C 210 ] 
or precipitate the water contained in its fubdance 
or, on the contrary, to diffolve and Imbibe a greater 
quantity : but as it is by feparating the efFedts of na- 
tural caufes, that we are enabled to judge of thefe 
caufes, and from thence their effedls when again 
compounded 5 every attempt to afcertain the operations 
of a fimple caufe will have its value in the fearch 
into nature ; nor can we a priori determine the 
value of any new inftrument; for, if it fliould lead 
to a fingle difcovery, or even to afcertain a fingle 
fadt, this may again lead to others of great importance, 
of which we might have, either none, or an im- 
perfedt idea of before. For my own part, I have 
always looked on a thick fog, and the fweating, or 
condenfation of the water’s vapours upon the walls 
in the infide of buildings, to be the greatell marks 
of a moift atmofphere : whereas I have not always 
found the hygrometer aftedled at thefe times in 
the higheft degree. On the contrary at the clofe 
of a fine day, and the fall of the dew on the fud'- 
den approach of a frofl, I have found the hygro- 
meter more affedled by moifture than in fome of 
the preceeding cafes ; and flill more by a falling dew 
in the time of an hard froft. I juft mention thefe 
matters of hints for the enquiry of others ^ not having 
had length of time, fince 1 brought the inftrument 
to anfwer my iirtention, to mako’ any abfolute con- 
clu lions. 
I am forry I have been obliged to take fo much 
compafs, to defcribe and explain a very limple in- 
ftrument ; but as I meant at the fame time to give 
fome idea of what is to be expedled from it, I thought 
it 
