MR JOHN AITKEN ON DEW. 25 



those of the elevated ones get wet like the grass, the reason for this is that the 

 bodies on the ground as well as the elevated ones are constantly losing more heat 

 by radiation than they receive by absorption ; but those in contact with the 

 ground have heat communicated to them by conduction and by the condensa- 

 tion of vapour on their under surfaces; their temperature is thus prevented 

 from falling as low as that of the elevated bodies, which only receive heat 

 from the passing air ; the latter are thus cooled more by radiation than those 

 on the ground. Bodies out of heat communication with the ground thus tend 

 to cool more than those in contact with it; and while the former get cooled 

 below the dew-point and collect dew, the latter keep warmer than the dew- 

 point, and thus tend to keep dry, or if wetted to become dry again. 



These considerations suggest a simple method of testing whether the surface 

 of any particular part of the ground is giving off vapour or not. It is very 

 evident that so long as the temperature of the surface of the soil is above the 

 dew-point of the air, vapour will rise from the ground, and that if the 

 surface is cooled by radiation below the dew-point, evaporation will cease, 

 and vapour will condense upon it. In order to test this, all that is necessary is 

 to place on the ground, and in good heat communication with it, some substance 

 that is a good conductor, and shows dewing easily. A piece of metal 

 covered with black varnish does well. It is painted black, not in order to radiate 

 copiously, but because black shows any deposit of dew most quickly and easily. 

 So long as this test surface keeps dry while in contact with the ground, the 

 soil round it must be giving off vapour, because the temperature of its surface 

 is higher than the dew-point. But if the temperature of the ground falls 

 below the dew-point it will collect moisture, and this test surface will 

 collect dew also, and will thus tell us that the surrounding soil is receiving 

 moisture. In experiments such as these we are simply converting a small 

 area of the earth's surface into a condescending hygroscope, and our test 

 surface tells us whether the earth's surface at the place is cooled by radiation 

 below the dew-point or not. So long as no dew forms on the test surface 

 vapour is being given off. 



These test surfaces must not be large, at most only two or three centimetres, 

 because if large they would check the free passage of the vapour to the air, 

 and so prevent the soil under them from cooling to the same amount as the 

 surrounding ground ; and further, it is difficult to get good contact with large 

 surfaces, without which only a part of the test surface keeps clear, while 

 the part not in contact gets dewed, even though the temperature of the surface 

 of the ground is above the dew-point. This was confirmed by observations 

 made on a frosty night. On lifting each test plate, it was observed that the 

 soil was frozen to it under the clear parts, and no soil adhered under the parts 

 that were dewed. In my experiments I have used small copper discs covered 



VOL. XXXIII. PART I. D 



