MR JOHN AITKEN ON DEW. 27 



point; (2) the wind by its passage over the surface of radiating bodies 

 prevents these surfaces being cooled much below the temperature of the air; 

 the wind thus tends to prevent the air in contact with these surfaces being 

 cooled below the dew-point ; and (3) wind blowing over the surface of the 

 ground rapidly carries away the vapour rising from the soil, and mixes it up 

 with a large quantity of drier air. The wind thus tends to prevent an 

 accumulation of clamp air near the ground. 



To illustrate this third effect of wind, let us use the observations made on 

 the evening of October 12. The sky was clear, and there was a considerable 

 amount of radiation, but a slight wind was blowing. The bare soil in the test- 

 pan lost 22 grains and the corresponding turf lost 30 grains in about five hours. 

 Almost no dew was formed on the grass, but trays placed over the bare soil 

 and over grass had their inside surfaces covered with moisture, though not so 

 heavily as was generally observed on dewy nights. The reason why so little 

 clew formed on this occasion was, partly, that the wind prevented the tempera- 

 ture of the air near the ground falling as much as it would have clone if it had 

 been calm. In the screen the temperature only fell to 40°. On the grass, 

 however, it fell to 31°*5, and on the soil to 35° "5; but a good deal depended on 

 the exposure of the thermometer to the wind. From the above we see that, 

 though wind was blowing, the thermometer on the grass fell a good deal below 

 the temperature of the air, and showed a considerable amount of radiation. The 

 wind apparently prevented the formation of clew on this occasion, principally by 

 preventing an accumulation of moist air near the surface of the ground. The 

 inverted trays showed that if the wind had fallen dew would have formed, 

 because it formed in the still air under the trays. The deposit was not so heavy 

 inside the trays on this occasion as was often seen in dewy nights, because the 

 wind prevented the radiation cooling the top of the trays to the same extent as 

 when it was calm. 



Dew and Vegetation. 



When I began to make observations on dew, one of the first things I did 

 was to make a tour of the garden on a dewy night, and to examine the 

 appearance of the plants. A very short survey was sufficient to show that 

 something else was at work than radiation and condensation to produce the 

 effects then seen. Let me briefly describe what I saw, and what at once 

 struck me could not be explained by the ordinary laws of radiation and con- 

 densation. Certain kinds of plants were found to be covered with moisture, 

 while others were dry. Many plants of the Brassica family were heavily 

 covered with glistening drops; while beans, peas, &c, growing alongside them, 

 were quite dry. Again, in clusters of plants of the same kind some were wet, 

 while others were not; and not only so, but some branches were wet, while 



