to driness and moisture, so that with one breathing upon it, I have made it untwist a whole bout, and the Index or Hand has shew'd or pointed to various divisions on the upper Face or Ring of the Instrument, according as it was carried neerer and neerer to the fire, or as the heat of the Sun increased upon it.
Other trials I have made with Gut-strings, but find them nothing neer so sensible, though they also may be so contriv'd as to exhibit the changes of the Air, as to driness and moisture, both by their stretching and shrinking in length, and also by their wreathing and unwreathing themselves; but these are nothing neer so exact or so tender, for their varying property will in a little time change very much. But there are several other Vegetable substances that are much more sensible then even this Beard of a wilde Oat; such I have found the Beard of the seed of Musk-grass, or Geranium moschatum, and those of other kinds of Cranes-bil seeds, and the like. But always the smaller the wreathing substance be, the more sensible is it of the mutations of the Air, a conjecture at the reason of which I shall by and by add.
The lower end of this wreath'd Cylinder being stuck upright in a little soft Wax, so that the bended part or Index of it lay horizontal, I have observ'd it always with moisture to unwreath it self from the East (For instance) by the South to the West, and so by the North to the East again, moving with the Sun (as we commonly say) and with heat and drouth to re-twist; and wreath it self the contrary way, namely, from the East, (for instance) by the North to the West, and so onwards.
The cause of all which Phænomena, seems to be the differing texture of the parts of these bodies, each of them (especially the Beard of a wilde Oat, and of Mosk-grass seed) seeming to have two kind of substances, one that is very porous, loose, and spongie, into which the watry steams of the Air may be very easily forced, which will be thereby swell'd and extended in its dimensions, just as we may observe all kind of Vegetable substance upon steeping in water to swell and grow bigger and longer. And a second that is more hard and close, into which the water can very little, or not at all penetrate, this therefore retaining always very neer the same dimensions, and the other stretching and shrinking, according as there is more or less moisture or water in its pores, by reason of the make and shape of the parts, the whole body must necessarily unwreath and wreath it self.
And upon this Principle, it is very easie to make several sorts of contrivances that should thus wreath and unwreath themselves, either by heat and cold, or by driness and moisture, or by any greater or less force, from whatever cause it proceed, whether from gravity or weight, or from wind which is motion of the Air, or from some springing body, or the like.
This, had I time, I should enlarge much more upon; for it seems to me to be the very first footstep of Sensation, and Animate motion, the most plain, simple, and obvious contrivance that Nature has made use of to produce a motion; next to that of Rarefaction and Condensation by heat
