-POWER OF AN ENGINE

Multiply the internal diameter of the cylinder by itself and strike off
the last figure of the quotient. The diameter is

20" X 20"
20
____
400. The H.P. is 40.

The following rules will be found more professionally accurate from
an engineering standpoint, though the term "horse-power" is not now
generally employed.

_To find the Nominal Horse-power_.--For _non-condensing_ engines:
Multiply the square of the diameter of the cylinder in inches by 7 and
divide the product by 80. For _condensing_ engines: Multiply the square
of the diameter of the cylinder in inches by 7 and divide the product by
200.

_To find the Actual Horse-power_ of an engine, multiply the area of the
cylinder in square inches by the average effective pressure in pounds
per square inch, less 3 lb. per square inch as the frictional allowance,
and also by the speed of the piston in feet per minute, dividing the
product by 33,000, and the quotient will be the actual horse-power.

"SCALING" COPPER PLATES

To "scale" copper plates they may be put over a charcoal or coke fire
to slowly sublimate the quicksilver. Where possible, the fireplace of
a spare boiler can be utilised, using a thin red fire. After the entire
evaporation of the quicksilver the plates should be slowly cooled,
rubbed with hydrochloric acid, and put in a damp place overnight, then
rubbed with a solution of sal ammoniac and nitre in equal parts, and
again heated slowly over a red fire. They must not be allowed to get red
hot; the proper degree of heat is indicated by the gold scale rising
in blisters, when the plates should be taken from the fire and the gold
scraped off. Any part of the plate on which the gold has not blistered
should be again rubbed with the solution and fired. The gold scale
should be collected in a glass or earthen dish and covered with nitric
acid, till all the copper is dissolved, when the gold can be smelted in
the usual way; but after it is melted corrosive sublimate should be put
in the crucible till a blue flame ceases to be given off.

_A Second Method_

The simplest plan I know is to have a hole dug nine inches deep by about
the size of the plate to be scaled; place a brick at each corner, and
on each side, halfway between, get up a good fire; let it burn down to
strong embers, or use charcoal, then place the plate on three bars of
iron extending between the three pairs of bricks, have a strong solution
of