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[ 52 ] rated for a while, at 10 o'clock there was some dew, and the hygrometer fell sensibly till eleven : but af- terwards the clouds closing again, the heat encreased, and the humidity evidently diminished.

102. I take it for granted here, that the most common and most plentiful dew proceeds from the air, and not from the earth, as some philosphers have imagined I should produce the proofs I have collected of this fact from a multitude of experi- ments, if it had not been done in an excellent paper, written by Professor le Roi, On the elevation and suspension of water in the air *. These phaenomena of the dew become very interesting examined with the help of the hygrometer, and joined to observations of the degrees of saturation of the air with respect to water, which have been so ingeniously imagined, and begun by the auror of this memoir. If this part of natural philosophy is ever cleared up, as I hope it will be, we shall be much indebted for it to the sagacity of this true philosopher.

103. I shall only mention one more observation I have endeavoured to make with my hygrometer, which ought not to be omitted as it is connected with the principles upon which the instrument is constructed. It has likewise a reference to medicine, in as much as one of the objects of that science, in its inquiries to preserve our health, to determine the effects of water at different degrees of heat upon our organs. Ivory being an animal substance, the effects produced upon it by water at different degrees

• Mem. de l'Ae. Des Sc. de Paris, for the year 1751.

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