1820.] Mr. Dalton on Meteorology. 259 



stances, as is evident to any one, on inspecting the logarithmic 

 curve, or on considering that the increments of a geometrical 

 profession, are in proportion to the terms. Hence the reason 

 why rains are heavier in summer than winter, and in warm coun- 

 tries than in cold. 



We may now inquire into the cause why less rain falls in the 

 first six months of the year than in the last six months. The 

 whole quantity of water in the atmosphere in January is usually 

 about three inches, as appears from the dew point, which is then 

 about 32°. Now the force of vapour at that temperature is 0-2 

 of an inch of mercupy, which is equal to 2*8 or three inches of 

 water. The dew point in July is usually about 58° or 59°, cor- 

 responding to 0*6 of an inch of mercury, which is equal to seven 

 inches of water ; the difference is four inches of water, which 

 the atmosphere then contains more than in the former month. 

 Hence, supposing the usual intermixture of currents of air in 

 both the intervening periods to be the same, the rain ought to be 

 four inches less in the former period of the year than the average, 

 and four inches more in the latter period, making a difference 

 of eight inches between the two periods, which nearly accords 

 with the preceding observations. 



In the preceding estimations of the whole quantity of water 

 in the incumbent atmosphere of any place, I take for granted, 

 that an atmosphere of steam is blended with tiie general atmo- 

 sphere throughout, in the same vertical column, and subject to 

 the common law of rarefaction in ascending. This is a view of 

 the aqueous atmosphere, which no one seems to have enter- 

 tained but myself. I have been making experiments almost 

 annually on the subject since 1802, on the mountains in the 

 north of England, and particularly on Helvellyn. These experi- 

 ments have been materially facilitated of late years by masses of 

 snow, which have been found near the summit, in the month of 

 July ; but it has often happened that the cold springs of water 

 near the summit have been adequate. By one or other of these, 

 the dew-point of the air may be found at any required elevation 

 on the mountain, and the law by which it is regulated in the 

 ascent may be investigated. On some future occasion, I intend 

 to draw up a memoir on this subject. In the mean lime I may 

 observe, that all the phenomena concur in exhibiting the same 

 variationof density in the aqueous vapour atmosphere in its present 

 mixed state, as would no doubt be observed in an atmosphere of 

 pure steam of equal density. 



It 2 



