A VERY SENSIBLE HYGROMETER. 27 



lake, and, at the same time, leave the hygrometer iii 

 possession of its extent and sensibility. 



A BCD fig. 1. represents a piece of wood, about 

 fourteen inches long, three inches broad, and one 

 inch, and two-tenths thick : the upper part is cut 

 out, as in the figure, to the depth of two inches, 

 leaving the sides A and B, about three-tenths of an 

 inch thick : the wood, thus prepared, is morticed 

 into a .square board, which serves as its support. 



Fig. C. is an ivory wheel *, about an inch and two- 

 tenths diameter, and two-tenths of an inch broad at 

 the rim. A semicircular groove is made in the cir- 

 cumference, of such a depth, that the diameter of the 

 wheel, taken at the bottom of the groove, is one 

 inch. Through the axis, which projects on one side 

 four-tentlis of an inch, a hole is made, the size of a 

 comnion sewing needle ; and, on this, as a centre, the 

 wheel .should be carefully turned ; for, on the truth 

 of the wheel, the accuracy and sensibility of the in- 

 strument chiefly depend. From the bottom of the 

 groove, a small hole is made, obliquely through the 

 side of the wheel, to admit a fine thread. All the 

 superfluous ivory should be turned away, that the 

 wheel may be as light as possible. 



Fig. 3. represents a piece of brass wire, two inclics 

 long; on one end of which a screvr is made, an inch 

 and a half in length ; and, in the other, a notch is cut, 

 with a fine saw, to the depth of half an inch. This 

 part is tapered ofipj so that the notch, which is intend- 

 ed to hold the beard of grass, in the manner hereafter 

 described, may be closed, by means of a small brass 

 ring (a) which slides on the taper part of the wire. 



* In my first experiments I used awheel made of card paper, witJi 

 an axis of wood, which answered very well. 



