208 



MACniXERY IN^ CONNECTION WITH WATER. 



SPECIFIC GRAVITIES. 



Fig. 2M. 



In connection with Hydrostatics, the subject of the 

 specific gravities of bodies is one of importance. The 

 specific gravity of a substance is its comparative \i^eujht 

 with some other substance, an equal bulk of each being 

 taken. Water is usually the standard for comparison. 



To ascertain the specific gravity, weigh the body both 

 in and out of water, and observe 

 the difference; then divide the 

 whole weight by this difference, and 

 the quotient will be the specific 

 gravity sought. For example, if a 

 stone weighs 12 lbs. out of Avater 

 and 7 lbs. in water, divide 12 by 

 5, and the quotient is 2.4, which 

 shows that the stone is 2*1,^ times 

 heavier than water. Figure 234 

 shows the mode of weighing the 

 body in water, by suspending it be- 

 neath a balance on a hair or thread. 



It was in a similar way that Archimedes is said to have 

 succeeded in detecting the suspected fraud in the manu- 

 facture of the golden crown of the ancient king of Syra- 



Instrument for taking Specific 

 Gravities. 



cuse. lie first 



weighed 



it, and then found that it dis- 



placed more water when plunged in a vessel just filled, 

 than a piece of pure gold, and also that it displaced less 

 than silver, whence he inferred the mixture of these two 

 metals. 



When the specific gravity of a substance lighter than 

 water is to be ascertained, it is loaded down by a weight, 

 so as to sink in water, for which allowance is made in the 

 calculation. A very simple way to determine this in dif- 

 ferent kinds of wood is to form them into rods or sticks 

 of uniform size throughout, and then to observe what 

 portion of them sink when placed endwise in water. 



