48 Mr. Daniell on Evaporation. 
object of the present paper, I shall proceed to the second case of 
evaporation. 
When the evaporating fluid is of a higher temperature than the 
surrounding air, but not so high as to emit vapour of equal elas- 
ticity to it, the exhalation is proportionate to the difference of tem- 
perature. The gaseous fluid, in contact with the surface, becomes 
lighter by the abstraction of portions of the excess of heat, and, 
rising up, carries with it, inits ascent, the entangled steam. This, 
as in the former case, is precipitated, and, in the form of cloud, 
exposed to the third species of evaporation. This process is not 
only proportioned to the difference of temperature, and the elasti- 
city of the vapour, but is also governed by the motion of the air. 
A current or wind tends to keep up that inequality of heat upon 
which it depends, and prevents that equalization which would 
gradually take place in a stagnant air. Such is the evaporation 
which often takes place in this climate, in Autumn, from rivers, 
lakes, and sea, and which is indicated by the fogs and mists 
which hang over their surfaces. 
It is, however, the third modification of circumstances, which 
is the most interesting in the point of view which I have suggested, 
and from which I have merely distinguished the preceding, to 
free the subject from ambiguity. When the temperature of water 
is below that of the atmosphere, it still exhales steam from its 
surface; but, in this case, the vapour, neither having the force 
necessary to displace the gaseous fluid, nor heat enough to 
cause a circulation, which would raise it in its course, is obliged 
to filter its way slowly through its interstices; and the nature of 
the resistance it meets with in this course is the first object of 
investigation. 
The force of vapour, at different temperatures, has been deter- 
mined with great accuracy, and the amount of evaporation has 
been shewn to be ceteris paribus, always in direct proportion to 
this force. The quantity is also known to depend upon the at- 
mospheric pressure, but I know of no experiments which esta~ 
blish the exact relation between the two powers. I attempted to 
elucidate the point as follows:— 
