C 433 ] 
fcale, happens at the fame time to be the thinnefe 
of them ail. What connexion there may be be- 
tween thefe two circumftances, muft be determined 
by experience. But in the mean time it appears to me 
that if the fibres of the ivory are interwoven with each 
other i they will make fo much the lels refinance 
either to the being feparated or brought clofer to 
each other, in proportion as- the bundles of thefe 
fibres have a lefs degree of thicknefs. Whether 
this remark is of confequence, or not, we fhall at 
lead run no rifque in making thefe ivory pipes al- 
ways exadtly of the fame thicknefs. This indeed 
was my intention in thofe which I have made, but 
unfortunately I thought I fhould have been able 
to turn them upon cylinders of hard wood ; and 
found too late, that no accuracy could be expedted 
from this method- It was to remedy this incon- 
venience, that, in fpeaking of the manner of turn- 
ing this piece, I have recommended brafs cylinders 
(3 O'- .... 
67. The fame precaution is likewife necefiary to 
be taken, that we may be certain of giving to every 
pipe an equal degree of thicknefs throughout the 
whole of its circumference: a circumftance no lefs 
dfential than the former; fince I have obferved in 
thofe of my hygrometers whofe pipes have not an 
equal thicknefs, that they bend, more or lefs ac- 
cording to the degree of humor to which they are 
sxpoied. 
This is probably the principal caufe why thefe 
indruments do not always prelerve the feme, pro- 
portions to each other (58). For the pipes not bend- 
ing according to the fame law, there muft be an ir- 
regular 
