Donovan — On a Comparable Hygrometer. 485 
as do those of Hooke, E-egnault, and others, nor until the index have 
passed rapidly though the degrees of moisture which existed in the 
open air previously to its exposure. The index then pauses, or passes 
slowly for a while, and shows the real existing state. The ab- 
sorption of water into the pores of a hard elastic substance cannot 
be instantaneous. 
The hygrometric range of the instrument may be assumed to con- 
sist of any number of degrees at pleasure, provided that the number be 
comprised within the turns which the gut line is capable of giving to 
the index. The gut line selected as the proper medium for this 
hygrometer, when subjected alternately to the two extremes, in my 
trials gave to the index ten rounds of a circle divided into 100 degrees. 
Neglecting a few redundant degrees, there are in its whole range 1000 
degrees between the two extremes. This number is therefore the 
denominator of a fraction of which the numerator is any number of 
degrees indicated by the revolutions and parts of a revolution of the 
index on any occasion. Thus, the gut line having been previously 
well dried by exsiccants, suppose the index to have moved twice round 
the dial, and then stopped, the fraction of saturation would be yS^^ 
or 0-2. Or if, after an absence, I find two spherules in the receiver, 
the index perhaps pointing to 40° besides, then the fraction of satura- 
tion would be 0-24. If the index go round ten times, the atmosphere 
is saturated or 10; but this is a rare occurrence, and never happened 
in my trials with this instrument unless by contrivance. So far of no- 
tation ; now as to the value of the degrees. 
A degree on this hygrometer represents the presence in its local at- 
mosphere of the one thousandth part of the moisture which would be 
necessary to produce saturation. When the h5^grometer, having been 
previously well dried, marks 100° (or better decimally 0-1) that is 
one round of the index on the dial, I know that the atmosphere 
contains one-tenth part of the water that would saturate it. When 
the hygrometer marks 0*5, the observer learns that he is in an 
atmosphere midway between the extremes of moisture and dryness. 
If the degree indicated be 0*84 it means that the index has gone 
round the graduated circle eight times and 40 degrees besides, in all 
0-84. Were the hygrometer to mark any degree much below O*],. 
it would announce an intolerably dry atmosphere, which, if permanent, 
would prove to human beings dangerously unwholesome or destructive 
to life. 
It need scarcely be observed that the foregoing degrees of dampness 
are convertible into degrees of dryness by subtracting them from unity, 
and taking the difference. 
As the agreement of two hygrometers, placed under the same 
circumstances, leads to a presumption of the correctness of both, and 
as comparability is desirable for many purposes of research, I made 
several experiments with a view of ascertaining if the hygrometer here 
described possessed that important quality. 
