THEORIES OF DEW.
Referring to our recent article on ‘[A New Theory of Dew]’ (No. 126), a correspondent at Beaumaris writes as follows:
‘You will see from the following experiment, one of many carried out by Mr Du Fay in Paris towards the end of last century, that Mr Aitken’s ideas regarding the origin of dew are not strikingly new, and only go to prove the old adage that “There is nothing new under the sun.”—“Mr Du Fay, at Paris, placed two ladders against one another, meeting at their upper ends, and spreading wide asunder below. Their height was thirty-two feet. To the several steps of these he fastened large panes of glass, so disposed as not to overshade one another. With this apparatus exposed to the air, he found that the lower surface of the lowest pane of glass was first wetted with dew, then its upper surface, then the lower surface of the pane next above it was wetted, and so on, until all the panes to the very top of the ladders became covered with dew. Mr Du Fay maintained that this was an unanswerable proof that dew was formed from vapours ascending from the earth during the night, rather than from the descent of such as had been raised in the course of the day.” Dr Wells’s theory is doubtless the more generally accepted; but many men, more especially such as have sojourned in tropical climes, hold to Du Fay’s opinion, namely, that the moisture causing dew emanates more from the soil than from the circumambient air.’