THE BEDSHIEL KAIMS 307 



way over the rock with a steady sliding motion, and with a 

 uniform direction of movement, for a period of great length. 



Some few years ago, I ventured to point out the bearing 

 which the results of some old experiments upon ice had 

 upon this difficult matter. To put this briefly, the result 

 of those experiments was to prove that ice contracts with 

 a fall of temperature, and expands with a rise, more than 

 any solid substance known. 



(It may be remarked that it is this principle which 

 accounts for the bursting of our water pipes during a very 

 hard frost. Ice forms in a cold part of the pipe, expanding 

 as the water freezes, so that 174 volumes of water become 

 184 volumes of ice. That expansion takes place with a 

 force equivalent to 1,110 lbs. on each square inch of the 

 surface of the water-pipe. When thfi temperature falls, the 

 ice contracts again, at the rate of about half an inch for 

 each degree in every thousand feet. In the case of our 

 water-pipes, the water from the unfrozen part follows up 

 this contraction. Presently, it occurs to someone to bathe 

 the frozen pipe with some hot water or some other means 

 of applying external heat, or perhaps there is a rapid thaw. 

 The ice immediately begins to expand on the application of 

 the heat, with a force which is practically irresistible, the 

 pipe bursts, and out comes the water. So the housekeeper 

 is not so far wrong after all in saying that it is the thaw 

 that bursts the pipes.) 



Now, bearing this pi-inciple in mind, we can see at once 

 that a thick mass of ice in contact with rock below, and with 

 the air at a very low temperature above, will contract at the 

 surface about half an inch in every thousand feet for each 

 fall of temperature of one degree. The middle part of the 

 ice will remain practically unaffected, as being in a position 

 corresponding to the zone of invariable temperature in the 

 •arth's crust ; and the lower part, being in contact with the 

 rock, will receive the whole of the heat which is continually 

 fl.owing outwards from the earth. That is to say, the 

 bottom of the ice will be warmer than the top, and will 

 therefore expand more. The greater the thickness of the 

 ice the more difference in temperature will there be between 

 the top and the bottom, 



