espondence" between vane and dial is easily
established. An advantage attaching to the use of a compass needle is that
the magnet repels the wrong end of the needle.

[Illustration: FIG. 161.--General arrangement of electric wind recorder.]

The brush and segments must be protected from he weather by a cover, either
attached to the segment platform or to the tube on which the vane is
mounted.

The spaces between the segments must be filled in flush with some
non-conducting material, such as fibre, vulcanite, or sealing-wax; and be
very slightly wider than the end of the contact arm, so that two segments
may not be in circuit simultaneously. In certain positions of the vane no
contact will be made, but, as the vane is motionless only when there is no
wind or none to speak of, this is a small matter.

XXIX. A STRENGTH-TESTING MACHINE.

The penny-in-the-slot strength-testing machine is popular among men and
boys, presumably because many of them like to show other people what their
muscles are capable of, and the opportunity of proving it on a graduated
dial is therefore tempting, especially if there be a possibility of
recovering the penny by an unusually good performance.

For the expenditure of quite a small number of pence, one may construct a
machine which will show fairly accurately what is the value of one's grip
and the twisting, power of the arms; and, even if inaccurate, will serve
for competitive purposes. The apparatus is very simple in principle,
consisting of but five pieces of wood, an ordinary spring balance
registering up to 40 lbs., and a couple of handles. The total cost is but a
couple of shillings at the outside.

Fig. 162 is a plan of the machine as used for grip measuring. The base is a
piece of deal 1 inch thick, 2 feet long, and 5-1/2 inches wide. The lever,
L, is pivoted at P, attached to a spring balance at Q, and subjected to the
pull of the hand at a point, R.

The pressure exerted at R is to that registered at Q as the distance PQ is
to the distance PR. As the spring balance will not record beyond 40 lbs.,
the ratio of PQ to PR may conveniently be made 5 to 1, as this will allow
for the performances of quite a strong man; but even if the ratio be
lowered to 4 to 1, few readers will stretch the balance to its limit.

The balance should preferably be of the type shown in Fig. 162, having an
indicator projecting at right angles to the scale through a slot, as this
can be very easily f