40 Proceedings of the Royal Physical Society. 
The following Table for July will serve to summarise the 
results for this month, and will also introduce the third 
conclusion. 
TABLE II. 
feneral Results for the Nile in July. 
Air. 
| 
| 
Isotherm. Observation.) | 
Locality. Lat. N. | 
| 
Abu Simbel, . | 22° 20 
95:0 | 921 | 78°8 
ele : -| 27° 37’| 865 | 86:3 | 78°4 | About eight degrees, 
Cairo, . Messe al oen0 85:0 | 79:0 | Four to six degrees. 
At the mouths, | 31° 30’; 80-0 | 79:0 
| One degree or less, 
Water. | Relative coolness of the Nile. 
| Thirteen to sixteen degrees. 
3. The diminution in the summer coolness of the river in 
relation to the air, as the Nile descends from the Second 
Cataract to the Mediterranean, is almost entirely due to the 
fall in the air temperature. The air and water temperatures 
do not in fact change together, the former altering consider- 
ably, and the latter to a much less extent. This feature 
is brought out in Table I., where we note that the reduction 
in the relative summer coolness of the river from 11°7 
in latitude 22° 20’ to 7°-4 in a locality about five degrees of 
latitude farther north, is due principally to the drop of the 
air temperature, the air cooling more than six degrees, and 
the river barely two degrees. We are of course here com- 
paring seasons of different years, and it is probable that 
some portion of the slight cooling here exhibited by the Nile 
is due to seasonal variation higher up the river. We know 
from the isothermal lines that this drop in the air tempera- 
ture is characteristic of this region, but we have no such 
guide for the Nile. Therefore, in order to obtain a less 
obstructed view of the matter, I have devoted Table II. to 
the results for a single month, namely July. 
In its columns we can follow the Nile in its course 
northward during July from the Second Cataract to the 
Mediterranean. During our imaginary descent we traverse 
more than nine degrees of latitude, and the temperature of 
the air drops from 95° to 80°. We look to the river for 
