250 Transactions. 



most were : March, T'-i" ; September, 6'6° ; and A2)ril, 5-.')°. Mean 

 difference for whole year, 4-3°. Thus it will be seen that the 

 months in which the temperatures of the air and water most 

 nearly approximated were those in which the day was at the 

 shortest and the longest. In other words, there were two maxima 

 and two minima of diff'erence between the temperatures of the air 

 and water, the former occurring in the months of March and 

 September, the equinoctial months ; and the latter in December 

 and June, the months of the winter and summer solstice. The 

 former fact is easily explained, but it is rather a curious circum- 

 stance that the same thing should hold good of the month in which 

 the sun is longest above the horizon, and most vertical. The 

 explanation, I have no doubt, is that in the latter part of June and 

 the first part of July, when there was a period of drought and 

 warm weather, which lasted more than three weeks, the river fell 

 to its lowest level, and the current was very sluggish. Hence the 

 water became more heated than in ordinary circumstances, and its 

 temperature more nearly approached that of the air. 



Though as a rule the temperature of the air was higher than 

 that of the water, there were a good many exceptions to this rule, 

 especially in the months of May, June, July, November, and 

 December. Thus there were five days in May in which this 

 occurred, six in June, and four in July, with an aggregate excess 

 in the temperature of the water of 37°. In November and December 

 there were also 15 days with an aggregate excess of 304°, the 

 greatest number being in December, viz., 10 days, while on other 

 two days of that month the temperature of air and water was 

 equal. The conditions under which this state of things was ob- 

 served were, as a rale, in summer, when the temperature was 

 lowered by cloudy and wet or dull and foggy weather, or by the 

 prevalence of cold winds ; and, in winter, when the conditions 

 were similai', or when frost set in. The most extreme diff'erence 

 was observed on July 7th, when the reading of the air temperature 

 was 53°, and that of the water 65° — a diff'erence of 12°. This was 

 at the close of the period of drought before alluded to. The 

 greatest excess in the temperature of the air above that of the 

 water occurred in March, when on 16 days it was higher by more 

 than 7°, ranging from 7° to 14'5°; and the next in September, in 

 which month there were 13 days in which the diff'erence exceeded 

 7°, ranging from 7° to 13*2°. On these occasions the weather was 



