206 Proceedings of the Royal Physical Society. 
autumn the amount of a pond’s daily fluctuation of tem- 
perature will be often reduced, and in winter it almost 
disappears. 
It follows from these facts that the comparison of the 
thermal conditions of a pond and of a river is a task of much 
complexity. Although the mean temperature of a river is 
readily ascertained on account of the small daily range and 
the slight variations of temperature in the different parts of 
the moving mass of water, it is almost impossible, at least in 
the warm half of the year, to obtain such an estimate for a 
pond. At its margins it exhibits, in the great daily fluctua- 
tions of temperature, the thermal conditions of a ditch; at 
the bottom, in the central portions, it resembles a river in its 
small daily range; and in the rest of its mass it displays a great 
variety of temperatures, due largely to the different depths, and 
to the arrangement and habits of the plants that there thrive. 
In order to illustrate the great diversity in the thermal 
conditions of ponds full of submerged plants, I have incor- 
porated some of my results in Table I. The Black Pond, 
Oxshott, where Scirpus fluitans abounds, together with 
Hypericum elodes, Sphagnum, and other plants, is typical of 
a pond in a boggy district. Ceratophyllum demersum is the 
prevailing plant of its habit in the Home Park Pond; whilst 
the Bushey Park Pond is largely occupied by Myriophyllum. 
There are usually one or two plants in a pond that, in giving 
their character to the plant-life, also mainly influence the 
thermal regime. Such plants I have just named. In the 
following table the temperatures of these ponds are contrasted 
with those of the Thames and of the air. The extremes 
only are here represented, and every intermediate condition 
is to be found between the cool bottom-waters and the heated 
marginal shallows. The water-temperature always rises 
where the submerged plants are densely gathered. Thus, 
in the centre of the Black Pond, where the surface was clear 
of plants, its temperature in the afternoon of June 19th was 
79°°8, whilst the thermometer placed in a dense floating mass 
of Scirpus fluitans indicated 82°; and in another part of the 
pond, where Hypericum elodes and Bog Moss filled up a depth 
of 18 inches, the surface was 88° and the bottom 73°, or a 
