189 



and the observations, as shown by the series of figures in Table I. 

 This diminution, it will be seen, takes place in all the series, approxi- 

 mately in a geometrical ratio, so that the density is reduced nearly in 

 an equal proportion for each 2000 feet of ascent, namely, from TOO 

 to '96, that is by T o, onDalton's hypothesis ; from I'OO to -84, that 

 is by y 1 ^-, according to Dr. Hooker ; and from I'OO to '82, that is 

 by -U^ according to Mr. Welsh. Now it follows, from an obvious 

 mathematical law, that the entire quantities of vapour in these dif- 

 ferent cases are inversely proportional to the constant reduction of 

 density ; so that the quantity on Dalton's hypothesis, which is that 

 represented by the observed tension at the surface, is to the quantity 

 according to Dr. Hooker, as sixteen to four, and to the quantity ac- 

 cording to Mr. Welsh, as eighteen to four, a result nearly identical 

 with the former. The subtraction of the observed tension of vapour 

 from the total barometrical pressure, in the hope of obtaining the 

 simple gaseous pressure, must consequently be denounced as an ab- 

 surdity ; and the barometrical pressure thus corrected, as it is called, 

 has no true meaning whatever. 



In conclusion, I would remark that the consideration of the small 

 quantity of vapour that is disseminated in the upper parts of the 

 atmosphere, shows us that inequalities of level on the earth's surface, 

 which are insignificant when viewed in relation to the dimensions of 

 the globe, become objects of the greatest importance in connexion 

 with the atmosphere which surrounds it. Three-fourths of the whole 

 mass of the air is within range of the influence of the highest moun- 

 tains ; one-half of the air and nearly nine-tenths of the vapour are 

 concentrated within about 19,000 feet of the sea-level, a height 

 which hardly exceeds the mean level of the crest of the Himalaya ; 

 while one-fourth of the air and one-half of the vapour are found be- 

 low a height of 8500 feet. Thus, mountains even of moderate mag- 

 nitude may produce important changes in very large masses of the 

 atmosphere, as regards their movements, their temperature, and their 

 hygrometric state ; and especially in those strata that contain the 

 great bulk of the watery vapour, and that have the greatest effect 

 therefore in determining the character of climate. 



VOL. XI. 



