218 KIPLING'S SUSSEX
absent. Amid the babel of voices a general principle or two may be heard which may give the clue. The upper surface of the water in the pond is cooled by radiation, and convection currents are thus set up. In other words, the colder layers of the surface descend, and the warmer water from below rises to the top. This goes on until the contents of the pond are colder than the surrounding rocks and soil, when condensation of the aqueous vapour, whencesoever arising, goes on at a rapid rate."
In some dew-ponds it will be observed that a tree or bush is planted on the south-west side. This helps the supply, for the drippings from the leaves fall to the pond.
Straw forms part of the ground work of other ponds. It is placed between layers of mixed lime and clay and broken chalk ; or between layers of cement. Straw is used to act as a non-conducting agent, for it isolates the clay from the heat generated by the earth. The greatest care must be used to brick up the margins of straw dew-ponds, for the water must not get at the straw. That is ruinous, for wet straw ceases to draw the mist.
The Sussex rustic is a slow person, though there is no reason to suppose that he is slower than any other rustic ; indeed, one is inclined to think that the proverbial slowness of all rustics covers a