274 NATURE'S TEACHINGS. 



being submerged below it. We have all seen the multitudinous 

 bubbles of soda-water, or of any effervescing liquid, and have 

 noticed how they are very small when generated, but enlarge 

 quickly, and rise to the surface with a rapidity equal to their 

 enlargement. The same phenomena may be observed in any 

 water-fall, or even in the very familiar and unpoetical opera- 

 tion of pouring beer from a jug into a glass. 



The reader will see that in the plumb-rule, the level, and 

 the spirit-level one single principle is employed, namely, the 

 attraction of matter towards the centre of the earth. In the two 

 former instruments this attraction gives a vertical line, and in 

 the latter it gives a horizontal line, but the principle is the 

 same in both. 



CALLIPERS. 



WE conclude the history of measuring tools with the Callipers. 

 For ordinary purposes, and upon a plane surface, the Com- 

 passes answer every purpose. But there are various arts, espe- 



JAWS OP 8IALI8. 



cialiy sculpture, in which the compasses, with their straight 

 legs, are absolutely valueless, and their place must be supplied 

 by a differently shaped instrument. For example, no ordinary 

 compasses could measure the exact distance from the nostril to 

 the back of the head, or even touch two points at opposite sides 

 of a limb, and it is therefore necessary to have compasses with 



