4^4 Afr. de Luc on 
92. There the porcupine quill thews no retrogr delation ; how- 
ever, confident with its tribe, it had fome in other experi- 
ments. Its laft fteps have the unfteadinefs of the ftationary 
date, and thereby are fubjed to anomalies. From the fame 
caufe, none of the other threads have exadly the fame fteps in 
any two experiments, though on the whole their march 
remains eftentially the fame. The march here given of the 
hair hygrometer comparatively with mine, is the mean refult 
of three experiments, with three different fets of inftruments ; 
one of the hair hygrometers that I have employed was fent 
to me by Mr. Paul, of Geneva, and its point of extreme 
moifture was determined in a fog. The fmall and change- 
able retrogradation of the thread of whalebone and of hair 
might have been overlooked, were it not for other threads in 
which the retrogradation begins before that period where the 
ftate of moifture is difficult to afeertain ; but from thefe threads , 
that phenomenon is placed in a clear light, which is refleded 
on the others. I have marked with an * the greateft elonga- 
tion of each of them, and with a f a point near which their 
elongation begins, and to which they return at laft. Thefe figns 
will guide the eye in the above table, which fhews clearly, 
that no thread can be trufted to for the hygrometer. 
Table 
