River Temperature. 47 
regions or in lakes, possess a temperature which, as repre- 
sented by the curves of the mouthly means, is usually 
warmer than the air all through the year. This is true of 
such rivers as the Thames, the Seine, and the Marne, which 
are from three to five degrees warmer than the air in 
summer, and from one to three in winter. Rivers of this 
size, possessing no great bulk of water, soon attain a stable 
temperature, depending mainly on the climatic conditions of 
the district traversed. Change of latitude can but slightly 
affect the mutual relation of the air and water temperatures 
in a river that, by reason of its small size, so soon responds to 
changes in the surrounding atmospheric conditions. The 
sreat rivers of the earth, such as the Amazon, the Congo, the 
Mississippi, and the Nile, have, on the other hand, no 
consistent thermal behaviour, except such as depends on 
similarity in the direction of their flow; and in their case 
the difference of flowing with the latitude and with the 
longitude, or between a course from high to low and low to 
high latitudes, is strikingly illustrated. As I hope sometime 
to deal at length with each of these large rivers, I will here 
treat briefly of each of them, and in the first place I will take 
the Amazon. 
During his descent of the Amazon from the junction 
of the Huallaga to the mouth of the Madeira, between 
September 1851 and February 1852, Lieutenant Herndon 
made a number of observations on the air and water 
temperatures, which are given in the narrative of his travels. 
During the whole descent of some 1400 miles, amounting in 
fact to about two-thirds of the river’s navigable course, the 
water temperature only varied between 79° and 83°, or four 
degrees. The results, however, are not worked up by the 
author in his book; and after some labour I have prepared 
the following table, from which we learn that the water was 
always a few degrees warmer than the air, the mean excess 
being a little over three degrees. This is close to the 
estimate obtained by Maury from the same observations. 
In an introductory note to Lieutenant Marr’s report on the 
Mississippi at Memphis, he remarks as follows: —“ The 
Amazon, according to Lieutenant Herndon’s descent in 1852, 
