s an ordinary hair--a sufficiently
familiar object, and one that is generally spoken of as if it were
rather fine. Much finer than this is the specimen of copper wire now
on the screen (Fig. 2), which I recently obtained from Messrs. Nalder
Brothers. It is only a little over one-thousandth of an inch in
diameter. Ordinary spun glass, a most beautiful material, is about
one-thousandth of an inch in diameter, and this would appear to be an
ideal torsion thread (Fig. 3). Owing to its fineness, its torsion
would be extremely small, and the more so because glass is more easily
deformed than metals. Owing to its very great strength, it can carry
heavier loads than would be expected of it. I imagine many physicists
must have turned to this material in their endeavor to find a really
delicate torsion thread. I have so turned only to be disappointed. It
has every good quality but one, and that is its imperfect elasticity.
For instance, a mirror hung by a piece of spun glass is casting an
image of a spot of light on the scale. If I turn the mirror, by means
of a fork, twice to the right, and then turn it back again, the light
does not come back to its old point of rest, but oscillates about a
point on one side, which, however, is slowly changing, so that it is
impossible to say what the point of rest really is. Further, if the
glass is twisted one way first and then the other way, the point of
rest moves in a manner which shows that it is not influenced by the
last deflection alone: the glass remembers what was done to it
previously. For this reason spun glass is quite unsuitable as a
torsion thread; it is impossible to say what the twist is at any time,
and therefore what is the force developed.

[Illustration: FIG. 4.]

So great has the difficulty been in finding a fine torsion thread that
the attempt has been given up, and in all the most exact instruments
silk has been used. The natural cocoon fibers, as shown on the screen
(Fig. 4), consist of two irregular lines gummed together, each about
one two-thousandth of an inch in diameter. These fibers must be
separated from one another and washed. Then each component will,
according to the experiment of Gray, carry nearly 60 grains before
breaking, and can be safely loaded with 15 grains. Silk is therefore
very strong, carrying at the rate of from 10 to 20 tons to the square
inch. It is further valuable in that its torsion is far less than that
of a fiber of the same size of met