CATALOGUE. — METEOROLOGY, EVAPORATl'ON. 



46r 



Comparison of ihe expansion of drj' Air, alnd Air saturated witli moisture, from Schmidt. 



Tlie Barometer being at 29.84. 



It is however very improbable that the expansion of 

 moist air in high temperatures was so great as Schmidt 

 makes it, and in the lower temperatures his experiments 

 agree witli Deluc and Dalton. Dalton says, that he found, 

 by numerous experiments, that the expansion of moist air 

 was exactly proportional to the effect of the elasticity of the 

 vapour. Still, however, the best experiments on the speci- 

 fic grayity of steam make it not more than -^^ as dense as 

 water, and the specific gravity of vapour at 212°, appears 

 from the experiments of Saussure, Schmidt, and Dalton to 

 be about -pLj, or about -^ at 50°, so that it is difficult to re- 

 concile these results with the opinion of the perfect inde- 

 pendence of vapour ; unless we suppose steam to be much 

 more expanded by heat than air. Pictet found the specific 

 gravity of pure steam, at the common temperature of the 

 atmosphere, about yj^j. 



Chi Evaporation. 



Dalton asserts, that the quantity of any liquid, that evapo- 

 rates in agiven time in the open air, is directly as the force of 

 vapour at the same temperature, deducting only the pressure 

 of the vapour of the same kind which is already in the at- 

 mosphere ; that the atmosphere does not contribute to pre- 

 serve liquidity, that it only retards evaporation a little, but 

 does not diminish its quantity ; that the evaporation of alco- 

 hol and ether requires no deduction for pressure. He found 

 the evaporation of a disc, 3 J inches in diameter, from ao to 

 45 grains in a minute, at 212'' ; in a high wind it would 

 probably have be«n 60 grains ; at 180°, from 18 to 22 ; at 

 152°, from 8 to 12 grains, being always proportional to the 

 •lasticity of steam at the given temperature. 



Hence he calculates a table for a disc of 6 inches, making 

 120,154, and 189 grains the least, mean, and greatest evapo- 

 ration in a minute, at 212° ; taking 35, 45, and 55 as the 

 evaporation, under similar circumstances, from his disc of 



3| inches. But it is much simpler and more convenient t» 

 estimate the depth of the water evaporated in a day, aiyl i* 

 happens that the column of mercury, equivalent to the elas- 

 ticity of the vapour, expresses accurately enough the mean 

 evaporation in 24 hours: for 45 X 60 X 24 := 64800 

 grains, or 256.6 cubic inches, which would make a cylin. 

 dcr 30.9 inches in height, on a base scinches in diameter' 

 and this differs only Jj from the height of the column of 

 rrtercury : we may therefore assume, that the mean daily 

 evaporation is equal to the tabular number expressing the 

 elasticity of the vapour ; sometimes exceeding it or falling 

 short of it about one fourth ; and we may readily allow for 

 the effect of the moisture of the atmosphere, by deducting 

 the number corresponding to the temperature of deposition. 



Mr. Dalton says, that the annual evaporation at Manches- 

 ter, from avessel kept full, was 44.4, from the ground 23.5 : 

 that the point of deposition for any time may be calculated 

 from the evaporation, when the temperature is given. In 

 July 1800 and 1301, the mean point of deposition was 53", 

 the highest 62°; in August 1800, the mean about 55°: 

 1801, 54.5°; in September 1301, the mean was 54°; in 

 December, the highest 44", the lowest is". 



We may also infer, that at Liverpool, where Dr. Dobson 

 found the annual evaporation from water 36.78 inches, » 

 that is, a tenth of an inch daily ; the mean temperature be- 

 ing, according to Dr. Dobson, 54°, but more probably 

 somewhat lower ; the mean temperature at which the air 

 began to deposit its moisture was about 7" lower than that 

 of the air; or considering the exposed situation of the vessel, 

 perhaps nor more than 6", so that the mean temperature of 

 deposition wag 47 or 48°. Mr. Dalton says, that the point 

 of saturation is generally trom I to 10° below the mean heat 

 of the 24 hours. 



Mr. Dalton found that ice lost, at 32", 33 grains in ami. 



