teresting to boys that a sketch is
given of a Prony Brake, which is the simplest form of the Dynamometer,
as these measuring machines are called.

[Illustration: _Fig. 133. Prony Brake_]

In the drawing (A) is the shaft, with a pulley (A'), which turns in the
direction of the arrow (B). C is a lever which may be of any length.
This has a block (C'), which fits on the pulley, and below the shaft,
and surrounding it, are blocks (D) held against the pulley by a chain
(E), the ends of the chain being attached to bolts (F) which pass
through the block (C') and lever (C).

Nuts (G) serve to draw the bolts upwardly and thus tighten the blocks
against the shaft. The free end of the lever has stops (H) above and
below, so as to limit its movement. Weights (I) are suspended from the
end of the lever.

[Illustration: _Fig. 134. Speed Indicator_]

THE TEST.--The test is made as follows: The shaft is set in motion, and
the nuts are tightened until its full power at the required speed is
balanced by the weight put on the platform.

The following calculation can then be made:

For our present purpose we shall assume that the diameter of the pulley
(A') is 4 inches; the length of the lever (C), 3 feet; the speed of the
shaft (A) and the pulley, 210 revolutions per minute; and the weight 600
pounds.

Now proceed as follows:

(1) Multiply the diameter of the pulley (A') (4 inches) by 3.1416, and
this will give the circumference 12.5664 inches; or, 1.0472 feet.

(2) Multiply this product (1.0472) by the revolutions per minute. 1.0472
x 210 = 219.912. This equals the _speed_ of the periphery of the pulley.

(3) The next step is to get the length of the lever (C) from the center
of the shaft (A) to the point from which the weights are suspended, and
divide this by one-half of the diameter of the pulley (A'). 36" / 2" =
18", or 1-1/2 feet. This is the _leverage_.

(4) Then multiply the _weight_ in pounds by the _leverage_. 600 x 1-1/2
= 900.

(5) Next multiply this product (900) by the _speed_, 900 x 219.912 =
197,920.8, which means _foot pounds_.

(6) As each horse-power has 33,000 foot pounds, the last product should
be divided by this figure, and we have 197,920.8 / 33,000 = 5.99 H. P.

THE FOOT MEASURE.--How long is a foot, and what is it determined by? It
is an arbitrary measure. The human foot is the basis of the measurement.
But what is the length of a man's foot? It varied in different countries
from 9 to 21 inches