on Evaporation. 407 
experiments on the formation of steam and its effects in our 
vessels, we should be ignorant of its functions in the atmo- 
sphere, if it were not for its property of producing moisture, 
by which we may trace it wherever it is, and determine its 
quantity. Here then a new field is open for experiments and 
observations ; since by connecting hygrometry with hygro- 
l°gy, the hygrometer is for us in the atmosphere, what the 
manometer is in close vessels. 
The particular experiments which I have to relate, have 
that connection in view ; as they will shew, that in a close 
vessel, either filled with air, or free from it, the product of 
evaporation affects, at the same time, the hygrometer and the 
manometer; the former by moisture, the latter by pression. 
But I must here take this as a fact, in order not to interrupt 
the course of the theory, which, from hygrology, leads to hy- 
grometry, by certain laws demonstrated in the fluid which is 
the cause of moisture. 
Of the Laws of Hygrometry. 
1. The science of hygrometry derives its origin from the 
cause whence proceeds, that the density of steam has different 
maxima, according to the temperature. I have shewn, in my 
last work on meteorology, that in the particular association of 
water and fire which produces steam, both of those ingredients 
retain the faculty of producing their respective equilibrium 
between the medium and bodies ; that also the particles of 
water retain a tendency of uniting together, which begins to 
be effectual at a certain distance : and that their union takes 
place when they come to a degree of proximity, by which 
3 G & 
