River Temperature. 305 
depths, and its greater velocity, that the main river has a 
larger range. An instance of this occurs in reference to the 
Loire at Tours, and its tributary the Loir at Vendome. 
Though the Loire is naturally a larger and also a more 
rapid river, it has, says M. Renou, a notably greater diurnal 
range than its tributary. The discounting of size here is 
evidently due to the greater velocity and shallower depths 
of the Loire, for though it has a larger sectional area and 
a much greater water-discharge than the Loir, it is only 
about half as deep. A rapid current hastens the mixing 
of the stream, whilst shallow waters, owing to their larger 
proportion of exposed surface, are heated up more easily 
during the day, and cool more readily during the night, and 
both influences have combined in this case to give the larger 
river the greatest diurnal range. 
The subject of the daily fluctuations of a river would be 
complex enough if it ended here; but there is much beside 
that is indeed perplexing. Spring-sources add their share 
to the difficulties; but, as I will show in a later paper, there 
are many methods, and notably the study of the curves of 
the monthly mean temperatures of air and water, by which 
we may detect their influence. By far the most puzzling 
feature of these daily changes is to be found in the circum- 
stance that, although in temperate regions the river’s diurnal 
range is usually included within that of the air, there are 
instances in warmer regions where the river exhibits regular 
daily fluctuations, with its range altogether outside that 
of the air.. The Brahmaputra is a case in point. As shown 
by the tables of Dr Griffith, its daily range of temperature 
during September was always outside and below the range 
of the air. As before observed, after emerging from the 
Himalayas it still retains its mountain coolness on reaching 
Sadiya, some 40 miles away; and there, strange to say, it 
regularly increases in temperature by day, and as regularly 
cools at night, though, as illustrated in Plate VIIL, the air 
is warmer during the whole twenty-four hours. Its mean 
diurnal range is about 2° (F.), whilst that of the air is 
about 9°. 
The opposite state of things may be found in some 
