14 MR JOHN AITKEN ON DEW. 



against this method of experimenting it is rather a reason for it. We must 

 remember the tray does not heat the ground ; it does not add anything to its 

 store of heat, and enable it to evaporate more moisture ; it simply prevents so 

 much of its store of heat escaping. Now heat escapes from the ground at 

 night in two ways — first, by radiation, and second, by absorption — to supply the 

 latent heat of evaporation. From the area covered by these trays radiation 

 goes on much as at other places ; the painted metal will radiate as much heat 

 as the grass, but evaporation is checked, as there is but little circulation under 

 the trays ; and further, there is the heat recovered by the condensation 

 inside the box. It would thus appear that the reduced evaporation and heat 

 of condensation will be the principal causes of the higher temperature inside 

 than outside ; so that the trays, instead of increasing the evaporation, would 

 rather seem to decrease it; and that the lower temperature outside is due to 

 the greater evaporation there taking place, as both surfaces are exposed to the 

 same loss by radiation. 



There is an objection that might be made to the whole theory that dew is 

 formed from vapour rising from the ground. It might be urged that it is 

 impossible for the vapour to rise from the ground, and that these trays interfere 

 with the conditions existing in nature. On a cold clear night, for instance, when 

 the grass gets cooled before the dew-point, it might be said that it is quite 

 impossible for the vapour to rise up through it, as it would be all trapped on 

 its passage to the surface by contact with the cold blades, and that the trays 

 placed over the grass prevent this condensation by stopping the radiation from 

 the grass, and thus they allow the vapour to come up. 



A little explanation will, however, show this objection to be groundless. 

 On a dewy night no doubt the top of the grass is at a temperature below the 

 clew point, and if Ave may take the temperature of a thermometer placed on the 

 grass to be the same as that of the grass, which we may do without sensible 

 error, if we then remove the thermometer and place it among the stems of the 

 grass, the thermometer will rise; and if we place the bulb among the stems close 

 to, but not in the ground, we shall find it to be very much warmer than at the 

 surface. On dewy nights I have frequently found it as much as 10 to 12 

 degrees warmer. From this we see that the warm air diffusing upwards with 

 its burden of vapour only meets with a very small amount of surface cooled 

 below the dew-point, so that the greater part of the vapour is free to escape 

 into the air. 



Fairly considered, I think these trays more nearly represent natural 

 conditions than might at first sight appear. Indeed, precisely similar results 

 have been observed with natural conditions. If we examine plants with large 

 blades, we shall often find, on dewy nights, that those leaves which are close to 

 the ground have their under surfaces heavily dewed, while their upper surfaces 



