River Temperature. 297 
full of submerged plants than in one comparatively clear. 
Pond-plants, to a great extent, regulate the temperature 
of the waters in which they thrive. Dense growths of 
such submerged plants as Myriophyllum, Ceratophyllum, 
Zannichellia, and Scirpus fluitans, favour surface-heating by 
obstructing the circulation of the water, and thus tend to 
produce the thermal conditions requisite for their flowering 
and the maturation of their seeds. On the other hand, the 
floating leaves of the water-lilies and of some of the 
Potamogetons keep the pond cool by day and warm at night. 
River and pond temperature in warm climates must be 
often greatly influenced by the floating masses of Pistia, 
Salvinia, and other water-plants. 
I pass on now to consider the times at which, in the course 
of its daily fluctuations, a river attains its minimum, maxi- 
mum, and mean temperatures. With regard to the hour of 
minimum temperature, most observers agree in their sunrise 
or early morning observations, but Renou is my principal 
authority in this matter. From his hourly observations on 
the Loir from 6 A.M. to 10 P.M, excepting 8 and 9 P.M., he 
places the hour of lowest temperature at 7 A.M. from April to 
October, at 8 A.M. in February, March, and November, and 
at 9 AM. in December and January. For the Seine at 
Choisy-le-Roi he states the time to be after sunrise, at 5 or 6 
A.M. in summer, and 8 or 9 A.M. in winter. For the Thames 
at Kingston I found the lowest temperature to be reached at 
sunrise or during the two succeeding hours. The Arve, the 
most considerable of the Alpine tributaries of the Rhone, 
behaves in a peculiar fashion, due probably to its mountain 
course as well as to its draining a basin, rather over one- 
tenth of which is occupied by glaciers and perpetual snow. 
According to Professor Chaix, its temperature in summer 
continues to fall until 9 or 10 a.m. From the table which 
Dr Borius gives for the Senegal, it is evident that above 
tidal influence the coolest temperature occurs not later than 
6 aM. On the whole, I am inclined to think that during 
the hour after sunrise would be found the most suitable 
time for observing the lowest temperature of rivers in most 
parts of the world. 
