nded cables to a single ultro-microscope line may be developed.

And now that I have explained that these fibers have such valuable
properties, it will no doubt be expected that I should perform some
feat with their aid which, up to the present time, has been considered
impossible, and this I intend to do.

Of all experiments, the one which has most excited my admiration is
the famous experiment of Cavendish, of which I have a full size model
before you. The object of this experiment is to weigh the earth by
comparing directly the force with which it attracts things with that
due to large masses of lead. As is shown by the model, any attraction
which these large balls exert on the small ones will tend to deflect
this 6 ft. beam in one direction, and then if the balls are reversed
in position, the deflection will be in the other direction. Now, when
it is considered how enormously greater the earth is than these balls,
it will be evident that the attraction due to them must be in
comparison excessively small. To make this evident, the enormous
apparatus you see had to be constructed, and then, using a fine
torsion wire, a perfectly certain but small effect was produced. The
experiment, however, could only be successfully carried out in cellars
and underground places, because changes of temperature produced
effects greater than those due to gravity.[2]

[Footnote 2: Dr. Lodge has been able, by an elaborate
arrangement of screens, to make this attraction just evident to
an audience.--C. V. B.]

Now I have in a hole in the wall an instrument no bigger than a
galvanometer, of which a model is on the table. The balls of the
Cavendish apparatus, weighing several hundredweight each, are replaced
by balls weighing 13/4 pounds only. The smaller balls of 13/4 pounds are
replaced by little weights of 15 grains each. The 6 foot beam is
replaced by one that will swing round freely in a tube three-quarters
of an inch in diameter. The beam is, of course, suspended by a quartz
fiber. With this microscopic apparatus, not only is the very feeble
attraction observable, but I can actually obtain an effect eighteen
times as great as that given by the apparatus of Cavendish, and what
is more important, the accuracy of observation is enormously
increased.

The light from a lamp passes through a telescope lens, and falls on
the mirror of the instrument. It is reflected back to the table, and
thence by a fixed mirror to the sca