MR JOHN AITKEN ON DEW. 53 



similar stimulating effect produced on vegetation when tools were left lying on 

 the surface, but was unable to account for it till he read the abstract of the 

 first part of this paper, when he at once recognised the manner in which the 

 tools lying on the surface acted in hot and dry climates by checking the 

 escape of vapour at night. No doubt much of the increased fertility observed 

 would be due to the wetting the plants received by the vapour condensed on 

 the under sides of the tools ; still it will be admitted something must also be 

 owing to the reduced evaporation during the day, produced by the metal tools 

 checking the escape of the vapour. 



This conservation of moisture by stones and other bodies lying on the 

 surface of the ground, and its up and down movement during night and day, 

 may be easily seen in the experiment with the slates. Examined at night the 

 road under the slates is as dry as the exposed parts, while the under sides of 

 the slates are wet ; but if examined at a certain time in the morning, the under 

 sides of the slates will be found to be dry, while the road under them is wet. 

 But, if the slates are not examined till a later hour, both the slates and the 

 road under them will be found to be dry, the moisture being driven deeper and 

 condensed among the stones lower down. 



Many people, after reading the conclusions arrived at in the first part of this 

 paper, seem to have a difficulty in accepting the theory that vapour is con- 

 stantly rising from the ground during night as well as day, and that the dew 

 on grass is formed of this rising vapour, because it seems to them to contradict 

 the teaching of Dr Wells, and it also appears to be at variance with their 

 experience. A little consideration will show that the results established by 

 Dr Wells are in no way affected by it. That investigator certainly did 

 not think that much vapour rose from the ground at night, yet he was well 

 aware that some might rise ; his investigation was however principally con- 

 fined to the condensation of the vapour after it is in the air, and he gave 

 comparatively little attention to its source ; whereas in this investigation 

 Wells' results are accepted with regard to the condensation, and an attempt is 

 made to extend the subject by investigating some parts not worked out by him. 



Others again have made a difficulty to the acceptance of this theory by 

 extending the conclusions arrived at in this paper to other conditions than those 

 for which they are true. They have assumed that if dew on grass and on bodies 

 near the ground is formed of vapour rising at the time, then the dew found on 

 bodies higher up in the air must also be formed of vapour rising at the time 

 from the ground immediately underneath ; and as this conclusion is opposed to 

 experience, they seem inclined to dismiss the whole theory as unworthy of 

 consideration. When we come to investigate what is taking place in nature 

 we will see that this extension of the conclusion is by no means justified. 



The reason why it is concluded that the dew on the grass, and on bodies 



