River Temperature. 205 
principles there laid down, it will be assumed that the 
reader has previously become acquainted with the data there 
given. 
A river of ordinary swiftness, unchecked in its course by 
locks and weirs, displays about the same temperature through- 
out its mass, or at most the variation would be only one 
or two degrees in its different parts even on a hot summer 
day." On the other hand, a pond, four feet deep and well 
stocked with submerged aquatic plants, varies considerably in 
temperature. On a warm summer afternoon the difference 
between the marginal and bottom temperatures would be 
from 15° to 20°, whilst the surface temperature would be 
9° or 10° higher than that at the bottom. In spring and 
autumn the variations in temperature between different 
parts of a pond are usually less, and in winter they almost 
disappear. When a pond is clear of such weeds the contrast 
in its temperatures is much less than when it is full of 
plants, on account of the circulation of the water being un- 
impeded. <A similar effect is produced by the floating leaves 
of such plants as the water-lilies, the Limnanth, Polygonum 
amphibium, and Potamogeton natans, which, in summer, 
preserve the coolness of the water and maintain much the 
same temperature all over the pond. However, I am dealing 
here only with ponds full of submerged plants, such as 
Ranunculus aquatilis (circinatus), Myriophyllum spicatum, 
Ceratophyllum demersum, Zannichellia palustris, Potamogeton 
erispus, P. obtusifolius, Scirpus fluitans, ete., a circum- 
stance that must not be forgotten whilst perusing this 
paper. 
Rivers also differ greatly from ponds in their small daily 
range. This contrast is, of course, greatest in sunny weather. 
Whilst the Thames even in summer does not usually vary 
more than 2° in its temperature during the twenty-four 
hours, a pond will vary 6° or 7° at its surface, 1° or 2° at 
the bottom, and some 20° at its margins. In spring and 
1 Since writing the first part of this paper, a perusal of Dr Forster’s recent 
Memoir on the Rivers of Europe has convinced me that the Thames owes its 
marginal heating to its locks and weirs, Left to nature, the river would 
display but little heating at its margins, 
VOL. XIU R 
