MR JOHN AITKEN ON DEW. 15 



are dry. The effect of the trays is very much the same as that of these large 

 leaves on a perfectly calm night. The only difference is, the trays will lose 

 more heat on account of their better conducting power, and more vapour will 

 be condensed under them than under the bad conductor, while the temperature 

 of the soil will be more nearly reduced to what it would have been if no large 

 close surface prevented the free evaporation. 



The experiments described were made in August and September, when the 

 ground was very dry, owing to the unusually small rainfall during the previous 

 months. On all occasions the inside of the tray was dewed, however dry the 

 soil, and the inside was always more moist than the outside. 



After these experiments were made, another method of testing the point 

 under investigation suggested itself, and though, unfortunately, rather far on 

 in the season for satisfactory work of this kind, I at once proceeded to carry it 

 out, as it afforded a means of checking my previous experiments with the 

 trays ; but by this time October had arrived, and the conditions had very much 

 changed. The temperature had fallen considerably, and the rainfall had greatly 

 increased the humidity of the soil. 



It is very evident that if vapour continues to rise from the ground during 

 dewy nights, as well as during the day, the ground giving off vapour must 

 lose weight. If this could be shown to be the case, it would prove in a more 

 satisfactory manner than the previous experiments that vapour does rise from 

 the ground during night, and that, therefore, dew on bodies near the surface 

 of the ground is really formed from the vapour rising at the time, and not from 

 the vapour that rose during the day. 



In the first week of October experiments were begun to test this point, by 

 weighing a small area of the surface of the ground, before and after dew had 

 formed, to see whether the ground continued to give off vapour or not while 

 dew was forming. For this purpose a number of shallow pans 6 inches 

 (152 mm.) square and \ inch (6 - 3 mm.) deep were prepared. One of these 

 pans was selected, and a piece of turf slightly smaller was cut from the lawn 

 and placed in it. The pan with its turf was then carefully weighed with a 

 balance sensitive enough to turn with ^ grain; but in experiments of this kind, 

 which must be done quickly, accuracy of only one grain was aimed at, lest the 

 time required for more accurate weighing might cause loss of weight by evapora- 

 tion. To prevent loss from this cause, the weighing was clone in an open shed. 



The turf was cut at sundown, and when dew began to form. The earth 

 was removed from it till it weighed exactly 3500 grains (226*79 grammes). 

 The pan with its turf was then rapidly restored to the lawn, and put in its 

 place, where the turf had been cut out, and in as good contact with the ground 

 as possible. The pan and turf were then brought back, the under side of the pan 

 carefully cleaned and dried, and all weighed again to make sure nothing was 



