River Temperature. 211 
some of our Potamogetons, it is the characteristic feature of 
ponds filled with plants like Myriophyllum, Ceratophyllum, 
Zannichellia, Scirpus fluitans, ete. It is indicated in 
Table ITI. that in sunny summer weather the surface of such 
a pond is, on the average, between six and eight degrees 
warmer than that of the river. 
In concluding this part of the paper, I would point out 
that, as yet, we have everything to learn concerning this 
important subject. For the student of the conditions of 
aquatic life, the real inquiry has yet to be begun; and it 
would be dangerous to attempt to harmonise facts of obser- 
vation, such as are given in this paper, with the habits of 
aquatic plants and animals without a general grasp of the 
intricate questions involved. When we investigate the 
thermal economy of a pond, or of a stream, we are but 
examining a single portion of a great thermal system which, 
in the first case, comprises both the road-splash and the 
inland sea, and, in the second, begins with the mountain 
rill and ends in the river.' 
1Jn Science Gossip for October 1894, I have attempted such a bird’s-eye 
view of the subject. For further reference to the influence of springs, see a 
paper by the author in Quart. Jour, Roy. Meteor, Soc., January 1895, 
[Taste I. 
