304 Proceedings of the Royal Physical Society. 
With the Thames tributaries, the Thurso river in the north 
of Scotland ought to be related in size. From the tables 
given by Mr J. Gunn in his paper on this river, the mean 
daily rise for the year comes out at just 2° (F.); but as the 
morning observations were made between 10 and _ half-past 
10, a good proportion of the rise would not be included: 
the afternoon observations of between 2 and 3, would in 
summer be too early for the maximum temperature; and, 
guided by Renou’s data for the Loir, I would place its mean 
diurnal range for the year at about 3°°5, or more than double 
that of the Thames at Kingston, which is 1°5. In summer 
the contrast would be still greater, and the Thurso’s mean 
diurnal range for June would be about 6°, or quite three 
times that of the Thames. I may here refer to the Rhone’s 
tributary, the Arve, which, according to the observations of 
Professor Chaix, has a daily range in summer of 4° or 5°. 
Then, again, the Kundeel, a small tributary of the 
Brahmaputra, has a range of 3°°7, which is nearly double 
that of the main river. 
I have assumed above in the case of small rivers and 
streams, like the Thames and its tributaries, that their 
diurnal range is regulated in amount by their bulk; but this 
can only be true in the strictest sense, when the rivers or 
streams have about the same plane of temperature as well 
as proportionally similar depths and velocities. First, as 
regards the plane of temperature, I will take the Mole, 
which, though not one-tenth of the size of the Thames, will 
display, when it has a lower day’s mean, a range less than 
that of the main stream. Then, with regard to depth and 
velocity, the diurnal range of the same stream may vary 
greatly from these causes in localities only a few hundred yards 
apart. Thus, within the same twenty-four hours, the Mole, 
where it was 35 feet broad, 5 feet deep, and very sluggish, 
ranged 1°5, whilst a little distance away, where it was 80 
feet broad, 2 feet deep, and more rapid, its range was 3°:0. 
Though the main stream would usually have a smaller 
. diurnal range than its tributary, it may, therefore, happen 
from any of the three causes above stated, namely, the 
superior position -of its temperature-plane, its shallower 
