River Temperature. 53 
always assisted migration between the Nile and the Mediter- 
ranean; the very reverse has been the case as regards the 
Mississippi and the Gulf of Mexico. 
12. The Nile exhibits an unusually large daily range of 
temperature. This matter can be only touched on here; but 
I may remark that very valuable materials for its study 
have been afforded to me by Mr Hay’s extensive series 
of sunrise and sunset observations, the results of which will 
be found in Table XV. It will be there seen that in summer, 
between Beni Hassan and Siout, the daily rise in summer is 
on the average from two to three degrees. In winter, accord- 
ing to the observations of Dr Schnepp in January, between 
Girgeh and Assouan, it is reduced to 1°1; and in the spring, 
in about the same locality, it is shown by Dr Marcet in 
March to be 1°6. When we turn to other large rivers, 
we find that as a rule they display but slight change in the 
day and night. According to Dr Dowler, in the Mississippi 
at New Orleans there is scarcely any variation in tempera- 
ture during the whole twenty-four hours, and he shows, in a 
few observations made in June, that it did not then exceed 
half a degree. The Niger below the confluence, according to 
the observations of Dr Baikie, has an average daily rise in 
August of about one degree; and the Senegal, in August also, 
has, as indicated in the observations of Captain Borius, a 
similar range. From the data obtained by Mr Wallace, the 
Rio Negro, the large tributary of the Amazon, exhibited an 
average daily rise in September of 1°°3. The Brahmaputra 
at Sadiya possesses, according to Dr Griffith, an average 
daily rise in September of 1°°5. Probably enough, the un- 
usually large diurnal oscillations of the Nile’s temperature 
are to some extent connected with those agencies that produce 
the great daily range of the temperature of the air. But it is 
very remarkable that, like the Brahmaputvra (see Part I., p. 305), 
the Nile, when far below the temperature of the air in summer, 
exhibits the usual daily rise and nocturnal fall, although it is 
colder than the air during the whole twenty-four hours. This 
matter will be dealt with in a subsequent paper. 
13. The Nile attains its highest temperature in August, and 
its lowest in January, usually in the latter part of those 
