250 TRANSACTIONS. 
most were: March, 7:4° ; September, 6°6°; and April, 55°. 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 difference 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 rule, 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 similar, or when frost set in. The most extreme difference 
was observed on July 7th, when the reading of the air temperature 
was 53°, and that of the water 65°—a difference 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 difference exceeded 
7°, ranging from 7° to 13°2°.. On these occasions the weather was 
