[ 434 ] 
regular change in their capacity, and confequently 
in the height of the mercury in the tubes. The 
differences of this kind which I have had occafion 
to obferve, are not indeed very confiderable ; but, 
however trifling the caufe of an imperfection may 
be, it is ftill ufeful to remove it ; were it only to 
aflift us in difcovering caufes of greater imperfections, 
by making their effeCts the more evident. 
68. But to make the ivory pipes keep ftraight, 
we muff attend to a circumflance (till more impor- 
tant; which is, that the texture of the ivory be the 
fame in the whole circumference of the pipe. There 
is alenfible difference in the organization of the ex- 
ternal, middle, ahd internal parts of the fame ele- 
phant’s tooth : nor is it impoffible that, befides this 
difference in the nature, and viflble arrangement of 
its fibres, there may be another arifing from their 
degree of ten lion ; fo that fome fibres may be more 
difpofed than others to relaxation, after the tooth 
has been cut to pieces. Suppofe then that any of thefe 
differences fhouid exifcina pipe, that is, if one of its 
fides fhouid be more porous, or of a weaker texture, 
than the other ; or if its fibres flhould be more diipofed 
to relaxation ; this pipe will take a bend, either for 
a conffancy, or for a time ; and the hygrometers in 
which it is ufed, will not of courfe agree with the 
others. We muff therefore endeavour to make thefe 
pipes with a part of the tooth that is homogeneous : 
that which I believe to be moft lo, within a certain 
extent, and which for that reafon I have advifed, is, 
the part which is between the center and the furface, 
and at fome inches diftance from the apex of the 
tooth (30). 
69. There 
