METEOROLOGY. 



169 



:'. .\ 



ouit. 



the sun's direct influence, and sheltered as much as pos- 

 sible from The results obtained in this way, 

 appear from the following facts to coincide nearly with 

 those of experiment* made to ascertain the actu . 

 {Miration from the earth. 



Rraport- a vessel .situated as above described, we found 



tion fran the annual evaporation in latitude 55^, for the last two 



bciteicd yean, to be as follows : 



fot- 1817 . . . 20.991 inche* 



1818 . . . 20.056 



being a mean < The amount from September 



\ugiMtI8: >.">. The annual 



evaporation, therefore, may lie stated at 20.6 inche*. 



, .:n the experiment* institute I l>y Mr. Dalton in 



rfice of 1796, and which have been detailed under the article 



the ground. EVAPORATION, it was found that the quantity evaporat- 

 ed from the - n an average of three yean, 

 amounts to 25.158 inches. There u reason to believe, 

 however, from certain circumstances in the mode of 

 _r the experiments, that this quantity is ra- 

 ther above than below the truth ; but even admitting 

 that there was no source of fallacy in the observation*, 

 the result, making allowance for difference of latitude, 

 which may be about S*, coincides very nearly with 



Both coin- those given above. Both results approximate to the 



UiMBrtT quantities deduced by Mr. Anderson from the law of 

 aqueous solution. By the table of evaporation former- 



lv a!lui!--d tn, tlic <jii intiry c.>rr.--;)iiiiii n_' tol.il: tn !'.">. 



i 22.35, a: greater in 



the one case, and .62 less in the other than what i* 

 found by actual observation. 



iay perhap* appear at first sight, that the near 

 approximation to equality between the quantity of rain 

 and that of evaporation, which some or the preceding 

 y^*_ remark* seem to imply, i* inconsistent with the un 

 sally received opinion respecting the or 



1 1 t'u- c\ ;ipi>rrit :<m lx- ; - ^'. t t ' .r ti . (!' | t u i ; i ' -i , > 



small a* ha* been stated, it may be alleged that there i* 

 not a sufficient supply of water to keep permanent 

 springs and rivers continually flowing. It U to be re- 

 membered, however, that the quantity of rain that falls 

 on mountains, and elevated tract* of country, greatly 

 exceed* what i* deposited in plains, or what would fall 

 if there were no mountains ; and that it is chiefly from 

 the latter that springs and rivers derive their supply. 

 Mr. Dalton ha* calculated, that the quantity of water 

 M th annually discharged into the sea by all the rivers in 

 England and Wales, amount* only to about 13 inches 

 of rain over the whole surface ; and when it i* oonei- 



< I'TL 1 - 1 t : . f thr .% :*'T ' irji MM 'ii ': ! ''i ; it.in - \ rrv BJBJaJh 

 ly arrive* at rocks, which, from their stratified position, 

 convey it with little waste to the point where it is to 

 issue in a spring, there seems to be little difficult v in 

 conceiving, that the superabundant moisture which 

 falls there is fully adequate to the supply . There i* no 

 doubt, in all such cahilation* a* those of Mr. Dalton, a 

 vagueness and want of precision ; but hi* estimate of the 

 quantity of water annually discharged by all the river* in 

 sTngJanrt, i* just a* likely to be above a* below the truth. 

 MM Another phenomenon, intimately connected with the 

 retw. subject now under consideration, and which ha* of late 

 occupied no small share of the attention of philoso- 

 phers, is the formation of rfr. From a very early pe- 

 riod, k ha* bean abaarved that dew is always most co- 

 pious during clear and calm night* ; and an idea seems 

 also to have long prevailed, that it is accompanied with 

 a reduction of temperature. 'I he late Dr. Wells, how- 

 ever, was unquestionably the first person who propo- 

 sed a distinct and satisfactory theory upon the subject. 



VOL. XIV, FAIIT I. 





(.by 

 r. Well*. 



That gentleman has shown, by a great variety of well- Meteoro/o. 

 conducted and accurate experiments that dew is never 

 formed upon any Mil^t.mce till the temperature of that S ""*""" 

 substance has been reduced below the temperature of 

 tha surrounding atmosphere-; and has consequently 

 proved, that the cold which accompanies dew. precedes 

 instead of following the prmluction of that fluid. This 

 indeed might have been interred from the well-known 

 . that vapour, in passing from the aeriform to the 

 fluid state, gives out instead of absorbing caloric; 

 and the formation of dew, therefore, so tar from increas- 

 ing must diminish the cold of contiguous bodies. Hut 

 Dr. U'. '.Is ha* also proved, in a manner equally satisfac- 

 tory, that the surface of the ground, as well as other 

 bodies, has it* temperature reduced below that of the 

 atmosphere, by being freely exposed to the clear sky, 

 and that whatever screen* h from this expos ir 

 vents the reduction of temperature. Thus he found, 

 that on clear and calm nights a thermometer, in contact 

 with the grass, Frequently stood from 7 to 12, and on 

 one occasion 15 n lower tlian another situated 4 feet above 

 the surface ; and of two thermometers which were both 

 in contact with the grass, one being freely exposed, and 

 the other sheltered by a screen of pasteboard, the form- 

 er sometime* indicated a temperature 10 below the 

 latter. In cloudy weather, and particularly when ti 

 was wind, he seldom observed much diller. .11 



the temperature of the grass and that of the air : but, in 

 such weather, a clear interval seldom failed to produce 

 a great reduction of temperature on the surface of tin 

 ground, while a passing cloud over a clear sky gene- 

 rally raised the thermometer on the grass several de- 

 grees. Dr. Wells observed farther, that when bodies, 

 which had been equally exposed to the air, were exa- 

 mined at the same time, those which had suffered th 

 greatest reduction of temperature had also collected 

 the greatest quantity of dew; and that substances 

 > had been exposed to the sky were uniformly more 

 dewed than those which had been screened. In proof 

 of this, we shall give the result of one exj>erhncnt 

 among many that might be produced, and which ap- 

 pear* to be perfectly satisfactory. 



" I Ixsnt a sheet of pasteboard," says Dr. Wells, Eitay 

 on Dew, p. 15, 1st edition, " into the shape of a house 

 roof, making the angle of flexure 90 degrees, and 

 leaving both ends open. This was placed one evening 

 with its ridge uppermost upon the same grass pi 

 the il -in- wind, as well as tliis could be as- 



certained. I then laid 10 grains of wool on tin- mid- 

 die of that part of the grass which wassheltered t>y t!i< 

 roof, and the same quantity on another part of the grass 

 plat fully exposed to the sky. In the morn in.:. 

 sheltered wool was found to have increased in v 

 only g grains, but that which had been exposed ' 

 sky 16 grains. 



From these experiments, repeated in a great variety Th . 

 of ways, Dr. Wells deduced the following tl,. dur ,V'* 



production of dew. On the common! ' and Dr.' 



well-established fact, that bodies have a tendency to from hi. . 

 throw off caloric by radiation to other bodies, whose 

 trm]>erature is lower, he supposes that the earth is con- 

 tinually radiating its heat to the high nnd coldrr re- 

 gions of the atmosphere; that, in the day time, tl: 

 fects of this radiation are not sensible, being more than 

 counterbalanced l>y the greater influx of heat fron> 

 direct influence of the sun ; but that during the i 

 when the counteracting cause is removed, these effects 

 become sen 1 produce the rcduct :per- 



ature above stated, unless where clouds interpose, whirl 



