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92. The first circumstance in these observations that deserves to be noticed, is the difference in the sinking of the two hygrometers when they were exposed to the air, before the sun shone in the garden. They both of them fell considerably, but one of the two 7 degrees and a half less than the other. One of the causes of this disparity is probably in the instruments themselves, and is owing to their being differently affected by the action of the humor. There is a difference of the same kind observable in the thermometers, which are likewise more or less sensible to the impressions of the heat even when the bulk of their liquid is the same; this is to say, they are acted upon more or less quickly by the degree of heat which surrounds them, accourding to the thickness, or even according to the nature of the glass of which the ball is made. Consequently it is possible that the different thickness or porosity of the ivory may have had some influence on the going of the hygrometer in this observation (66 and 69).

93. But these differences in the ivory pipes must produce a much greater different in the sensibility of the hygrometers, than those of the glass balls can produce in the thermometers because it is much more difficult for the humor to penetrate the ivory, than for the heat to get through the glass. So that any encrease of the obstacles retards the introduction of the humor, much more than that of the heat; and consequently the difference of sensibility must be more difficult to be prevented in the hygrometers, than it is in the thermometers.

This slowness of the humor in pervading the bodies into which it insinuates itself, makes it a de-

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