^6 DESCRIPTION OF 



the whole, was turned round, by means of the notch 

 on the outside, till the index, or coloured end of the 

 straw, pointed to Zero. 



To obtain extreme moisture, it was at first thought, 

 that the best mode would be to fill the box with 

 water ; but, on trial, this was found to disturb the in- 

 dex too much. The beard of grass was therefore 

 thoroughly wetted, with a fine hair pencil dipt in 

 water, and the number of revolutions, made by the 

 index, observed. Care was taken, to allow suflficient 

 time, for the water to have its utmost effect ; and, in 

 this manner, the hygrometer I have at present was 

 found to make three and a half revolutions, between 

 extreme dryness and extreme moisture. 



As some fixed mode of division is necessary, to ren- 

 der observations, made by different persons, intelli- 

 gible to others ; I imagine the scale of this hygrome- 

 ter to be divided into 1000 equal parts, commencing 

 at extreme dryness. 



Tlie sensibility of this instrument is very great. 

 The efiluvia of a finger, applied about a quarter of 

 an inch from the grass, causes the index imme- 

 diately to shift eighteen divisions ; and, on withdraw- 

 ing the finger, it instantly returns to its former situa- 

 tion. Gently breathing on it, at the distance of about 

 a foot, moves the iudtxjifty'sia: divisions. It is equal- 

 ly susceptible of the moisture of the atmosphere; 

 scarcely ever remaining steady, for any length of 

 time, in variable weather. 



Such was the construction I at first employed ; but 

 on applying it to use, so much difficulty occurred, in 

 ascertaining the number of revolutions made by the 

 index, that I was induced to attempt to give the 

 whole a more convenient form, and to adapt to it a 

 scale, such ^ might preclude the possibihty of mis- 



