This so oddly constituted Vegetable substance, is first (that I have met with) taken notice of by _Baptista Porta_, in his _Natural Magick_, as a thing known to children and Juglers, and it has been call’d by some of those last named persons, the better to cover their cheat, the Legg of an _Arabian Spider_, or the Legg of an inchanted _Egyptian fly_, and has been used by them to make a small Index, Cross, or the like, to move round upon the wetting of it with a drop of Water, and muttering certain words. But the use that has been made of it, for the discovery of the various constitutions of the Air, as to driness and moistness, is incomparably beyond any other, for this it does to admiration: The manner of contriving it so, as to perform this great effect, is onely thus: Provide a good large Box of Ivory, about four Inches over, and of what depth you shall judge convenient (according to your intention of making use of one, two, three, or more of these small Beards, ordered in the manner which I shall by and by describe) let all the sides of this Box be turned of Basket-work (which here in _London_ is easily enough procur’d) full of holes, in the manner almost of a Lettice, the bigger, or more the holes are, the better, that so the Air may have the more free passage to the inclosed Beard, and may the more easily pass through the Instrument; it will be better yet, though not altogether so handsom, if insteed of the Basket-work on the sides of the Box, the bottom and top of the Box be join’d together onely with three or four small Pillars, after the manner represented 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.