^BARNBS] TEMPERATURE OF THE LACHINE RAPIDS 25 



remained practically at the freezing point until after midnight, when it 

 became cooled slightly below the freezing-point. The currents and quiet 

 water showed very little difference in temperature. 



About 6. -40 p.m. the following readings were obtained, which will 

 serve to show how uniformly the temperature of the river Avas falling to 

 the freezing-point : 



Quiet water down 3 feet. + -0037 ° C. 



" " " 8 inches. + 0034° C. 



Current + -0037° C. 



This tends to show that the cooling by the surface abstraction of 

 heat, which would cause the upper layers of water to be cooler than the 

 lower layers, was also probably aided by radiation, which would cause the 

 .mass to sink more uniformly. 



Later readings between 7 p.m. and 11 p.m. showed the temperature 

 remaining practically at 0° C. At midnight, when the sky was about at 

 its clearest, experiments were tiled with the thermometer stem at different 

 depths in the quiet water. The temperature at the bottom was, as near 

 as could be measured, at the freezing-point. Differing the other readings 

 from the reading of the thei-mometer on the bottom, we have : 



Within 1 foot of surface —0016° C. 



Same reading 15 minutes after; stem not disturbed — 0023° C. 



At 3 feet —0018 ° C. 



These observations apparentl}' show that the thermometer, when left 

 undisturbed, was being actually cooled by radiation below the temperature 

 of the surrounding water, a condition further borne out by the formation 

 ■of ice actually on the stem itself. At about 3 a.m. measurements were 

 made of the difference in temperature of the back-water and current. 

 The quiet water was apparently colder than the curi-ent by -0058° C. The 

 only difference in the temperature of the two bodies of water seems to be 

 explained best in admitting again the possibility of radiation influencing 

 the readings of the thermometer. This is a condidion not so easily at- 

 tained in the swift current. It may be of interest to state that the writer 

 had actually to remove ice from the stem of the thermometer which, from 

 its nature, was apparently formed in situ. 



During this night the radiation did not continue so strong as it some- 

 times does, as was shown by the small amount of ice made on the bottom. 

 The radiation, although small in amount, became strong at times when the 

 sky was clearest, and had an important influence in determining the tem- 

 peratui'e of the water, as already shown. 



Towards morning the current became cooled slightly below the freez- 

 ing point. Observations at 5.50 a.m. give — -0058° C, and at 6.30 a.m. give 

 — 0046° C. This was no doubt due to the continued action of the cold 

 .atmosphere, as well as to the effect of many hours of radiation. Curiously 



