K K, in the upper Figure, represent the two Channels or Clefts opened, which reach
from the Bottom to the Angle C, all along the writhed Part, and are twisted round with
it, as at the Letters K K, &c. L L, &c. in both the Figures. These Channels are filled
up with a kind of spungy Substance.
PLATE XII. FIG. 3.
A transverse Section of the Wild-Oat Beard
ON cutting the twisted Part across, to examine its Pitb, with the Form and Disposition
of the Pores thereof, the Appearance was as A B C C E F.
K L represent the two Clefts or Channels, which as it were divide the Beard, its whole
Length, into two unequal Parts, they wind very oddly in the inward Part of the Writhe.
C C shew the Pores or Sap-Vessels running the long Way.
PLATE XII. FIG. 4.
An Hygrometer made with a Wild-Oat Beard
A A, B B, is a Kind of Box or Frame, the Top and Bottom Plates whereof are held
together only by four small Pillars, that a free Passage for the Air between them
may no ways be obstructed.
C is a small Hole in the Middle of the Under-Plate B B, into which Hole the Bx-
tom of the Oat-Beard is fixed, upright, with soft Bees-Wax, in the Manner of ab;?
while the Upper-End thereof passes through another Hole exactly opposite in the Top-
Plate A A A A.
On the Top of the Beard at e, a small and very light Index, f g, made of a thin Slip
of Reed or Cane, must be fastned with a Piece of sine Silk, or a Touch of hard Wax or
Glue.
This Instrument is so extremely sensible of the least Alteration in the Constitution of
the Air, as to Dryness or Moisture, and does so certainly twist or untwist itself in propor-
tion thereto, that it will frequently untwist, and thereby turn the Index a whole Round,
only by breathing on it, or twist and thereby turn it as much the contrary Way by letting
it approach the Fire, or placing it in the Sun-shine.
And because, in Times of great Dryness or Moisture, the Index f g, moves some-
times twice or thrice round, and may thereby make it difficult to form a right Judgment
of it, the following Contrivance has been employed with good Success, to know certainly
what Number of Revolutions have been made.
The Index f g being raised to some Distance above the Surface of the Plate A A,?
small Pin b, was fixed downwards pretty near the Middle of it, in such a manner that it
might almost touch the Surface of the Plate A A. And then another Pin being also
fixed in a convenient Part of the said Plate, whereon a small Piece of Paper, shaped like
the Figure i k, was placed, by making a Hole through its Center ; which Paper having a
convenient Number of Teeth, every Turn or Return of the Pin b moved its little indented
Circle a Tooth forwards or backwards ; whereby, as the Teeth were marked, it was?
to ascertain how many Revolutions the Index made.
This little Circle may be made of thin Pastboard, Vellum, or Parchment, as well as
Paper ; but great Care must be taken that it be exceeding light, and move very ea? up-
on the Pin, otherwise the whole Operation will be spoiled. The Box may be made of
Brass, Silver, Iron, Wood, or Ivory, and Degrees marked upon it as every one chuses : and
the Index may be contrived various Ways, to shew not only the Number of Revolu-
tions, but the minute Divisions of each.
BAPTISTA PORTA informs us, in his Book of Natural Magic, that some?
by Means of the Beard of a Wild-Oat, (which, to make it the more sorprizing, they
called the Leg of an Arabian Spider or an Egyptian Fly) used to make a small Index,
Cross, or the like, to move round, by putting a Drop of Water to it privately : though
they pretended it was in Obedience to certain Words they muttered.
Twisted Cord, Cat-Gut, and some other Things may be contrived to shew the Changes as
to Drought or Moisture in the Air, as well by stretching and shrinking, as by untwi?ing
and twisting : But these are not near so sensible or exact ; their varying Property also gra-
dually diminishes. The Beards of Geranium Moschatum, and also of some other Species
of Cranes-Bill, are at least as casily affected as that of the Wild-Oat : And it is farther
observable, that the smaller the writhing Substance the quicker its Sensibility of every little
Change.
