VOL. XLIII.] FHILOSOPHICAL TRANSACTIONS. 35 



glass ; and, by lifting up the sash, they can be taken in, as occasion re- 

 quires. 



Of the Hygrometer. — ^Mr. P. had, for some time, used Dr. Hooke's hygro- 

 meter, made of the beard of a wild oat, set in a small box, with a dial-plate and 

 an index ; but he soon found an inconvenience, without the remedying of which 

 no dependence could be had on this machine ; viz. its making more than one 

 revolution in a night. He endeavoured to remedy this by the following method, 

 described in fig. 17, pi- 1. 



At the vertical point, from which moisture and dryness are graduated, a 

 small circle is described; the lower arch of which should just intersect with that 

 arch, round which the index of the oat described its circuit. In the centre of 

 this small circle is placed a pin, easily turning in the central cavity, and furnished 

 with a flat piece of thin ivory on its head. This piece of ivory, intersecting with 

 the index of the oat, by it was turned either to the moist or dry side of its gra- 

 duation, as the index made a double revolution. Mr. P. flattered himself with 

 success ; but soon found, in the great fogs in the winter, that the wild oat 

 is not a safe material to make an accurate hygrometer of. So he immediately 

 turned his thoughts to some other for the diary, and reserved this for his study ; 

 where, or in any inclosed place, it does well enough, and may be very useful. 



As a succedaneum to this, he thought on a statical one ; it recurring to his 

 mind, that the weight and moisture of the air being two properties of one and the 

 same body, a statical hygrometer promised the best assistance towards a more 

 complete knowledge of the barometer, which acts on statical principles ; and 

 that these two machines must have a reciprocal correspondence with each other. 

 Remembering that Mr. Boyle had mentioned something of this nature ; after con- 

 sulting whom, he made the following machine, acting on his principles, but 

 formed in a manner differing from his. 



He procured a balance to turn with half a grain, the axis of the balance drawn 

 out to the length of one inch, and its end furnished with a male screw, to which 

 a light index with a female screw might be fixed. He had this balance fastened 

 in a wainscot box, 1 1 inches in length, Q in diameter, and 4 in depth at top, 

 but gradually widening towards the bottom, with a back to slide up and down in 

 a groove. The axis of an inch length, came through a hole in the front of the 

 box, and then had the index fastened on, which described the segment of a 

 circle on a brass plate, silvered and graduated into 1 80°, as if it had consisted of 

 a perfect semicircle, or two quadrants. 



The beam turning with half a grain ; and every such turn, after repeated 

 trials, moving the index somewhat more than one degree of the 1 80 described 

 on the plate ; he made a weight of QO grains, which he fixed with a thread to 

 one brachium of the balance, without any scale, the several threads or silk strings 



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