4 1 8 Mr. de Luc on 
proccfs be found by which known quantities of moifture may 
be fucceffively produced in the medium itfelf. The ufe of that 
procefs for hygrometry will be, as M. de Saussure has begun 
to do it, to obierve, on feme hygrometer , the fucceffive effects 
of thofe known quantities of moifture , from which may be 
formed a table of the correfpondence between the equal degrees 
of the fcale of the chofen inftrument, and the real quantities 
of moifture in the medium ; and that table will ferve to correct 
as well paflt as future obfervations made with that inftru- 
ment. Therefore it matters not what that hygrometer (hall be, 
provided it is convenient in other refpedts. Let us then exa- # 
mine which of the flaps pofikftes the mod effential properties of 
an hygrometer , fuch as fhould be in common ufe for compara- 
tive obfervations, and to which confequently future diicoveries 
in refpeef ot the real proportions between the quantities of 
moifture itfelf would be applied. 
97. Steadinefs is furely a firft requifite for fuch an inftru- 
ment; and in that refpedt no flip comes in competition 
with that of whalebone . That property was the firft motive 
of my choice ; and as an inftance of it I (hall only mention, 
that 1 have juft now plunged into water an inftrument of that 
fort, of above ten years (landing, which is come to its point 
of extreme moifture as if it had been fixed yefterday ; for, 
without regard to the diftance of obfervations, there may be 
between them a difference of fome tenths of a degree . Some 
other flips may be brought to a certain degree of fteadinefs by 
ftudying what is the -degree of Jtretch which they may bear; 
but that attention is not necefiary for the flip of whalebone : 
if, for inftance, when its point of extreme moifture has been 
fixed while it was Jir etched to a certain degree, that flretch is 
5 much 
