River Temperature. 207 
change of 1° for each inch in depth, a common feature 
of the thermal regime of a ditch in a summer day. The 
observations on the Home Park Pond are of particular 
interest with regard to Ceratophyllum demersum. During 
this hot summer of 1895, I frequently took the temperature 
in the shallows where this plant was thriving, and here, 
where the afternoon temperature was frequently over 80° 
and sometimes as high as 90°, it flowered and produced 
abundant mature fruit. Incidental reference to the thermal 
conditions required by this plant for reproducing itself from 
seed will be found in the first part (vol. xii., p. 296) of this 
paper, but a full discussion of the matter will be found in 
Science Gossip for November 1894. 
In sunny weather the thermal regime of a pond is to a large 
extent regulated by the plants that live in it. Submerged 
plants like Myriophyllum, Ceratophyllum, and Zannichellia 
favour the heating process by impeding the free circulation 
of the water; and, as before remarked, plants with large 
floating leaves, like the water-lilies and the Zimnanth, keep 
the temperature cool. A multitude of other influences, 
however, are always at work. Upon the position of a spring 
and on the situation of an affluent great differences often 
depend. Whilst the temperature at one end of a pond may 
be under the influence of the agencies regulating that of the 
air, at the other end it may be far more equable on account 
of the control of a perennial spring. A fresh breeze on a 
sunny day causes the lee side to be some degrees warmer 
than the opposite border. A pond with a shallow outlet 
loses its heated surface-waters, whilst one with a deep outlet 
loses its cool bottom-waters. Ponds without affluents or 
effluents heat up rapidly. In fact, as I have elsewhere 
observed, “no two ponds possess precisely the same thermal 
regime, the determining conditions being almost infinite in 
their variety” (Science Gossip, October 1894). 
On referring to Table I. we notice that, however much a 
pond may differ from a river in its thermal behaviour, its 
bottom-waters exhibit much the same conditions all through 
the year. The resemblance is not only concerned with the 
small daily range of one or two degrees, but also with the 
