208 Proceedings of the Royal Physveal Society. 
actual readings of the thermometer. Observations on other 
ponds point to the same conclusion, This is only true, 
however, of ponds of ordinary depth, namely, 4 or 5 feet. 
As indicated in Table I., a pond 7 or 8 feet deep, as in the 
case of the Queen’s Mere, Wimbledon, has a bottom-tempera- 
ture that is in summer four or five degrees cooler than the 
Thames, the diurnal range being almost nil. 
Although, as before remarked, it is not possible to compare 
the mean temperature of a pond in its entirety with that of 
a river, yet, by contrasting their several features, we can bring 
certain facts into prominence that deserve the attention of 
the student of the conditions of aquatic life. The observations 
on which the estimates for ponds, as given in Table IL., are 
based, were for the most part systematically made. But a 
large number of occasional observations have also been here 
employed, and, in fact, during two years I rarely went out 
without taking a thermometer with me.! It will be noticed 
in this Table that, contrasted in the monthly means, the 
thermal conditions of ponds and rivers appear in their true 
proportions. Isolated observations, such as occur in Table L., 
are rather apt to exaggerate the differences. The divergence, 
however, between a pond and a river is sufficiently striking 
in these respects; and in the varying temperature of the 
one, and in the uniform temperature of the other, we have 
conditions that would very differently affect aquatic life. 
It is probable that the diverse thermal conditions of ponds 
and rivers find their fullest expression in the varying habits 
of a water-molluse. It should, however, be remembered that 
the contrast is greatest in summer and least in winter, when, 
in fact, there is but little difference in this respect between 
a pond and a river, as is evidenced in Tables I. and II. 
Except in very sluggish rivers, a mollusc enjoys much the 
same temperature in any part of a river, whether at the 
surface, on the bottom, or at the borders. All the year 
round the temperature does not usually vary more than one 
or two degrees in the twenty-four hours, and the change 
‘ IT may here take the opportunity of expressing my sorrow at the death 
of Mr John Gunn, who, as Secretary of the Society, gave me valuable 
assistance in the early part of this inquiry. 
