THE PHILOSOPHY OP DEW. 



whilst the varnish beneath the small disc can only cool by contact. And we find that 

 the central part of the varnished circle remains perfectly dry, whereas the outer metallic 

 band is wetted to its very edge, if the atmosphere is exceedingly damp. 



But there is another circumstance which we could not so easily have foreseen. I mean 

 the exact repetition of the same appearances on the surface of the disc turned next the 

 ground. Dew begins to appear on this surface exactly opposite the little exterior varnish- 

 ed band ; a light whitish circle suddenly appears on the polished metal, and after becom- 

 ing better defined, spreads little by little, sometimes as far as the edges of the plate in 

 one direction, but never reaches the center, which remains quite dry and brilliant like the 

 corresponding portion on the other side of the disc, and the small circular roof which covers 

 it without touching it. 



From this very simple experiment we draw the following conclusions. Dew does not 

 fall from the sky, because the upper disc is always dry, and the greater part of the lower 

 one is wetted. It does not arise from the ground, because if the exterior part of the lower 

 surface of the great disc is covered, the middle part of it remains perfectly dr}"- and bril- 

 liant. jMctals do not repel aqueous vapor which forms dew, nor do they cause its evapo- 

 ration, as it is deposited upon them, since we see some parts of the metal quite wet, and 

 othei's quite dry. 



The appearance of the dew first on the uncovered band of varnish, and its gradual ex- 

 tension to the other adjacent and opposite parts of the great disc, together with the fall 

 of temperature observed on the varnished cases of the thermometers exposed freely to the 

 air, prove that dew is a pure consequence of nocturnal radiation, which gives to good ra- 

 diators the degree of cold necessary for the condensation of the elastic invisible aqueous 

 vapor which pervades our atmosphere. 



Thus far all the facts agree perfectly with the generally received theory of dew. In a 

 second letter I shall bring forward others which are not capable of explanation in the usual 

 way, but which, nevertheless, are connected in a fortunate manner to the principle of 

 Wells. 



SECOND LETTER. 



In this letter I shall show, as I before promised, that although Wells' principle of the 

 origin of dew is correct, it is impossible to explain all the thermometrical and hygromet- 

 rical phenomena which are produced during a calm fine night, without taking into account 

 a new circumstance, as yet entirely neglected, which is of the greatest consequence in the 

 nocturnal cooling of bodies. But before proceeding to this point, I shall offer a few re- 

 marks on two series of experiments, advanced by those who maintain that dew arises 

 from the soil, and which they say completely refute all theories of dew based upon radia- 

 tion. 



A certain number of thermometers, as nearly as possible equal, both in size or sensibi- 

 lity, arc taken, of which some are coated with lampblack, varnish and Indian ink, while 

 others are covered with gold, silver, tin, copper, and leaves of plants. These instruments, 

 thus prepared and exposed to the free atmosphere on a calm fine night, indicated at first 

 temperatures differing a little the one from the other; but after a certain space of time, 

 the}' all indicated very nearly the same temperature. The experiment was varied — on the 

 ends of glass tubes placed upright in the soil, plates of zinc, copper, glass, were laid; in the 

 center of each of these plates was a depression in which the bulb of the thermometer was 

 placed, wliile its stem, supported by an iron wire, remained in a vertical position; another 



ermometer suspended freely between the plates marked the temperature of the air 

 gain the instruments, when exposed to the air in the evening, indicated differences. 



