CH. X.] ICE AND ITS WORK. 151 



of large size, are thus rent from the rock and ready to 

 tumble down at the next thaw, exactly as the flakes peel off 

 a stuccoed wall after a hard frost. Nor should it be for 

 gotten that frost does excellent work for the farmer in 

 breaking up hard ground. A stiff soil is more or less 

 loosened after a thaw, and is thus brought easily within the 

 reach of other denuding agents. 



In addition to the mechanical force exerted by water 

 during freezing, there are other ways in which ice assists in 

 the destruction of the land. In a country with a mild 

 climate, like that of Britain, the effects of ice are extremely 

 feeble ; yet they are not altogether wanting, even within the 

 basin of the Thames. It has been explained in Chapter IV., 

 that when a body of water is cooled, it shrinks in bulk, like 

 other substances ; but it shrinks only when cooled down to 

 a certain temperature. In fact when water is reduced to 

 about 39° f'ahr. (4° Cent.)^ its molecules are packed more 

 closely together than at any other temperature, so that 

 whether you raise or lower the temperature above or below 

 this point, precisely the same efitect is produced ; the bulk 

 of the liquid is increased. At 39° Fahr., therefore, water 



* The thermometer commonly used in this countr)' is graduated 

 according to a plan introduced by Daniel Gabriel Fahrenheit, a native 

 of Dantzic, who settled at Amsterdam in the beginning of the last 

 century, and became famous as a thermometer maker. In Fahrenheit's 

 instrument the distance between the freezing and boiling points of water 

 is divided into 180 equal parts, or degrees, and the zero or starting- 

 ]>oint of the scale is arbitrarily placed 32 degrees below this freezing 

 point. On the Continent another scale is commonly used, known as 

 the centigrade scale, since the distance between the freezing and boiling 

 points of water is divided into one hundred degrees. The centigrade 

 scale is now frequently used in scientific investigations in this country. 

 As a given temperature is indicated by different numbers on the two 

 scales, they are distinguished by addition of " P'ahr." and " Cent." to 

 the readings or simply by the initials F. and C. 



