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[ 2 ] meter, the scales of Fahrenheit, Delisle Reaumur, &c.

3d. Constancy in the variations produced by the same differences of humidity.

2. I perceived, moreover, that it were to be wished, that the hygrometer should give a true indication of the relation between the real quantities of the humi- dity, or at least between their differences: but this last point I rather considered as a desirable degree of per- fection, than as an indispensable requisite; the essen- tial point being, that observers might understand each other, when mentioning degrees of humidity ; and this seemed to be sufficiently provided for by the abovementioned conditions.

3· Having thus planned to myself the work I had to go through, I first attended entirely to the first point, and laid aside all the others. This I again subdivided. I had soon perceived that I must begin by thinking much less of the hygrometer than of the different phænomena of humidity. For this pur­- pose it was necessary to find out a fixed state, either of bodies in general, or of same body in particular ; and this fixed state might either be extreme humi­- dity, or dryness, or any intermediate point­.

4· Knowing that the extremes in nature are com­- monly very difficult. and sometimes impossible to hit, I conceived at first greater hopes of intermediate degrees. But in vain did my imagination fatigue it­ self in a road, which I was forced to abandon.

5· I then came to the extremes, and that of abso­- lute dryness was the first I was induced to try. But having found no other way to procure it but by fire, and fire not producing it in all bodies which appearet do.