Temperatures of Great Lakes and St. Lawrence. 81 



exposed for an eighth of a mile to the sun's rays, and slowly 

 flowing over a succession of limestone ledges, where, in 1.5 

 inches of water, the mercury on a warm June afternoon 

 could be seen rising and falling between 81° and 83° F. 

 Here there were some exceptional causes, but in the line of 

 outflow from Lake Ontario to the St. Lawrence, the fluctu- 

 ations are rather to be ascribed to the evaporation at the 

 surface, and to the cooler waters beneath ascending to sup- 

 ply the place of the evaporated water. As the evaporation 

 would be irregular, varying with the passing clouds, the 

 gusts of wind, and the features of the land, the ascending 

 currents would also be irregular. These ascending waters 

 would give rise to a slight inflow at the bottom from deeper 

 and cooler parts of the lake to take their place, and both 

 these currents would be affected by the general onward 

 flow of the lake waters towards the entrance of the St. 

 Lawrence. 



Bottom Currents in Georgian Bay. 



On 20th August, 1886, Commander Boulton, in a series 

 of soundings diagonally across the centre of the Georgian 

 Bay, in a somewhat southerly direction, found the tempera- 

 ture of the water at the bottom at one point (31 fathoms 

 deep) 39.5° F., at another (47 fathoms) 38.25° F., and at a 

 third (42 fathoms) 37.75° F. — the distance between the 

 extreme points being about 40 miles. On 10th July, 1889, 

 nearer the Bruce peninsula, the readings in 70 fathoms 

 gave 38.75° F., and on 8th September following, at another 

 point in 63 fathoms, the reading was 39° F. In all these 

 different cases, the surface water varied from 59.75° to 68° — 

 the last being on 8th Sept., at 10. 10 a.m. As the tempera- 

 ture of water at its maximum density is 39'2° F., and 

 below that, the density again diminishes, there would be a 

 tendency in these bottom strata of water to rise until they 

 intermingled with water of a higher temperature and 

 equivalent density. It is thus necessary to seek some 

 explanation of this singular fact that the bottom tempera- 

 tures in this extensive bay are in summer as low in places 



