( 33 ) 
Art. IV. — Description of a New Hygrometer^ made of the in-^ 
ternal membrane of the Arundo Phragmites"^ , By Alex- 
ander Adie, F. R. S. Edin. Communicated by the Author. 
Xn the winter of 1816, I made many trials of different sub-, 
stances, for the purpose of ascertaining their hygrometric powers, 
in order, if possible, to find one which should possess sufficient 
sensibility, and, at the same time, not be liable to change the 
extent of its contraction between the extremes of dryness and hu- 
midity. Among the various substances which I tried, those 
that changed their bulk in a considerable degree by a change of 
humidity were Rottenstone, Chalk, unbaked Clay made very 
thin, and Mountain Cork. Though, from the friable nature 
of the three first, it was found difficult to use them, yet I am 
of opinion that they may be advantageously employed in the 
construction of hygrometers, and there is reason to think that 
they will not be subject to any alteration in their scales* 
Charcoal, from its known durability, likewise presented itself 
as a proper substance for the above purpose ; and it was found, 
upon trial, to be sensibly hygrometric, although its range was 
very limited. 
Most of the above substances were formed into hollow cylin- 
ders, and cemented to the end of thermometer tubes ; and their 
expansibility was tried by filling the cylinder and tube with mer- 
cury, in the usual maimer. 
But the substance which was found to possess by far the 
most delicate sensibility, and extensive range, was the in- 
ternal membrane of the Arundo Phragmites. A small bag, 
made of this membrane, is attached to the lower end of a 
thermometer tube, so as to form, as it were, its bulb. It is 
then nearly filled with quicksilver, which rises and falls, in con- 
sequence of the contraction and dilatation of the membrane, by 
any change of moisture ; and these changes are indicated upon a 
scale attached to the tube, the zero of this scale marking abso- 
lute humidity, and the other extremity of the scale absolute dry- 
* This Paper was read before the Royal Society of Edinburgh, on the 1st of 
February 1819.* 
