River Temperature. 37 
two localities. As regards the temperature of the Nile in 
July between Cairo and Thebes, these two observers may be 
therefore considered as in accord with each other. This is a 
matter of some importance, since meteorologists are already 
acquainted with the average conditions of the air temperature 
in this region, and the point to be established is the average 
temperature of the Nile. In support of the estimates for the 
river afforded by the observations of Mr Hay above the First 
Cataract, we look for the agreement of the observations of 
M. Coutelle. This meteorologist remarks that during his 
sojourn on the Isle of Phil (latitude 24°), the temperatiire 
90, 
of the Nile in the afternoon was 83°°7. Corrected for the 
daily range, this estimate falls to about 82°°7. Unfortunately 
no date is mentioned, but we are informed that at the same 
time the air in the shade was constantly 107°4-109°6, 
thermometric conditions characteristic of July or August, 
and indicative of a mean daily temperature of not under 90°. 
Taken as they stand, these observations give general support 
to those of Mr Hay at Abu Simbel. 
Another mode of demonstrating the summer coolness of 
the Nile in relation to the air is to be found in the comparison 
of the observations made at or about sunrise, a method that 
is free from some of the difficulties connected with the 
estimation of the daily means. Given the difference between 
the air and water temperatures at sunrise, and postulating 
the relatively large daily rise of the air temperature, and 
the correspondingly limited daily rise of the water tempera- 
ture, we can, by the aid of these two daily observations 
only, approximately determine the relation in the day’s mean 
between the air and water temperatures. For example, I 
will assume that a certain river during the month of June 
has on the average about the same temperature as the air at 
sunrise. We have no further data of our own; but we learn 
from the meteorologist that the average daily rise of the air 
temperature for this region in June is about twenty degrees, 
and we are informed by those who have specially studied the 
temperature of rivers, that the average daily rise of the water 
temperature could not under these circumstances be more 
than two or three degrees. Here it is at once evident that 
