[ 95 3 
I. ExtraSi of a Letter from Mr. William 
Arderon F. R. S. to Mr. Henry Baker, 
F. R. i S. with the Defer iption of an improved 
Hygroleope. 
Dear Sir , 
Read Feb. 2 7 r~B 'HE great Mr. Boyle has taken much 
>745- • Jp Pains to bring the Hygrometer to 
Perfection ; and Mr. Roger Pickering, one of your 
ingenious Members, has * lately made an Improve- 
ment to it: But, as the Inftrument I ufe differs from 
them both, I fhall beg Leave to deferibe it to you. 
Some Years ago I applied my Thoughts to confider 
the Nature of Hygrolcopes, and compared many dif- 
ferent Sorts together, in order to determine which I 
might employ with the greateft Certainty ; when none 
appeared to me to come nearer the Truth than that 
recommended by Mr. Boyle, of weighing a Piece of 
Sponge in a Pair of Gold Scales. But the Difficulty 
and Time, which I found, upon Trial, were requifite 
to adjuft the Weights, and difeover the true State of 
the Air, fet me upon contriving another Method, 
whereby at all Seafons I might perceive, by InfpeCtion 
only, the moft minute Alterations with refpeCt to 
Moifture orDrynefs; and the following Drawing will, 
I believe, fufficiently deferibe what I found moft ef- 
fectual for that Purpofe. 
* See Phil, Tran/, n. 473. 
N 
Tab. 
