OF ICE. 



457 



Squirrel takec the latter between its fore-paws, presses it 

 against its upper incisor teeth, and makes it revolve rapidly. 

 In a second or two the kernel is perfectly peeled, and is then 

 eaten. 



In this case the incisor teeth of the Squirrel take the part of 

 the knife, the muscles of the leg that of the spring, and the 

 sharp edges of the upper teeth that of the knife. The struc- 

 ture of the Rodent teeth has already been explained in 

 page 233. 



THE wonderful effects of water in breaking up the hardest 

 rock have already been described. We will now proceed to 

 another branch of the same subject. 



Perhaps some of my readers may have wandered along our 



FROST-OLEFT ROCK. 



8TOXE- SPLITTING. 



rocky coasts, and have seen how large masses of rock are con- 

 tinually detaching themselves, though they are so harl that a 

 cold chisel is needed to make any impression upon the.n. 



Then they fall into the sea, and are rolled backwards and 

 forwards until they become smoothed and rounded, and are 

 called pebbles, while the portion that is rubbed off them is 

 called sand. The phenomenon is well shown in the wonderful 

 Pebble Ridge of North Devon. 



The real agent is ice. 



We all know that, when water freezes, it expands consider- 

 ably. This accounts for two phenomena. 



First, as it expands, it becomes lighter than water, and con- 

 sequently floats on the surface. 



Next, there are few of us who have not seen water-bottles 



