River Temperature. 4] 
some response to these great changes in the atmospheric 
conditions, but we get no reply. Its temperature has varied 
only one or two degrees between Abu Simbel and Damietta, 
and we are brought face to face with an enigma that might 
have been offered by the Sphinx. 
4, The Nile, in summer, is therefore cooler than the air in 
nearly the whole of its course. This follows from the fact 
that its relative coolness, which is very small at the mouth, 
perhaps not one degree, increases as we ascend the river to 
the Second Cataract, down which it flows, during the months 
of May, June, and July, at a temperature between twelve 
and sixteen degrees cooler than the air. 
5. The Nile, between the First and Second Cataract, is in 
the middle of winter still cooler than the air; but the 
difference between the air and water temperatures is much less 
than wm summer, and disappears altogether as the river 
proceeds north, so that between Assouan and Minieh the water 
is nearly two degrees warmer than the air, an excess that ws 
Surther increased at Cairo. This conclusion is based on the 
general results of a number of observations made in January 
by Hay, Russegger, Chaix, Schnepp, and Villiers Stuart, 
which will be found in Table III. In Table IV. I have 
subjected these observations to a more detailed analysis, in 
order to determine the locality of the crossing of the air and 
water temperatures; and it will be noticed that the locality 
varies somewhat, a result that might have been expected 
from observations made in different years. Thus, from the 
data obtained by Schnepp in 1860, it may be inferred that 
the crossing occurred between Assouan and Thebes; and 
from those of Hay in 1832, and Russegger in 1837, we should 
be justified in placing its locality in the vicinity of Thebes; 
whilst those of Chaix, in 1847, indicate its having taken 
place above the First Cataract. According to the observa- 
tions of Villiers Stuart in the latter part of December and 
the early part of January 1878-79, the temperatures crossed 
in the neighbourhood of Kalabshi. The locality of the 
crossing of the air and river temperatures would, therefore, 
appear from these observations to vary in January between 
23° 30’ and 25° 30’ N. lat. From year to year the climatic 
