170 



METEOROLOGY. 



Meteorolo- 

 gy- 



The opi- 

 nion that 

 dew rises 

 from the 

 ground re- 

 futed. 



Theory of 

 Dr. Wells 

 objected to 

 by Mr. 



I.eilie. 



operate as a screen in arresting the rays of heat. Dr. 

 Wells seems to think, that the interposition of clouds 

 operates not only as a screen in preventing radiation 

 from the earth, but by actually radiating heat back to 

 the ground, and thus restoring a portion of what had 

 been formerly abstracted. This, we apprehend, may 

 possibly take place when the clouds are low and 

 dense, to which indeed Dr. Wells's remark chiefly ap- 

 plies ; but, in general, their elevation is such, that their 

 temperature must be lo.wer than even the reduced tem- 

 perature of the ground. But however this may be, 

 the cold produced by radiation to a clear sky is suffi- 

 cient to account for all the phenomena of dew. We 

 have already seen, that the mean point of deposition in 

 the atmosphere is about 6 below its mean minimum 

 temperature, and that below this point it is no longer 

 capable of holding its moisture in a state of solution. 

 When the ground, therefore, is cooled down from 7 to 

 12* below the atmosphere, the portion of the latter in 

 contact with the surface will in general be reduced be- 

 low the point of deposition, and must consequently part 

 with a portion of its humidity, the quantity deposited 

 being proportional to the reduction of temperature, and 

 the previous state of the air with regard to moisture. 

 It appears accordingly from Dr. Wells's experiments, 

 what might be inferred front the theory, that though of 

 two similar substances examined at the same time, the 

 coldest was always the most dewed, it did not uniform- 

 ly happen, that on different nights, or even at different 

 times of the same night, the quantity of dew was pro- 

 portional to the reduction of temperature alone. " Thus 

 during two nights," says Dr. Wells, p. 43, " on which 

 grass was 12 or 14 colder than the air, there was little 

 dew ; while on the night which afforded the most copi- 

 ous dew ever observed by me, the cold possessed by 

 the grass beyond that of the air was for the most part 

 only 3 or 4." It is deeply to be regretted, that in 

 none of his experiments did Dr. Wells employ Leslie's 

 hygrometer, the state of which, at the time of his obser- 

 vations, could not have failed to give them a precision 

 which they can never otherwise possess. 



The opinion which has sometimes been entertained, 

 that dew is entirely produced by vapour emitted from 

 the earth and vegetable substances, has been satisfac- 

 torily refuted by Dr. Wells, at least in so far as he has 

 demonstrated, that a great proportion of it is deposited 

 from the atmosphere. He has also examined at consi- 

 derable length, some of the objections that may be 

 urged against his theory, and has thus been led to dis- 

 cuss various collateral subjects of considerable interest. 

 But for these we must refer to the treatise itself. We 

 would only observe, that the principle which he has as- 

 sumed as the foundation of his theory, affords a satis- 

 factory explanation of all the phenomena accompanying 

 the production of dew, and that none of the apparent 

 anomalies hitherto observed are sufficient of themselves 

 to set that principle aside. It is in the adoption of 

 this principle indeed, that the chief merit cf Dr. Wells 

 consists, as many of the facts which be states had been 

 previously made known by the experiments of Mr. Six 

 of Canterbury, and Dr. Wilson of Glasgow. 



When the Essay on Den first made its appearance, 

 it excited considerable interest in the scientific world, 

 and for some time the accuracy of the theory there ad- 

 vanced seemed to be generally admitted. Of late, how- 

 ever, a very formidable attack has been made on the fun- 

 damental principle of that theory, and from a quarter 

 which renders it particulary deserving of notice. Mr. 

 Leslie,.after having assigned to Dr. Wells"the merit of be- 



mgthefirstwhodistinetly attributed the formation of dew Meteorol*- 

 to the previous cold induced on the ground from the as- KJ- 

 pect of the sky," and admitted that " this cooling of V "V"*' 

 substances from exposure is at least one great source of 

 dew," proceeds to express his regret, " that the expli- 

 cation of this primary phenomenon should have been 

 sought for from the very loose, cumbrous, and vision- 

 ary hypothesis, concerning what is gratuitously called 

 radiant heat ;" an hypothesis which he considers as " re- 

 pugnant to all the principles of sound philosophy," and 

 as " obtaining favour only from the blind admiration 

 which the multitude are prone to entertain for whatever 

 lulls the reasoning faculty, and appears cloudy and mys- 

 terious." The principle which Mr. Leslie proposes to 

 substitute for radiant heat, in explaining the phenome- 

 na observed by Dr. Wells, is that of cold pulsations 

 from the higher regions ; but to do justice to the hy- 

 pothesis, it will be necessary to state it in the author's 

 own words. After explaining the diffusion of heat Leslie'i 

 through the atmosphere, in a manner similar to the theory, 

 propagation of sound, he thus proceeds: " But the 

 same pulsatory system will enable the atmosphere to 

 transmit likewise the impressions of cold. The shell of 

 air adjacent to a frigid surface becoming suddenly chill- 

 ed, suffers a corresponding contraction, which must ex- 

 cite a concatenated train of pulsations. This contrac- 

 tion is followed by an immediate expansion, which 

 withdraws a portion of heat from the next succeeding 

 shell, itself now in the act of contracting ; and the tide 

 of apparent cold, or rather of deficient heat, shoots for- 

 wards with diffusive sweep." 



Now it must appear obvious, we think, that upon More ob- 

 this principle the frigorific pulses which Mr. Leslie sup- jectionabla 

 poses are showered down from the higher regions, never th tbat 

 can communicate to the earth a greater degree of cold ' 



than they had previously communicated to the atmo- 

 sphere. When the last pulsation reaches the surface of 

 the ground, it will deposit its load of cold, if we may 

 use such an expression, or abstract from the earth a 

 quantity of caloric proportional to its reduction of tem- 

 perature below that of the earth ; but if the latter be 

 the colder of the two, instead of abstracting, the shell of 

 air will deposit heat, and tend to bring the ground to 

 the same temperature with itself. The moment that 

 the lowest film, or indefinitely small wave of air, conies 

 in contact with a colder surface, it is precisely in the 

 same circumstances with the highest one where the 

 pulsation commenced ; and should the temperature of 

 the ground, therefore, be reduced by any means below 

 that of the atmosphere, a concatenated train of pulsa- 

 tions ought, upon Mr. Leslie's principles, to commence 

 at the surface of the earth, and proceed upwards till 

 they arrive at a still colder region. Nor will it at all 

 remove this objection, to say, that the pulsations from 

 above more than counteract those from below, in con- 

 sequence of the former being at a lower temperature 

 than the latter. Admitting that the descending wave 

 is the colder of the two, and not only arrests the pro- 

 gress of the other upwards, but actually propels it back 

 again towards the earth, it still remains to be explained 

 how the surface of the ground comes to have its tem- 

 perature reduced below that of the body by which it is 

 cooled, as was found to be the case by Dr. Wells and 

 other observers. According to Mr. Leslie's theory, the 

 atmosphere is not merely the medium by which the cold 

 pulsations are conveyed, without having its own tem- 

 perature affected by them. These pulsations are, in 

 fact, the contractions which take place in the atmo- 

 sphere itself, in consequence of the successive abstrac- 



