56 On HYGROMETERS. 
Now what I would beg leave to recommend to you, is, 
that you would recolle, if you can, the fpecies of mahoga- 
ny of which you made my box, for you know there isa good 
deal of difference in woods that go under that name; or 
if that cannot be, that you would take a number of pieces 
of the clofeft and fineft grained mahogany that you can 
meet with, plane them to the thinnefs of about a line, and 
the width of about two inches acrofs the grain, and fix 
each of the pieces in fome inftrument that you can con- 
trive, which will permit them to contrac and dilate, and 
will fhow, in fenfible degrees, by a moveable hand upon 
a marked feale, the otherwife lefs fenfible quantities of fuch 
contraction and dilatation. If thefe inftruments are all 
kept in the fame place while making, and are graduated 
together while fubje& to the fame degrees of moifture or 
drynefs, I apprehend you will have fo many comparable 
hygrometers, which being fent into different countries, 
and continued there for fome time, will find and fhow 
there the mean of the different drynefs and moifture of the 
air of thofe countries, and that with much lefs trouble than 
by any hygrometer hitherto in ufe. 
With great efteem, 
Tam, dear es 
Your moft obedient, 
And moft humble fervant, 
B. FRANKLIN. 
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