River Temperature. 295 
running stream, and thrive only where the stream is rapid 
and sheltered from the sun. 
The difference between rivers, and ponds, and pools, with 
regard to surface-heating, has been remarked by Von 
Freeden, Fournet, Max von dem Borne, and many others, in 
its connection with the life of the plants and animals that 
there thrive. The distinction would be, probably, even 
more pronounced in the tropics than in the temperate 
regions. Thus, for instance, Livingstone refers to some 
pools in Central Africa in lat. 20° 8. which in March 
had a temperature of 100° (F.) at the surface, whilst water 
deliciously cool could be obtained by walking into the 
middle, and lifting it up from the bottom. The heated 
top-waters of these ponds must have had a temperature 
quite 20° higher than that of the Zambesi in the same time 
of the year, and by this single comparison an important 
distinction between ponds and rivers in the tropics is dis- 
closed. Having paid considerable attention to this subject 
in connection with plant life, whilst contrasting the tem- 
peratures of the rivers, streams, and ponds in the neighbour- 
hood of Kingston, I must be excused if I diverge a little 
from the subject immediately in hand, in order to show that 
the lack or absence of surface-heating in rivers, in place 
of being a meaningless distinction, has a very practical 
significance as regards the germination, flowering, and seed- 
ing of water-plants. 
In a small pool, in the middle of April, I noticed that the 
flower-buds of Ranunculus aquatilis, which were just sub- 
merged, were in water of a temperature of 61° (F.), whilst, 
9 inches down, the stems of the plants were in water of a 
temperature of 46°. At the beginning of May, when the 
flowers were open, the surface of this pool was 71°, a 
temperature not attained in the Thames until after the 
middle of June. My experiments show, that whilst the initial 
germinating temperature of this plant is below 49°, the produc- 
tion of the flower-bud requires a temperature of about 60°. 
In a hot August afternoon, with the air at 86° in the 
shade, and the Thames at 74°, I found, in the middle of one 
of the large ponds in Bushey Park, where Myriophyllum 
VOL. XII. U 
