50 Proceedings of the Royal Physical Society. 
either the Congo or the Mississippi. Here we notice that 
between the First and Second Cataracts the river is cooler 
than the air all through the year, the difference being great in 
summer and small in winter. Lower down, between Assouan 
and Minieh, the Nile is cooler than the air during two- 
thirds of the year, but in the winter period, from December 
to March, it becomes warmer than the air. 
11. When specially contrasted with the Mississippi at 
New Orleans, the Lower Nile differs from it in the 
following points :— 
(a) In the relation of the curves of the air and water tempera- 
tures, as just described, and as illustrated in the table of curves. 
They not only differ in this respect conspicuously from each 
other, but both depart widely from the prevailing type pre- 
sented by a river under the control of the climatic conditions 
of the regions traversed. Such a river, whether it be the Congo 
or the Thames, is warmer than the air all through the year. 
(b) Whilst, as shown in Table X., the Nile, judged by its 
monthly mean temperatures, has a range of temperature much 
less than that of the air, the range for the Mississippi is con- 
siderably greater than for the air. On looking more closely 
into this matter, it will be observed that the Mississippi 
extends its range beyond the limits of the air in winter, 
whilst the Nile contracts its range considerably within that 
of the air in summer ; and that the Mississippi behaves like 
other rivers only in the warmer part of the year, whilst the 
Nile resembles other rivers only in winter. I refer of course 
to the water temperature rising above the air in those 
seasons in the case of most rivers. 
(c) The Nile is true to the climatic conditions of rts latitude 
only in winter, and the Mississippi only in summer. The 
influence of the higher courses 1s displayed, in fact, in contrary 
seasons, the Nile being under their control in summer and the 
Mississippi in winter. 
(d) According to the annual mean temperatures (see Table 
X.), the Nile is some seven or eight degrees warmer than the 
Mississippi at New Orleans. 
(e) As indicated also in the mean temperatures for the 
year, the coolness of the Nile with respect to the air is 
