Hygrometry . 4 ay* 
Cdmpenfation for the changes of heat by a thin hrafs lamina : 
this being fooner affedted than the g/afs rods , by fudden changes 
of heat , the index moves firft one way ; then it recoils com- 
pletely, by the change being operated later in the glafs rods . 
86. The experiments briefly related in this Paper will, I 
hope, be fufficient to anfwer the following queftion, which 
has been made to me by fome obfervers of the two principal 
hygrometers here compared : 44 Why does the hair hygrometer, 
44 when expofed to the open air in day time, come fo often 
44 near its point of extreme mojfture , while the whalebone hygro* 
« 4 meter almoft never comes within 30 degrees diftance of that 
44 point in fummer, and very feldom within 20 in winter, even 
44 in rainy weather (if preferved from rain)?” The anfwer, 
according to the refults of thefe experiments, is this : 66 The 
44 general march of the hair hygrometer is much decreafing , 
44 comparatively to equal increafes of moijlure ; that march for- 
46 wards, ends in a fiationary (late, and is followed by a fmall 
44 retrogradation ; while the whalebone hygrometer has con- 
44 ftantly ^march, if not prop or t ion al^ at leaft conftantly Jtmilar 
44 to that of moifture itfelf.” 
87. There remains an objedl of inquiry, which is, a deter- 
mination of thofe ratios here generally exprefled. I have ex- 
plained, in the firft part of this Paper, the difficulties of that 
object, and what help may be found in comparing the marches 
of hygrometers with the acquifiiions of weight of their fub- 
ftances, of which procefs 1 gave fome examples', and here I 
fhall relate fimilar experiments on hair, whalebone, ' box, and 
aloes-pitta. But as I have already defcribed the whole procefs, 
and the manner of calculation of that clafs of experiments, I 
(hall only give here the refults of thefe laft. 
88. I 
