SCIENTIFIC RECREATIONS. 



density of any substance relatively to water," or as Professor Huxley says, 

 " The weight of a volume of any liquid or solid in proportion to the weight 

 of the same volume of water, at a known temperature and pressure." 



Water, therefore, is taken as the unit ; so anything whose equal volume 

 under the same circumstances is twice as heavy as the water, is declared 

 to have its specific gravity 2 ; if three-and-a-half times it is 3*5, and so on. 

 We append a few examples ; so we see that things which possess a higher 

 specific gravity than water sink, which comes to the same thing as saying 

 they are heavier than water, and vice versa. 



To find the specific gravity of any solid body proceed as above, in 

 the experiment of the lead. By weighing the substance in and out of water 

 we find the weight of the water displaced ; that is, the first weight less the 



Fig. 74. Over-shot wheel of mill. 



second. Divide the weight in air by the remainder, and we shall find the 

 specific gravity of the substance. 



The following is a table of specific gravities of some very different 

 substances, taking water as the unit. 



But we have by no means exhausted the uses of water. Hydro- 

 dynamics, which is the alternative term for hydraulics, includes the con- 

 sideration of many forms of water-wheels, most of which, as mill-wheels, are 



