56 Mr. Daniell on Evaporation. 
in the receiver, prevent its universal diffusion. This never-failing 
cause is inequality of temperature. As in the small experiment 
we found that the degree of dryness was proportioned to the energy 
of the absorbent mass, and that the existing vapour was equally 
diffused between it and the exhaling surface; so, in the larger 
operations of nature, we shall find that the state of saturation is 
dependant upon the point of precipitation, and that the aqueous 
atmosphere is nearly uniform between it and the source of steam. 
Now, it is well understood that the temperature of the gaseous 
atmosphere in its natural state must decrease with its density as 
we ascend to its upper parts; so that a great degree of cold is at 
all times to be found within a very moderate distance from the 
surface of the waters. It is this low temperature which determines 
the tension of the aqueous atmosphere ; and it is evident that the 
evaporation which is thus caused at the base of the aérial fluid, 
must be accompanied by a simultaneous and equal precipitation 
above. What then becomes of the precipitated moisture? Let us 
endeavour to trace the order of this phenomena. We will first 
suppose a calm state of the atmosphere, a temperature of 80°, and 
the barometer at 30 at the surface of the earth. By a calm state of 
the atmosphere is here meant, one that is free from any lateral 
wind, and in which, the only currents being in an ascending and 
descending direction, evaporation would proceed at the rate ex- 
hibited in the first column of Mr. Dalton’s table. The dew-point 
at the surface of the earth is 64°, and this is determined by the 
temperature at the height of about 5000 feet, where the barometric 
column would maintain itself at 24 inches. The degree of satura- 
tion below would therefore be 600, and the amount of evaporation 
1-74 grains per minute from a surface of six inches diameter. 
This quantity we therefore suppose condensed at the height before 
named. But the state of saturation in the atmosphere, above this 
point of precipitation, is again diminished; for we may suppose the 
force of the vapour to be determined by a temperature of 31° at a 
height of 15,000 feet, where the barometer would stand about 16 
inches. ‘The force of evaporation would, therefore, be 1.71 grains 
per minute, at the full atmospheric pressure ; and this amount 
