Mr. de Ltrcr 
4 16 
iny subsequent labour. That experiment is now made with 
a sufficient degree of regularity ; and the more so, as it has- 
been executed by Mr. Haas, in one of his air-pumps, with 
Some of my whale-bone hygrometers, made by himself : and 
I shall now give its result. 
Experiments on Evaporation , in Air and in Vacua. 
The purpose of these experiments being to shew, that the 
effects produced by evaporation on the hygrometer and the 
manometer, are the same in the absence of air, as when they 
take place in air ; I shall begin here by referring to Mr. 
Nairne’s paper, in the Philosophical Transactions for 1777, 
in which it is proved by experiments, that Mr. Smeaton's 
pear -gage is not a true manometer, since it does not indicate 
the presence of steam, when some remains in an exhausted 
receiver ; but that it is the only true measure of the quantity 
of air itself remaining in that space. 
That property of the pear-gage, proceeds from the fifth 
law of the above theory of hygrology. When, in the com- 
mon temperature of the atmosphere, steam is unprotected by 
a sufficient mixture of air, it is totally decomposed by the 
pressure of the atmosphere, when this acts by the interposi- 
tion of some sort of piston, as is the column of quicksilver in 
the pear-gage : therefore no fluid can remain at the top of 
that instrument when filled with quicksilver, except the 
quantity of air which was contained in its capacity before 
the introduction of the quicksilver. 
When some lasting cause of formation of steam happens 
to be within the air-pump, and that the pump is long worked. 
