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regular change in their capacity, and consequently in the height of the mercury in the tubes. The differences of this kind which I have had occasion to observe, are not indeed very considerable but, however trifling the cause of an imperfection may be, it is still useful to remove it; were it only to assist us in discovering causes of greater imperfections, by making their effects the more evident.

68. But to make the ivory pipes keep straight, we must attend to a circumstance still more important; which, is that the texture of the ivory be the same in the whole circumference of the pipe. There is a sensible difference in the organization of the external, middle and internal parts of the same elephant's tooth : nor is it impossible that, besides this difference in the nature, and visible arrangement of its fibres, there may be another arising from their degree of tension ; so that some fibres may be more disposed that others to relaxation, after the tooth has been cut to pieces. Suppose then that any of these difference should existing a pipe, that is, if one of its sides should be more porous, or of a weaker texture, than the other ; or if its fibres should be more disposed to relaxation ; this pipe will take a bend, either for a constancy or for a time ; and the hygrometers in which it is used, will not of course agree with the others. We must therefore endeavor to make these pipes with a part of the tooth that is homogeneous: that which I believe to be most so, within a certain extent, and which for that reason I have advised is, the part which is between the center and the surface, and at some inches distance from the apex of the tooth (30).

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