£43 Mr Blackadder on Circumstances connected with the 
scopic property ; but as, at least, part of the moisture is 
quickly taken into their substance, its presence on their surface 
is still less to be detected, than on the rough surfaces of the non- 
absorbents. Lastly, When a polished metal, of the same tem- 
perature with the air, is placed over a vessel containing water, of 
a somewhat higher temperature, vapour is condensed on its un- 
der surface ; and the same thing happens when it is placed on, 
or a little above, the surface of an open field after sunset. The 
vapour issuing from the ground is condensed on the side of 
the metal, which is directed to the earth, provided its superfi- 
cies be of a certain extent,— -for if very small, the mechanical ef- 
fect becomes neutralized. 
In this country, examples of the second mode in which me- 
tals become dewed, are less familiar than in such countries as 
Holland and the Netherlands generally. There, during the 
warm season, the cold produced by evaporation (as it is conceiv- 
ed), is seldom or never great, the air being usually so very 
damp that but a small reduction of temperature is requisite to 
bring it to a state of supersaturation. 
Musschenbroek had remarked, that a low haze or fog was a 
concomitant of dew in Holland ; and Dr Wells seems unneces- 
sarily to have objected to this observation of the Dutch philoso- 
pher. I never saw dew forming on the grass in the Low’ Coun- 
tries, without a haze being at the same time more or less appa- 
rent ; and, in our own country, if the eye be directed, on such 
occasions^ to the distant surface, it will be found that there is 
commonly a certain haziness of the lower air, though not 
so dense as to be perceptible within a considerable distance. 
44 Respecting this point,” Dr Wells observes, fc4 I can aver, 
after much experience, that I never knew dew to be abun- 
dant except in serene weather and again, 44 I can assert, after 
much attention to this point, that the formation of the most 
abundant dew is consistent with a pellucid state of the atmo- 
sphere. Hasselquist makes a similar observation with regard 
to Egypt ; where, during the season remarkable for the most 
profuse dews, the 4 nights,’ he says, 4 are as resplendent with 
stars in the midst of summer, as the lightest and clearest win- 
ter nights in the North.’” From this it is pretty obvious, that 
