18 



HYDROSTATICS. 



oil is to that of the water as nine to 

 ten. 



If there are various liquids of differ- 

 ent specific gravities, and which do 

 not mix together, all poured into the 

 same vessel, they will arrange them- 

 selves so that the heaviest will make 

 a layer at the bottom, then the next 

 heaviest, and then the next heaviest; 

 and they will all lie parallel and level. 

 Thus if mercury, olive oil, and water, 

 be poured together, there will be a 

 layer of mercury at the bottom, then 

 one of water, and then one of oil, and 

 a fourth may be formed above the oil 

 by pouring spirit of wine upon it. The 

 pressure of the whole upon the bottom 

 will in this case be equal to the sur- 

 face of the bottom multiplied by the 

 product of each layer's depth into its 

 specific gravity. 



" If the lighter fluid be poured in 

 first, and then the heavier, the lighter 

 will rise through it and float on the 

 top ; and if any thing be done to alter 

 the weight of the lower layer, it will 

 rise through the upper one. Thus, 

 while the oil is floating on the water, 

 if, by heating the bottom of the vessel 

 you heat the water, it becomes lighter 

 and rises through the oil, until the oil 

 too gets heated, when it rises through 

 the water. So when a vessel containing 

 any liquid is placed on the fire, the 

 parts of the liquid next the fire get 

 heated, and rise up through the colder 

 parts which are heavier: and this is 

 found to be the princ-ipal manner of 

 communicating heat to ail the parts of 

 a liquid ; for if the heat is applied at 

 the top, it can only with great diffi- 

 culty be conducted through the liquid 

 either sideways or downwards ; but when 

 applied below, the parts as they are 

 heated become enlarged and lighter: 

 they rise up to the top, and they heat 

 the others in their progress, while those 

 others, being still somewhat heavier, 

 sink down and are heated fully in their 

 turn. By degrees the whole liquid gets 

 so hot that the parts next the bottom 

 are converted into steam or vapour, 

 which rises through the rest of the 

 liquid in bubbles to the top, and there 

 flies off till the whole liquid is evapo- 

 rated. This is the common process of 



When a solid body floats in a liquid, 

 its weight is equal to the weight of li- 

 quid which the body displaces : but in 

 order that it may float at rest, and not 



roll round, its centre of gravity must 

 be in the perpendicular, which runs 

 through the centre of gravity of the 

 part of the liquid displaced by it ; for 

 the pressure upwards of the liquid is in 

 this line, and unless that pressure is in 

 a direction running through the body's 

 centre of gravity, it cannot support the 

 centre, and when the centre is not sup- 

 ported, the body is not at rest, but must 

 fall ; or if the centre is kept up by the 

 fluid, the body must still turn round 

 when the upward pressure bears on some 

 other part of it. When the hne which 

 joins the centre of gravity of the whole 

 body and the centre of gravity of the 

 part under water, is perpendicular to the 

 surface of the water, the body will float 

 at rest in the water. In order, there- 

 fore, to make a body of any figure float 

 steadily, and, as it were, balance itself 

 in the water, with a certain proportion 

 of its bulk under water, the depth to 

 which it must be sunk must be ascer- 

 tained by the proportion of its specific 

 gravity to that of the water, and the posi- 

 tion must be ascertained by making the 

 centre of gravity of the whole body and 

 the part under water be in the plumb-line 

 or line perpendicular to the upper sur- 

 face of the water. 



CHAPTER VI. 

 Specific Gravities of Bodies. 



BY experiments with hydrostatic ba- 

 lances, and with hydrometers of differ- 

 ent kinds, the comparative gravities of 

 bodies have been ascertained with great 

 nicety. The following Table exhibits 

 in one connected view the results of 

 those trials, collected from a great 

 variety of sources, and reduced to one 

 common measure. 



In consulting it, this may further be 

 borne in mind ; that water is taken as 

 the unit for solids and liquids ; atmos- 

 pheric air for gases. Thus water is 

 1.000; mercury, at the common tem- 

 perature, 13.58: whence we conclude, 

 that mercury is between 13 and 14 times 

 heavier than water. So common air 

 is 1.000; chlorine (or oxymuriatic acid) 

 2.500; and hydrogen 0.069. Whence 

 we conclude, that chlorine is two and a 

 half times heavier, and hydrogen be- 

 tween fifteen and sixteen times lighter 

 than common air. Again: one cubic 

 foot of water weighs 1000 ounces; 

 therefore all the numbers in the 



