River Temperature. 39 
small difference in the latitude, the air mean deduced from 
Hay’s observations practically corresponds with the isotherm. 
Whether indicated, therefore, by the isothermal line or by 
the accompanying air observations, the Nile during July, in 
the vicinity of Beni Hassan and Tel, is about eight degrees 
cooler than the air. Farther down still, opposite Cairo, in 
latitude 30° 3’, we may, following Newbold, place the mean 
temperature of the Nile in July at 79°. Judging from the 
position of Cairo, between the isothermal lines of 80° and 85°, 
the mean temperature of the air for July in this locality is 
about 835°. The mean temperature accepted for July at 
Cairo is, however, 85°; but whichever estimate is most 
correct, we shall not be far wrong in assuming that the 
Nile at Cairo in July is from four to six degrees cooler than 
the air. The isotherm of 80° for July crosses the mouths of 
the river in latitude 31° 30’. The water temperature there 
would probably be much the same as at Cairo, so that at the 
mouths the water would only be a degree cooler than the air, 
or perhaps there would be no difference at all. 
These various considerations have led me to the second 
proposition :— 
2. The coolness of the Nile in summer, in comparison with 
the air, diminishes as the river flows north. This feature in 
the thermal economy of this river has been already indicated 
in several ways. It is clearly brought out in Table I, 
whether we take the individual months or the means for the 
two summers. We there see that whilst for this season the 
Nile at Abu Simbel, in latitude 22° 20’, was on the average 
11°°7 cooler than the air, this difference was reduced to 7°-+4 
about five degrees of latitude farther north. It is also 
exemplified in Table XV., and there receives independent 
support from the comparison of the sunrise observations. 
Tested by the isothermal lines, it is placed beyond reasonable 
doubt; and it has been above shown in the case of July by the 
evidence of the isotherms, as well as by that of the air observa- 
tions of Hay, that the difference of from thirteen to sixteen 
degrees at Abu Simbel, in latitude 22° 20’, is reduced to eight 
degrees at Tel,inlatitude 27° 37’,to some five degrees at Cairo,in 
latitude 30° 3’, and nearly disappears altogether at the mouths, 
