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the reach of our instruments. A fall of the temperature may 
bring on first cloud and then rain ; a rise of the dewpoint may do 
the same ; the wet-bulb making no move in the first instance, the 
dry-bulb making no move in the second instance. On the other 
hand a rise of the temperature or a fall of the dewpoint may have 
the contrary effect,—if rain is falling causing it to cease, and the 
clonds to disperse. Or to take another case, the air may be 
nearly saturated with moisture causing mist or fog, the temper- 
ature of the air and dewpoint being the same, but no rain falling 
so long as the wind is steady to one point; then let the wind 
shift a little, a mixture of airs of different temperatures ensues, 
and the fog turns to rain. 
The same considerations help us also to explain another circum- 
stance connected with weather phenomena, and that is the 
occurrence of very unequal falls of rain at different times during 
changeable weather, without any proportionate difference in the 
temperature of the air, the temperature of the dewpoint, or even 
in some cases the height of the barometer. Our meteorological 
instruments being ordinarily fixed at a very inconsiderable height 
above the ground, the indications of the wet-and dry-bulb 
thermometers relate of course only to that stratum of air in which 
they are placed. The barometer indeed tells us the whole weight 
of the atmospheric column above our heads; but the height of 
the mercury is determined mainly by the weight of the air alone, 
the additional height due to the elasticity of the vapour contained 
in the air being comparatively small. Consequently, though there 
may be a large increase of vapour at any time, this would not 
show itself by its effect on the barometer to the same extent that 
an increase in the volume of the air itself would. 
The meteorological registers kept at the Literary Institution 
supply us with many instances serving to illustrate what has been 
said above. 
Thus two rainy days sometimes occur in the same month, and 
standing almost next to one another, with scarce any difference 
