i? 
233 
Vapour and Humidity.—There is always more or less vapour 
diffused through the atmosphere, though it varies much, not 
merely in quantity but in the mode of its distribution. Some- 
times the vapour prevails most in the higher or cloud regions, 
sometimes in the strata of air just above the earth. Either the 
higher or the lower strata may be in a state of saturation, while 
the other strata are comparatively dry, the two opposite conditions 
of moist and dry passing into each other gradually. 
The presence and the amount of this vapour have such a 
marked influence upon weather and climate, that it is quite of 
equal importance to determine its variations, as it is to determine 
the variations of atmospheric pressure and temperature. Hence 
the dry-and wet-bulb thermometer, or some other form of 
hygrometer, though much less frequently attended to, is as 
necessary an instrument in a meteorological observatory as the 
barometer or thermometer. Without combining observations of 
the former instrument with those of the two latter, we are apt to 
mistake the indications of these last, especially when they run 
counter to commonly received notions. Persons frequently express 
their surprise at rain falling with a high barometer, or at the 
absence of rain with a low barometer. Or they see changes of 
weather taking place without either barometer or thermometer 
showing any particular sign ; even the wind perhaps remaining in 
the same quarter as indicated by the vane. In these cases they over- 
look the important part played by vapour, with its own separate 
influence upon the weather, and forget how necessary it is to 
know what is going on above our heads as well as near about us ; 
the barometer no doubt revealing the former to some extent, but 
its variations liable to be counteracted by the intermingling of 
opposite currents above, while the current next the earth remains 
unchanged. 
Thus persons rise in the morning and find it wet, the rain 
coming steadily down, yet the barometer high, and just where it 
was the night previous ; the thermometer also not very different. 
