llom, 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
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 _Phaenomena_, seems to be the differing texture of
the parts of these bodies, each