Rwer Temperature. 305 
are really comparing large tropical or semi-tropical rivers 
with rivers much smaller in size in the temperate zone. 
Doubtless, this similarity in the amount of the diurnal range 
for rivers of temperate and warm latitudes, is due in part to 
the increased heat of the tropics and sub-tropics being 
counterbalanced by the large size of the rivers, or it may 
be that it is concerned with the relatively small diurnal 
range of the temperature of the air in the tropics. There is, 
however, very little evidence of this last influence in the case 
of the Nile and the Senegal. Taking the diurnal range as 
the mean of the rise and fall, the Nile in March, according to 
Marcet’s tables, had a water range of 1°°6 (F.) to an air range of 
29°-5; whilst the Senegal in August had a water range of 1°°3 
to an air range of 14°, the ratios of water to air being in these 
cases widely different. Space will not admit of further notice of 
this subject, which will be subsequently dealt with at length. 
As regards the diurnal range of rivers smaller in size than 
the Thames or the Loir, it can here be noted that after these 
limits are passed, the amount increases rapidly with the 
diminution of the river’s bulk. The tributaries of the 
Thames, as far as I have been able to observe them, display 
a daily range considerably greater than that of the main 
stream. During the same September day when the Thames 
ranged 1°°5 (F.), the Hogsmill ranged 3°°7. In the middle of 
August, when the range of the Thames was 1°°8, that of the 
Mole was 3°:2, and that of the Ember 15° At the end of 
March, when the Thames ranged 1°, Beverley Brook ranged 
10°. The dimensions of each stream at the place of observa- 
tion, together with their proportional ranges, taking the 
range of the Thames as 1°, are here given. 
The Thames and four of its Tributaries. 
River. | Dimensions at Place of Observation. | ie tie 
Thames, ; . | Breadth 250 feet. | Depth 10 feet. 1° "OME 
Mole, . 3 ‘4 = 80 ,, | - Di aks alestey es 
Hogsmill, = = | ”? 40 ”? ”? ” . ”? 
Beverley Brook, . |, ,, il 4 gees 078 inches; |! LOO, ,, 
Ember, ° c ” 12 ,, ee 4D) ay 8°°3 cB) 
