124 Aqueous Vapour Atmosphere. 
appears to be a consequent rather than an 
agent in the formation and decomposition of 
clouds; or if a necessary agent it is equally 
so in the boiling of water or in the drying of 
piece-goods in a stove. 
General Observations. 
As the dew-point and temperature of the air approximate 
in proportion as we ascend, at some height they must be- 
come the same; and hence the cause why the upper regions 
of the atmosphere are so frequently clouded; also why the 
mossy summits of the mountains are generally moist. 
From occasional observations on the dew-point for 20 
years past, chiefly at Manchester, I have once observed 
it as high as 64°, once 63°, five times 62°, thrice 61°, 
and twenty times 60°, mostly in the months June, July 
and August. Its usual range is from 50 to 60° in those 
months. 
The following little Table may have its use perhaps : the 
numbers exhibit the drying power of the air, according 
as its temperature is elevated above the dew-point. The 
Table is derived from that of evaporation. (Memoirs, Vol. 
5, p. 585.) 
Temperature of the air above the dew-point. 
Dew- 
point.| 2° 42 6° 8° 10° 12° 14° 16° 18° 20° 
30°] 8 16 24 32 41 51 63 74 86 98 
30) 9.17 26°37 48 61 72;.85,.,99 114 
40 |10 22 33 45 57 71 85 100 117 135 
45 |12 24 37 51 66 S81 99 117 135 154 
60 [14 28 43 60 78 96 115 135 155 I78 
55 |16 33 51 69 88 109 131 155 I81 209 
60 (18 37 57 77 100 126 153 182 213 244 
