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[ 43 ] that of the town, I determined to observe at the same time the variations of the humor in the open air for a whole day together. There are doubtless many varieties in this respect; nor indeed shall I determine from this observation anything more than the state of the open air during one day, and in one parti- cular foot.

90. I made my observation the 13th of Septem- ber 1772, in a garden situated to the west of our lake, and only separated from it by another garden and some buildings. There I hung up a couple of hygrometers which I kept perfectly insulated, one of which had no other frame but aa scale fixt to its tube, and the other was in a frame whose opening at the height of the ivory tube was of considerable size. They were four feet and a half above the ground, and at the distance of t foot from each other. A piece of pasteboard about 12 inches in breadth, placed at a foot's distance from the hygrometer which was not mounted, was intended to shelter it from the fun Each hygrometer had a thermometer close to it, the ball of which was not in contact with any thing. 1 have proved in my work, that it is ne- cessary to keep this ball insulated, in order to observe the heat of the free air.

91. One of the hygrometers I made use of for this observation, was at 93, and the other at 96 1/2 in my room the night at before In order to correct this difference, which I should suppose to be in pro- portion to their height, I would always add about 1/1.to the height of that one which kept itself the lowest, that there might be no difference between them, but such as should be produced by the dif- G 2 ference