CAUSES OF DEW. @1 
ull other atmospherical precipitations. They all result from the 
influence of some refrigerating cause upon the air; such as the 
passage of a warm current into a cooler region; the influx of a 
cold wind; a cold-radiating chain of high mountains; a forest, 
and so forth. 
The very name of dew is refreshing, and calls forth a host of 
pleasing ideas, associated as it is with the memory of serene skies 
and sunny mornings. How beautiful are its diamonds glittering 
in all the colours of the rainbow, on verdant meads, or on the 
blushing petals of the rose. How suggestive of all that is lovely, 
pure, and innocent ! 
Poetry is of older date than prose, and bards have sung long 
before philosophers inquired. Thus, although the children of 
song from Homer and Theocritus to Byron and Wordsworth so 
frequently mention dew in their immortal strains, it is only in 
our time that its formation has been fully explained by 
Dr. Wells, who in a very ingenious’and masterly essay on this 
subject, first proved that it results from the ground radiating or 
projecting heat into free space, and consequently becoming 
colder than the neighbouring air. During calm and clear 
nights, the upper surfaces of grass-blades, for instance, radiate 
their caloric into the serene sky, from which they receive 
none in return. The lower parts of the plant, being slow 
conductors of heat, can only transmit to them a small portion 
of terrestrial warmth, and their temperature consequently 
falling below that of the cireumambient atmosphere, they con- 
dense its aqueous vapours. Clouds on the contrary compensate 
for the loss of heat the grass sustains from radiation, by reflect- 
ing or throwing back again upon the terrestrial surface, the 
caloric which would else have been dissipated in a clear sky, and 
this is the reason why dew does not fall, or but slightly falls 
during clouded nights. It is easy to conceive why none is formed 
in windy weather, as then the air in contact with the ground is 
constantly removed ere it has time to cool so far as to compel it 
to part with its moisture. We can also understand why dew is 
more abundant in autumn and spring than at any other season ; 
as then very cold nights frequently follow upon warm days; and 
why it is most copious in the torrid zone, as in those sultry regions 
the air is more saturated with moisture than anywhere else, and 
the comparatively cold nights are almost constantly serene and 
