DEW PONDS.
(To THE EDITOR OF THE " SpEcTiroR:1 Ste,--I am afraid the explanation of your correspondent as to the cause which determines the -filling of dew ponds does not bear scientific examination. Dew will not deposit on a surface warmer than the surrounding air. A dew pond in good order is generally filled, or partly filled, with water, so that neither the clay nor straw is exposed, and the surface of the water, as I know from personal observation on the spot, is warmer at night than the air (for, -being a bad conductor, it loses heat slowly). It is impossible, therefore, for moisture to deposit on this surface as dew. My own opinion is that the clay tamping is merely to render the chalk impervious, and the straw is added as a binding material, just as hair is added to plaster. If these ponds. were situated on low- lying ground instead of ou the tops and sides of the chalk Downs, the colder and denser air, flowing down during radiation after sunset, might produce a deposition of moisture by contact with the air saturated with moisture above the warm surface -of the water. But on the hilltops the air on a still, dewy night is warmer than in the valley, a fact which is rendered visible by the deposition of mist in the valley, a condition which is never seen on hilltops. Moreover, I was able to show some years ag that a thermometer will register a higher temperature in this early morning before sunrise on high than on low ground. So far as I know, no really satisfactory explanation of this remark- able phenomenon of the filling of dew ponds has ever been
[Any one who has slept out in hot weather on the Wiltshire Downs, say during autumn manoeuvres, will recall dews of astonishing potency. Clothes not covered up would be wringing wet in the morning. This is, we believe, as true of the tops of the Downs as of the lower slopes.—Ea. Spectator.]
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