CAUSES OF DEW. 71 



all other atmospherical precipifetions. 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 frorn 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 circumambient 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 



