1339. Heat produced by walls, 'Walls,' Dr. Wells continues, 'as far as warmth is concerned, are regarded as useful, during a cold night, to the plants which touch them or are near to them, only in two ways: first, by the mechanical shelter which they afford against cold winds; and, secondly, by giving out the heat which they had acquired during the day. It appearing to me, however, that, on clear and calm nights, those on which plants frequently receive much injury from cold, walls must be beneficial in a third way, namely, by preventing, in part, the loss of heat which the plants would sustain from radiation, if they were fully exposed to the sky, the following experiment was made for the purpose of determining the justness of this opinion. A cambric handkerchief having been placed by means of two upright sticks, perpendicularly to a grass-plot, and at right angles to the course of the air, a thermometer was laid upon the grass, close to the lower edge of the handkerchief, on its windward side. The thermometer, thus situated, was several nights compared with another, lying on the same grass-plot, but on a part of it fully exposed to the sky. On two of these nights, the air being clear and calm, the grass close to the handkerchief was found to be 4ﾦ warmer than the fully exposed grass. On a third, the difference was 6ﾦ. An analogous fact is mentioned by Garstin, who says that a horizontal surface is more abundantly dewed than one which is perpendicular to the ground.'
