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[ 49 ] 97. I shall not attribute intirely to the same cause, the great difference observed between my hygrome- ters, when one was exposed to the fun, while the other stood in the shade. The immediate action of the solar rays, or of the luminous heat, produces a variety of effects which, as I have said before, do not appear to follow the same laws as those of dark heat. And if I may be allowed to propose a con- jecture upon this particular point, before fuller ex- periments have been made, it should seem, that the immediate action of the solar rays must occasion a greater evaporation than what is produced by dark heat, even when they ho!J the thermometer at the same height. But let the cause be as it will, we see by this experiment that in a section of air about a foot wide, through which the solar rays did not immediately pass, the action of the humor upon the hygrometer was 233 degrees greater than in the place round about ; though that of the heat upon the thermometers was only a degree and a quarter less; which leads us to conceive how many apparently small causes may contribute to produce sensible dif- ferences in the distribution of the discrete humor.

98. Another use to be made or these observations is, to compare them with those that I have made in the mountains of Sixt ; in order to form a better judg- ment of the proportion between the different de- grees of humidity, in the superior and inferior parts of the atmosphere My hygrometer, held in the shade upon the summit of Buet, rose to 132 1/2, and was not yet stationary. This is pretty nearly the greatest degree of dryness observed in the hy- grometer exposed to the fun in the garden ; while H the