Temperatures of Great Lakes and St. Lawrence. 83 



waters would form some objection to their use for household 

 purposes, though not so serious an objection as their 

 contamination. 



Temperature in Eelation to Depth. 



It is impossible to lay down any general rule regarding 

 the changes of temperature with the increase of depth. 

 Apart from variations resulting at the different seasons, 

 surface readings are affected by sunlight and cloud, gusts of 

 wind, channel currents, the inflow of affluent streams, and 

 the physical features of the surrounding land. Eeadings 

 beneath the surface are affected by the depth of the water, 

 by ordinary currents resulting from changes of level, by 

 evaporation at the surface creating an upward flow of the 

 water underneath, by the contour of the bottom, and by 

 high winds which drive the surface waters before them, 

 creating return currents underneath to take their place. 

 Each case has to be judged by its own special circumstances. 

 Thus, in the Georgian Bay, between Cabot's Head and Cape 

 Croker, Commander Boulton, on 27th July, 1888, at 8.30 

 a.m. r obtained the following record : 



Surface 60-2° F. 



lOfms.. 45-7° 



20 " 41-4° 



35 " 41° 



66 " (bottom) 39-5° 



On 14th June, 1889, at 11 .25 a.m., one mile south-west of 

 Kingston, in the channel from the lake to the river, one of 

 the records was : 



Air in sun 79° F. 



Surface water 58.5°. 



6 feet 56.25°. 



18 " 54°. 



30 " 54.25°. 



60 " (bottom) 52°. 



On the 25th July following, at 4.15 p.m., at a point in 

 the same channel, two miles distant, the readings showed 

 not only a higher range, but a much nearer approach be- 

 tween the surface and bottom temperatures, thus : 



