ser a clear idea of the engine he
required. Savory was the first to suggest this comparison, but Watt knew
that horses differed in size and strength, and in order to be sure of a
safe standard for his engine power he experimented with big horses in
some London breweries, and after careful calculation and comparison he
fixed a horse-power at 32,000 lb., that is to say, that a horse could
lift that weight of water one foot above the ground in a minute for
eight hours per day. This standard has remained ever since, although it
is above the average of the power of the average horse, it is in favour
of the purchaser of an engine, as well as being capable of working more
than eight hours a day, or twenty hours if required.

44. _Question._--What is meant by "nominal horse-power"?

_Answer._--It is a rough and ready way of giving some idea of the power
of an engine or engines on the basis of the number of inches in the area
of the cylinder or cylinders, but when the process of taking the diagram
of the engine is gone through the term nominal is dropped, and indicated
horse-power is then expressed, because it was proved by actual
experiment and certainty.

45. _Question._--How is that performance accomplished?

_Answer._--In horizontal engines there are generally two gun-metal
screw-plugs on the top of the cylinder, one over each end and in front
of the piston; when a diagram is to be taken, these plugs are taken out
and other screws put in their places, to which a copper pipe is
attached; the screw plugs are 1 inch in diameter, also the copper pipes;
and exactly mid-way on the copper pipe is a small cylinder which moves
on a pivot, by means of a string with a turn round it. One end of the
string is fixed by a clip on the connecting rod, the other end anywhere
to keep the string tight, so that by the movement of the steam entering
the cylinder at either end, and the connecting rod working backwards and
forwards, the small cylinder is made to turn frontways and backways; and
within the small cylinder is another cylinder very much smaller; it has
a tiny piston within it, and as the steam presses on the little piston
at every stroke of the engine, a pencil from the outer cylinder is fixed
in a slot and marks the movements of the little piston on a roll of
prepared paper, slid over the inner cylinder for that purpose, the
pencil being kept up to the paper by means of a small steel spring. This
diagram on the paper cylinder,