183 



sphere inversely as the specific gravities of the atmospheres, that is, 

 as 1 to '625 or as 8 to 5. Thus by ascending ahout 19,000 feet in 

 the atmosphere, the barometric pressure is found to be reduced one- 

 half; and consequently it would be necessary to ascend to about f of 

 19,000 feet, or upwards of 30,000 feet, to produce a corresponding 

 diminution of pressure in a vapour atmosphere, or to reduce the ten- 

 sion, say from 1 inch to \ an inch. 



Now let us compare this result with the observed facts. This is 

 done in the annexed Table I., in which the ratio of the tensions of 

 the vapour at heights extending to 20,000 feet with the surface 

 tension, as actually observed, is set down in juxtaposition with the 

 ratios that should hold good in an independent vapour atmosphere. 

 We here see that in reality the tension is reduced to one-half of what 

 it is at the earth's surface by an ascent of about 8000 feet, instead 

 of 30,000 feet, as the hypothesis of the independent vapour atmo- 

 sphere would require. 



TABLE I. 



Comparison of Tensions of Daltons Hypothesis with those actually 

 observed. 



The facts, as indicated by the long series of observations, of which 

 the Table contains an abstract, are altogether in accordance with the 

 results of my own observations ; but I have thought it better to rest 

 my conclusions on the facts observed by others. Further, the discre- 

 pancy between the observations and the hypothesis is so great, and 

 so constant with reference to all the localities, the Himalaya, En- 



