sented in the 4. Figure of the 15. Scheme. Or, if you intend to make use of many of these small Beards join'd together, you may have a small long Case of Ivory, whose sides are turn'd of Basket-work, full of holes, which may be screw'd on to the underside of a broad Plate of Ivory, on the other side of which is to be made the divided Ring or Circle, to which divisions the pointing of the Hand or Index, which is moved by the conjoin'd Beard, may shew all the Minute variations of the Air.
There may be multitudes of other ways for contriving this small Instrument, so as to produce this effect, which any one may, according to his peculiar use, and the exigency of his present occasion, easily enough contrive and take, on which I shall not therefore insist. The whole manner of making any one of them is thus: Having your Box or frame A A B B, fitly adapted for the free passage of the Air through it, in the midst of the bottom B B B, you must have a very small hole C, into which the lower end of the Beard is to be fix'd, the upper end of which Beard a b, is to pass through a small hole of a Plate, or top A A, if you make use onely of a single one, and on the top of it e, is to be fix'd a small and very light Index f g, made of a very thin sliver of a Reed or Cane; but if you make use of two or more Beards, they must be fix'd and bound together, either with a very fine piece of Silk, or with a very small touch of hard Wax, or Glew, which is better, and the Index f g, is to be fix'd on the top of the second, third, or fourth in the same manner as on the single one.
Now, because that in every of these contrivances, the Index f g, will with some temperatures of Air, move two, three, or more times round, which without some other contrivance then this, will be difficult to distinguish, therefore I thought of this Expedient: The Index or Hand f g, being rais'd a pretty way above the surface of the Plate A A, fix in at a little distance from the middle of it a small Pin h, so as almost to touch the surface of the Plate A A, and then in any convenient place of the surface of the Plate, fix a small Pin, on which put on a small piece of Paper, or thin Past-board, Vellom, or Parchment, made of a convenient cize, and shap'd in the manner of that in the Figure express'd by i k, so that having a convenient number of teeth every turn or return of the Pin h, may move this small indented Circle, a tooth forward or backwards, by which means the teeth of the Circle, being mark'd, it will be thereby very easie to know certainly, how much variation any change of weather will make upon the small wreath'd body. In the making of this Secundary Circle of Vellom, or the like, great care is to be had, that it be made exceeding light, and to move very easily, for otherwise a small variation will spoil the whole operation. The Box may be made of Brass, Silver, Iron, or any other substance, if care be taken to make it open enough, to let the Air have a sufficiently free access to the Beard. The Index also may be various ways contrived, so as to shew both the number of the revolutions it makes, and the Minute divisions of each revolution.
I have made several trials and Instruments for discovering the driness and moisture of the Air with this little wreath'd body, and find it to vary exceeding sensibly with the least change in the constitution of the Air, as
