OF THE RIVER SAINT LAWRENCE. 



275 



shallow part of the lake, near its southern end, and where the volume is but small, the 

 temperature is very little above the freezing-point for some weeks, but in the deeper parts 

 of it, the water rarely falls below 40°. The mean temperature of the earth in Vermont, 

 contiguous to the lake, has been observed to be about 45° ; and it is not surprising, there- 

 fore, that so soon as the cooling influences of the severe winter of this region have been 

 mitigated, the large masses of water should first break up, and very rapidly convert the 

 ice ; and in support of this view, it may be remarked, that the latest disappearance of ice 

 may be almost always reckoned on in the comparatively shallow exposed surfaces of the 

 smaller lakes and ponds of the country. 



Nor, under the circumstances, is it a matter of surprise that these large masses of water, 

 these inland seas of the country, should exert a certain influence on the atmosphere sur- 

 rounding them ; and it has long been a matter of general observation, but is now reduced 

 to the most tangible form by Dr. Henry, of the Smithsonian Institution, at Washington, 

 who has given the results of the observations established throughout the United States 

 and Canada, and which I have elsewhere referred to at length; and the deflection of the 

 general direction of the mean isothermal lines of winter and summer, as given on the 

 map, shows the modifying influences of the large masses of water in the central and 

 western parts of Canada. 



Experiments arc wanting to show the temperature of the large upper lakes in winter: 

 these would probably afford most interesting results. 



Except in creeks, and small harbors and bays, these are never frozen much beyond the 

 bordage of broken ice, which varies in width from a few yards, as in the case of deep 

 water against bluffs, to a mile, or occasionally two miles, under circumstances favoring its 

 accumulation ; and hence it is that the cooling of large masses of water having such pro- 

 fundity as Lake Huron and Lake Ontario, is carried on to even a lower point than occurs 

 in the case of the smaller lakes, and some shallower lakes, where the smaller extent of 

 surface to be ruffled by tin; winds, admits of the early formation of a protective covering 



of ice. 



Observations have been made by the officers of the United States Topographical Engi- 

 neers during the progress of the lake surveys, and show very interesting results as to the 

 temperature of the lakes: for instance, on Lake Erie, in the middle of August, 1845, 

 the mean temperature of the air at noon was 76° ; that of the water at the surface was 

 73° ; while the temperature of the bottom was 57°, the depth of the lake being 76 J feet. 

 In another instance, while the temperature at the surface was 71 J°, that of the bottom, 

 at 81 feet, was 58° ; at double the depth, the water on the surface being 73°. Experi- 

 ments were made at the end of July, 1860, midway in the lake between Cape Hurdand 



