On HYGROMETERS. st 
or in a parallel line with the bottom, over which it im- 
pends fifteen or twenty feet, and that without columns or 
even a fingle pillar for its fupport. This circumftance, 
together with the grand circular walk between the front 
of the rock and the fheet of water falling from the fum- 
mit, exhibits fo noble and fingular an appearance, that a 
{pectator cannot behold it without admiration and delight. 
N° V. 
Letter to Mr. Natrne, of London. 
Pafy, near Paris, Nov. 13th, 1780. 
SIR, 
ee 1, poe qualities hitherto fought in a hygrome- 
ter, or inftrument to difcover the degrees of 
moifture and drynefs in the air, feem to have been, an 
aptitude to receive humidity readily from a moift air, and 
to part with itas readily toadry air. Different fubftances 
have been found to poffefs more or lefs of this quality ; 
but when we fhall have found the fubftance that has it in 
the greateft perfeCtion, there will ftill remain fome uncer- 
tainty in the conclufions to be drawn from the degree 
fhown by the inftrument, arifing from the actual ftate of 
the inftrument itfelf as to heat and cold. Thus, if two 
bottles or veffels of glafs or metal being filled, the one with 
cold and the other with hot water, are brought into a room, 
the moifture of the air in the room will attach itfelf in 
quantities to the furface of the cold veffel, while if you 
actually wet the furface of the hot veffel, the moifture will 
immediately quit it, and be abforbed by the fame air. 
And thus ina fudden change of the air from cold to warm, 
the inftrument remaining longer cold may condenfe and 
abforb more moifture, and mark the air as having become 
G2 more 
