196 WRITINGS OP JOSEPH nENRY. [1855- 



at a temperature of 20° above freezing, to the air below 32°, 

 it requires twenty minutes to reduce it to the point of con- 

 gelation, one hundred and forty minutes will be required to 

 solidify it — or seven times as long. 



In melting the ice, the same amount of heat has to be ab- 

 sorbed, so that a large extent of deep water becomes a regu- 

 lator of temperature, preserving the air immediately over it 

 at near 32°, though the atmosphere in the vicinity during 

 the winter may be far below zero ; conversely in the spring, 

 though the temperature of the same latitude may be 60° or 

 even 80°, that of the air immediately over the water will be 

 near 32°. It is evident from these facts that the deeper the res- 

 ervoir, the longer will be the continuance of low temperature 

 required to freeze the surface, and the longer the time neces- 

 sary for melting it again. These principles are illustrated 

 in our great lakes. The greatest known depth of Lake Su- 

 perior is 792 feet, and soundings of 300, 400, and even 600 

 feet are not uncommon. In the coldest weather, the water 

 over these deeper places is above 32°, and does not freeze, 

 while over the shallow parts a coating of ice is formed, which 

 gradually cooled by the slow diffusion of the water under- 

 neath, retains its solidity until the last of June. Indeed, ice 

 is sometimes found at the surface in the middle of July. At 

 this period of the year, or a little later, the smaller ponds of 

 water in the vicinity have a temperature of 72° to 74°. Lake 

 Erie, being much shallower, sometimes freezes entirely across, 

 and becomes in summer heated throughout its extent to 

 nearly the temperature of the supernatant air. At the 

 beginning of September, 1857, the temperature of Lake 

 Huron was 56°, while that of the water from Lake Erie, 

 which passed over the falls of Niagara, was 72°, precisely 

 that of the air. 



All bodies, as we have previously said, in passing from a 

 liquid to a solid state, tend to assume a regular geometrical 

 arrangement called crystals. This is particularly observable 

 when the process has been slow, and undisturbed by agita- 

 tions and tremors. The form peculiar to each substance is 

 exhibited when a portion only of liquid has assumed the 



