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KIDD'S OWN JOURNAL. 



it is said, to the opinion that this humidity 

 arises from the earth. Dr. Dufay, a French 

 philosopher, maintained this opinion, based 

 on the following experiment. Taking two 

 long ladders, he fixed them so that they met 

 at the top and were wide apart at the bottom, 

 and attached to the several rounds large panes 

 of glass. Observing that the lower surface 

 of the lowest pane was first wetted ; then the 

 upper, next the lower surface of the one 

 above it, then its upper, and so on to the top 

 of the ladders, — he deduced the conclusion 

 that dew is caused by the exhalation of vapors 

 from the earth during the night. 



On the other hand, it was urged, in proof 

 of the descent of vapor, that in cloudy 

 weather little or no dew is formed. The 

 fallacy of both these hypotheses has been 

 proved by Dr. Wells, by a most beautiful 

 inductive process ; in which he shows that 

 dew is produced by the condensation of the 

 atmospheric vapor surrounding the bodies on 

 which it is deposited. There were other dif- 

 ficulties still more perplexing connected with 

 the first question — Does the vapor producing 

 dew rise or fall ? For example, while some 

 substances receive the deposition of dew very 

 readily, there are others on which it cannot 

 be deposited. 



But every circumstance connected with 

 this phenomenon finds the most satisfactory 

 explanation in the beautiful theory proposed 

 by Dr. "Wells, and now universally adopted 

 by philosophers — a theory which depends 

 upon two principles, viz., the nocturnal radi- 

 ation of caloric and the condensation of invi- 

 sible vapor. One important lesson at least 

 is taught by the history of these opinions, 

 which is, the absolute necessity of basing our 

 theories upon authenticated and well- inves- 

 tigated experiments, carried out under the 

 guidance of legitimate deductions. 



According to the theory of Dr. Wells, there 

 are five essential requisites for the deposition 

 of dew. 



1. An atmosphere replete with moisture. 

 That the moisture must be in excess before 

 it can be deposited, is evidenced by the fact, 

 that in Egypt no dew is formed when the 

 winds blow from the south, over the extensive 

 tracts of sandy desert ; but so soon as the 

 wind changes to the north, laden with mois- 

 ture from the Mediterranean, the deposition 

 is remarkably great. 



2. The difference between the temperature of 

 the earth in the day and the night must be con- 

 siderable. Consequently, the deposition is 

 greatest when a sultry day is followed by a 

 cool evening ; and, for the same reason, the 

 dews are most abundant, in our climate, in 

 spring and autumn, as then the difference of 

 temperature is greatest. But hot climates 

 have more copious dews than temperate 

 countries, notwithstanding the difference be- 



tween diurnal and nocturnal temperature may 

 be less in the former — a fact that finds an 

 explanation in the circumstance, that an in- 

 crease of temperature is attended with more 

 than a corresponding increase of moisture. 



3. A serene and cloudless s7cy. Notwith- 

 standing the atmosphere may be in other 

 respects favorable, little or no deposition, if 

 the sky is veiled in clouds, occurs ; for, as the 

 caloric radiated from the earth i3 reflected 

 back by the clouds, the temperature of objects 

 on its surface is little diminished. Screens of 

 an opaque material, interposed between the 

 sky, and the surface of the earth, produced 

 the same effect ; and, accordingly, a thermo- 

 meter laid on a table, compared with one 

 placed on the ground beneath it, indicated a 

 lower temperature. Even fogs, which are 

 precipitated from the higher air, acting as 

 screens, are unfavorable to the deposition of 

 true dew, which is separated from the inferior 

 atmospheric stratum. 



4. Serene and calm weather. This follows 

 from the circumstance, that if the lower 

 atmosphere be in violent motion, it will main- 

 tain the general temperature of bodies on 

 the surface of the earth ; and hence, too, 

 every condition which favors radiation, as a 

 dark color or a rough surface, contributes to 

 the deposition of dew. 



5. The temperature of the body upon which 

 the dew is deposited must be considerably lower 

 than that of the ambient air. This is the most 

 essential requisite. Dr. Wells, in his experi- 

 ments, found the bodies on which dew formed 

 to be 10° or 15° colder than the atmosphere. 



Different bodies, according to their consti- 

 tution, possess different powers of radiation. 

 For instance, metals and vitreous substances 

 are, in this respect, in very opposite extremes. 

 Bad conductors or bad reflectors are, as a 

 general law, good radiators ; but the power 

 of radiation, as just remarked, depends greatly 

 upon the nature of the surface. Hence, a 

 piece of wool, or a plate of glass, placed in 

 a horizontal position, favors the deposition 

 of dew ; but a piece of polished metal will 

 retain its lustre, notwithstanding every blade 

 of grass around it may be drooping with the 

 pressure of condensed vapor. These facts 

 lead at once to the deduction, that during 

 the night, the temperature of different sub- 

 stances varies in accordance with their 

 respective powers of radiation and con- 

 duction. 



Thus have these deductions been developed 

 by Dr. Wells, in a long series of experiments, 

 as conclusive as they are ingenious. His 

 admirable work is well worth being consulted 

 by every one who takes an interest in phy- 

 sical facts as a science, or by the mere prac- 

 tical horticulturist. His extensive operations 

 have enabled him to apply many useful 

 precautions to the cultivation and preserva- 



