River Temperature. 45 
variation of the river temperature is less than two degrees, 
whilst the air temperature falls about nine degrees between 
these two localities. On referring to Buchan’s isothermal 
chart of the mean annual temperature of the globe, it will 
be observed that the same conclusions could be formed by 
employing the isothermal lines. The isotherm of 80° crosses 
the Nile between the First and Second Cataract in about 
latitude 23° 30',a region for which the air observations I 
have used give a yearly mean of 80°2. ‘That of 75° is drawn 
across the Nile in about latitude 27°, between Assouan and 
Minieh, a region for which the air mean deduced from my 
data comes out as 74°°5, 
TaBLE VI. 
Comparison of Mean Annual Temperatures of the Nile. 
| 
Locality. Lat. Isotherm. Air. | Water. W. diff. | 
| 
| | 
Between Second and | | 
| 
99°_94° ates fr re O)-_¢ ° , ys « _ 7 Q 
First Cataracts, . | 22°-24° | Lat. of isotherm of 80°=23° 30’ | 80°2 | 72:9 pes 
Between Assouanand | | 5,0 590 pee ana Pipe leat : 
Minieh, : sf = a9 ay ee 0 haa 
x . 70°=31° 71:2 | 71-0 | -0°2 
| Cairo, . : a |) . 30: 
Note.—The mean annual temperature of the air at Cairo, in accordance 
with which Buchan has drawn his isotherm of 70°, is 71°2; and from the 
little variation displayed in the monthly means of the Nile in January and 
July in the lower course of the river (Tables II. and III.), the year’s mean 
for the river at Cairo may be placed at 71°. 
9. During the four seasons of the year there is no great 
contrast in temperature between the water of the Nile and the 
surface-waters of the Mediterranean. In the accompanying 
table I have endeavoured to illustrate this. The data for 
the Mediterranean are taken from the Admiralty Chart of 
Surface-Temperatures, published in 1885 as one of the sheets 
of the Wind and Current Charts of the Atlantic, Indian, and 
Pacific Oceans. In framing the temperatures for the Nile, I 
have been not only guided by the scattered data given in 
Table XIIa.; but as it is evident that the Nile on the 
average varies but- little in its lower course, I have had 
