METEOROLOGY. 211 
Plane and horizontal pieces of glass are well adapted for observing the 
formation of dew. Pl. 23, figs. 44-47, show how the drops, small at first, 
afterwards increase and run together. Wool deserves the preference so 
far, that besides readily receiving the dew, it retains it for some time. 
The amount of dew produced depends not so much on the quantity of wool 
as upon the manner of its arrangement. A certain quantity of wool rolled 
up in a ball (fig. 48) will receive much less dew than if spread out flat. 
Metals become dewed very slowly, although there is a difference in this 
respect, platinum, iron, steel. and zinc, receiving a greater amount than 
gold, silver, copper, and tin. Metals purposely moistened sometimes become 
dry while other bodies are becoming coated. If a metal be combined with 
a substance of some thickness, capable of being strongly bedewed, the 
radiating power of the metal is not increased, as might be supposed, but 
diminished, and the resistance to the deposition of dew presented by the 
metal may even be communicated to other bodies lying upon it. Thus 
wool on a plate of metal becomes less wetted than if lying on a plate of 
glass. If, however, a piece of gilt paper be laid with its unmetallic side 
upon wood, the paper will be perfectly dry where it is in contact 
with the wood, and the metallic side will become bedewed (figs. 49-52). 
If a watch-glass be laid on a plate of polished tin, with the concave side 
up (pl. 23, jigs. 53* and 53°), a dry zone will be seen on the outer border 
of the glass, and a circular dry space in the centre, the two separated by a 
zone of dew, exhibiting the largest drop in the middle. An example of the 
regularity with which dew is deposited around a row of wafers laid on glass, 
themselves receiving no dew, is seen in fig. 54. Finally, figs. 56 and 57 
show the dewing of spiders’ webs. 
According to the experiments of Wells, it is not correct to assign 
morning and evening as the time of greatest deposit of dew. It seems 
rather to be deposited during all hours of the night, more abundantly, 
perhaps, after midnight than before it. In shaded places it appears to form 
even in the afternoon. Nevertheless dew is not deposited in all countries 
in equal quantities. It is most copious in the coast regions of warm lands ; 
for instance, along the Red Sea, the Persian Gulf, the coast of Coromandel, 
at Alexandria, and in Chili. On the other hand it is almost entirely wanting 
on the arid plains in the interior of continents, as in central Brazil, in the 
deserts of Sahara and Nubia, &c. It is very rarely seen at sea. 
That dew does not ascend, as supposed by the earlier philosophers, is now 
not a matter of controversy. Other things being equal, the amount of dew 
will be in proportion to that of moisture in the atmosphere, for which reason 
heavy dews are frequently the precursors of rain. With equal quantities 
of moisture in the air the dews of cold nights are more copious than those 
of warm. Dew-water, when collected, is found to be almost chemically 
pure, containing, however, some carbonic acid. The sticky deposits found 
on plants, called honey-dew and meal-dew, do not proceed from the 
atmosphere, but consist of the secretions of Aphides and other insects. 
ICONOGRAPHIC ENCYCLOP 2DIA.—VOL. I. 25 385 
