Part of a Letter from Mr. Wm. Arderon, F. R. S. to Mr. Henry Baker, F. R. S. concerning an Improvement of the Weather-Cord

On Examination, I find the five Appearances to answer the five Stamina on which the Apices are set; and from this Appearance, growing nearer and nearer to each other by degrees, they join at last all in one in the Stalk of the Flower. XIII. Part of a Letter from Mr. Wm. Arderon, F. R. S. to Mr. Henry Baker, F. R. S. concerning an Improvement of the Weather-Cord. SIR, Read April 24, 1746. THE Weather-Cord is an Hygrometer of a very ancient Invention, and, if properly constructed, may be made use of with very good Success, to shew the various Alterations of the Atmosphere, in respect to Moisture and Dryness; but, as commonly made, it never rises or falls sufficiently to point out such minute Changes as the Curious would be desirous to know. A Sense of this Defect set me upon endeavouring to find out some Method of removing it; and how far I have succeeded, will best appear upon casting your Eye upon the Drawings, Tab. II. Fig. 4. and Fig. 5. In the first of these Hygrometers which I made, as in Fig. 1. I only fixed the End of the Index $AB$ fast to the Silk $CE$ at $A$, leaving it lying loose upon the Point $D$; and in this manner the other End of the Index would nearly describe the Arch $FGH$: But then I soon perceived, that the Centre of Motion, whereon the Index turned, was changed whenever it moved ever so little; and, consequently, that the Arch struck by the End $B$ must be irregular. On considering this, I toothed two Pieces of Brass, as 1, 2, and 3, 4 (See Fig. 5.) to fit each other so exactly, that, upon the least Motion of the one, the other would move; then, fixing the Index upon the Centre $C$, its Motions were rendered much more regular. I placed likewise a little Collar of Brass at $B$, upon the Cord $SR$, and to that Collar tied the Silk, which gave Motion to the Index, that the Cord $SR$ might twist and untwist without any Impediment. If there is no Weight placed at Bottom, as in Fig. 4 X the Piece of Brass 1, 2 must be so heavy as to keep the Cord $SBR$ at a convenient Tightness, and also to counterbalance the End of the Index $CE$, provided it be heavier than the other. The Length of the Cord $SBR$, its Thickness, and the Manner of preparing it, are already described in so many Books which treat of Hygromscopes, that to mention them would seem unnecessary; and I shall only add, that I am Norwich, Dec. 21, 1745. Yours, &c. Wm. Arderon.