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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: immediatte action of the solar rays, or of the luminous heat, produces a variety of effects which, as I havesaid before, do not appear to follow the same laws as those of dark heat. And if I may be allowed to propse a con- jecture upon this particular point, before fuller ex- periments have been made, it should seem, that the immediate action of rh e: 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 le t the cause be as it will, we see by this experiment that in a section of air about a goot wide, through which the solar rays did not immediately pass, the action of the humpor 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