and float. 



To what is the ^17. The buoyaucy of liquids is in propor- 

 liquIdBpropor- tioii to their density or specific gravity, or, in 

 tionai? other words, a solid is buoyant in a liquid, in 



proportion as it is light, and the liquid heavy. 



Thus quicksilver, the heaviest, or most dense fluid known, supports iron 

 upon its surface; and a man might float upon mercury as easily as a cork 

 floats upon water. Many varieties of wood which will sink in oil, float 

 readily upon water. 



318. The principle that the buoyancy of liquids varies in proportion as their 

 specific gravity varies, furnishes a very ready method of determining the spe- 

 cific gravity of a liquid. This ia done by means of an instrument called the 

 hydrometer. 



