Page:Popular Science Monthly Volume 23.djvu/481

Rh continued throughout the night, the greater cold of the surface is found to be maintained until sunrise, and for some hours beyond it. Had the air been perfectly still during the observations, the nocturnal chilling of the surface would have been in this case greater: for you can readily understand that even a light wind sweeping over the surface, and mixing the chilled with the warmer air, must seriously interfere with its refrigeration.

''Hind Head, elevation 850 feet; sky cloudless; hoar-frost; wind light, from northeast. Course of temperature, March 4, 1883.''

Various circumstances may contribute to lessen, or even abolish, the difference between the two thermometers. Haze, fog, cloud, rain, snow, are all known to be influential. These are visible impediments to the outflow of beat from the earth; but we have now to deal with the powerful obstacle to that outflow to which reference has been already made, and which is entirely invisible. The pure vapor of water, for example, is a gas as invisible as the air itself. It is everywhere diffused through the air; but, unlike the oxygen and nitrogen of the atmosphere, it is not constant in quantity. We have now to examine whether meteorological observations do not clearly indicate its influence on terrestrial radiation: