44 Proceedings of the Royal Physical Society. 
6. The approximation and subsequent crossing of the air 
and river temperatures in mid-winter below the First Cataract 
is due mainly to the fall of the air tenperature. During the 
descent of the Nile in January from Abu Simbel to Cairo, 
between the 22nd and 30th parallels of north latitude, the 
temperature of the air drops at least ten degrees, 64° to 54°, 
whilst that of the water only varies one or two degrees, 60° 
to 62°. This is shown in the columns of Table III.; and we 
now perceive that in winter, as in suinmer, when we follow 
the Nile from the Second Cataract to Cairo, the air and water 
temperatures do not change together, and that in both seasons, 
whilst the air cools considerably, the water varies but little 
in its temperature. . 
7. It follows from all that has been previously remarked, 
as well as from the data for the spring and autumn to be 
found in Tables XII. and XIII., that between the First and 
Second Cataracts the Nile is cooler than the air all through 
the year, the difference being as much as thirteen degrees or 
more in the middle of summer, as in July, and reduced to as 
little as two or three degrees in the middle of winter, as in 
January. Lower down the river, between Assouan and 
Minieh, the relative coolness in summer with respect to the 
air is less marked, but is still eight degrees in June and 
July; whilst in mid-winter, as in January, the river becomes 
nearly two degrees warmer than the air. At Cairo, or in its 
vicinity, the summer coolness, though again diminished, is 
still five or six degrees in July; while in January the Nile 
is some five or six degrees warmer than the air. With this 
general conclusion attention is directed to the diagram of the 
curves of the air and water temperatures for the Nile. 
8. Most of the striking features in the thermal economy of 
the Nile below the Second Cataract are also brought out in the 
comparison of the annual mean temperatures of the air and 
water given in Table VI. Whilst between the Second and 
First Cataract (22°-24° lat.) the river is more than seven 
degrees cooler than the air, between Assouan and Minieh 
(24°-28° lat.) the difference is less than three degrees, and at 
Cairo (30° lat.) the river and the air possess about the same 
temperature. Between the Second Cataract and Cairo the 
