5'8 Mr Black adder on the Formation of Dew. 
dally when the supply of moisture, upon which that process de- 
pends, is furnished by the peculiar action of an organised body, 
the regular exercise of whose functions is greatly influenced by 
the presence or absence of a certain degree of heat. When the 
temperature of the grass is greatly, and, as often happens, sud- 
denly reduced, we may expect, from what is known to take place 
in similar cases, that its transpiration will be diminished, though 
not altogether suspended, and, consequently, that the evapora- 
tion from its surface will be less copious. 
On the other hand, as the temperature of the soil under the 
grass diminishes very slowly, owing to the facility with which 
it acquires heat by conduction, the evaporation from its surface 
must continue comparatively active, long after that from the 
grass has become almost extinct ; and even should the air be- 
come saturated with moisture, vapour will continue to be gene- 
rated at the earthy surface, until such time as the temperature 
of the latter comes to be in equilibrium with that of the con- 
tiguous air. 
When, therefore, the disposition of the air for receiving mois- 
ture, and the temperature of the grass, are such as to admit of 
the vapour generated at the surface of the subjacent soil being 
partly condensed into dew, as it comes into contact with the piles 
of grass, part of this vapour, as it rises through the cold air, 
will also be condensed into aqueous particles, and remain sus- 
pended in the air. At first, and on some occasions, these par- 
ticles are so thinly dispersed as to be almost invisible ; but af- 
terwards, and on other occasions, their numbers increase with 
such rapidity, as very speedily to produce a general haziness 
over the surface. When this occurs early or soon after sunset, 
and when the particles congregate, so as to form a mist, the 
lower surface of which is elevated from 15 to 30 feet above the 
ground, those who inhabit parts of the country, in the vicinity 
of extensive meadows, consider it as .an indication of approach- 
ing rain ; and they express it by saying, 44 The dew is rising, we 
shall soon have rain.'” In this instance, as in many others of a 
similar nature, the general observation is founded in truth, while 
the principle upon which it is grounded is manifestly erroneous. 
The appearance referred to occurs only after a continuance of 
hot weather, with the prevalence of southerly winds ; and at a 
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