!58 DESCRIPTION OF 



A little below the centres of the seiiiicircles A and 

 B, (fig. 1.) two holes are made, precisely in the same 

 direction : one of these is intended to receive the screw 

 {\s;. S, and the other a gold pin, which is to project 

 four-tenths of an inch beyond the inside of the part A. 

 The pin is made rather smaller than tlie hole in the 

 axis of the ivory wheel, and is highly polished ; in 

 order that the motion of the wheel may be the less 

 impeded by friction. 



Two line threads, about fourteen inches long, are 

 passed together through tlie hole in the groove of the 

 wheel, and aie prevented from returning, by a knot 

 on the outside. To the ends of these threads, two 

 weights are attaclied, exactly siniilar, and just heavy 

 enough to keep the threads extended. 



One of the threads having been wound on its cir- 

 cumference, tl)e wheel is to be placed on the pin, 

 about the tenth of an inch from the side A, as in fig. 4. 

 Two ii'lass tubes, of a sufficient bore to admit the 

 free motion of the weights, are fixed m grooves, m 

 such a manner, that each thread should fall exactly 

 in the axis of the tube. The tubes are so long as nearly 

 to touch the ivory wheel. 



The beard of the Ooheeiia hooloo being prepared, as 

 for the circular hygrometer^ by cutting oflr' that part 

 which is useless, is inserted about the tenth of an inch, 

 in the projecting end of the axis of the wheel, and 

 confined by a small Avooden pin, which is to be bro- 

 1<en off close to the axis; the other end is placed in 

 the notch of tlie brass screw, before described, and 

 secured by means of the sliding ring. 



It is evident, that when the grass untwists, the 

 wheel will turn on the gold pin; and the thread, 

 which is wound about it, with the wei§;ht attached, 



