^, 6 Mr. de Luc on 
go. This laft TABLE is the moft important, as it contains a 
ckvfs of hygrofcopes which pollefs in common the following 
firft requifites for an hygrometer ; ift, that they may indicate, 
without any illufion, both extreme drynefs and extreme ,moflure% 
2dly, that they move conftantly in the fame direction as moif- 
ture itfelf; 3dly, that they move always when moijiure changes. 
It ftiould feem as if the march of th e flip of horn taken length- 
wife, from its very decreafing progreffion, came very near that 
of the thin porcupine quill ; but, as I have faid, among the 
fteps of the latter there are accidental r degradations, and it 
fometimes has a final one ; and I have never obferved that dif- 
pofition in the former, which, in its laft fmall fteps, follows 
conftantly the motions of every other flip. 
94. The agreement of all the flips in this laft refpeft is a 
very effential circumftance in hygrometry, as it allures us, 
that we cannot miftake the cafes w’hen moifure is extreme in the 
atmofphere ; a very important point for difeovering the nature 
of many meteorological phaenomena. No flip will create decep- 
tion in that refped; while, on the contrary, every thread 
may deceive in dubious cafes, and even create great error, if, 
unknown to the obferver, it happened to be in the beginning 
of its elongation. There was, however, a queftion to be de- 
cided in that refped, namely, whether or not a great moifure 
in the medium was a caufe of alteration in the march 01 any 
hygrofeope , by producing in its fubftance a hidden irregular 
lengthening. T hat accidental queftion is anfwered in the nega- 
tive by all the hygrofcopes of both dalles : for, in refpect of the 
threads, inftead of lengthening luddenly in that period of moif- 
iure, they have then a retrograde motion, either continuing 
or only beginning ; and as for the flips, they, by lengthening 
in the fame period, only follow their former laws : the flips 
which 
