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[ 51 ] on the higher ones. I shall add, that though my hy- grometer was exposed to the open air on the moun- tain, as it was in the plain, yet it was not so much insulated there, being tied to the box of my portable barometer The difference observed, however, is so considerable that, notwithstanding the concurrence of all these particularcauses. I cannot bot ascribe it in some measure to that general one which I have suspected, namely, that there is comparatively a less degree of humidity in the upper than in the lower parts of the atmosphere.

101. The obfcrvation of the 13th of September seems likewise to throw some light upon the phae- nomena of dew. We know that when the sky is cloudy, there is liulc or no dew, and it has likewise been observed from this very circumstance, that the air is not fo much cooled after fun-fet. The cause of these differences appears to n1e to be, that when there are no clouds in the air at fun-fer, or when they are dispersed the heat of the inferior air, and that which rises from the earth, dissipates itself into the superior regions, and then the vapours which are dispersed throughout the air condense and all down again in dew J but when the clouds are continued, and thus seperate the inferior from the superior air, they prevent this dissipation of the heat, and the va- pours remain suspended. And if the sky grows cloudy some hours after the fening of the fun, and after the heat has sensibly diminished in the inferior air, it encreases again in it; because the heat, which continues to rise out of the earth, is accumulated in the inferior air. This appears in the observation I am speaking of. The clouds having been sepa- H 2 rated