1334. When bodies become cold by radiation, the degree of effect observed must depend, not only on their radiating power, but in part also on the greater or less case with which they can derive heat, by conduction, from warmer substances in contact with them. Bodies, exposed in a clear night to the sky, must radiate as much heat to it during the prevalence of wind, as they would do if the air were altogether still. But in the former case, little or no cold will be observed upon them above that of the atmosphere, as the frequent application of warm air must quickly return a heat equal, or nearly so, to that which they had lost by radiation. A slight agitation of the air is sufficient to produce some effect of this kind; though, as has already been said, such an agitation, when the air is very pregnant with moisture, will render greater the quantity of dew; one requisite for a considerable production of this fluid being more increased by it, than another is diminished.
