Trials of Traction- Engines at Wolverhamf.ton. 535 
in its one stroke or half a revolution, and if this be doubled for 
the whole revolution, then we shall get 20,000 lbs. through 1 foot 
as the force exerted by the engine, and supposing the engine to be 
making 100 revolutions in a minute, then 2,000,000 lbs. through 
1 foot would be the force exerted by the piston of such an engine 
in a minute of time ; this, divided by 33,000, gives 60 and 
about two-thirds horse-power, as what is called the gross indicated 
horse-power of such an engine. Before, however, the available 
power that the crank-shaft will deliver for pumping, threshing, 
ploughing, or for other purposes can be considered, an allowance 
must be made off the gross indicated horse-power for the friction 
of the engine. The diagram shown in Fig. 2 is a mere elemen- 
tary diagram, one that hardly ever occurs in practice, and if it 
does, is only justified by the desire to obtain the greatest possible 
power out of a given size of engine without regard to the highest 
economy. It will be seen that it supposes steam to have been 
admitted during the whole length of the stroke, and that no 
advantage whatever has been taken of the expansive use of the 
steam. 
Diagram, Fig. 3, shows steam working expansively. 
Assume, that, as in the former case, the pressure of the steam 
has been 100 lbs. above the atmosphere, and has raised the pencil 
from " F^ " to " " — assume also that during half the length of the 
stroke of the engine that steam has been admitted to the cylinder, 
but at that point, viz. at " P"," all further inlet of steam has been 
stopped by the action of the slide, — the steam now in the cylinder 
begins to expand, and as it expands it loses pressure ; by the 
time, therefore, the piston has got from " P" " to " P,"' " the steam 
will have lost pressure, so that the pencil will gradually have 
dropped along the curved line " P " P''''^" and by the time the 
piston has got to the end of the stroke "Q\" the pressure will 
still further have diminished, say down to " pi"".'' 
An inspection of this diagram will show, that although only 
half the steam was admitted that was admitted in the case of 
diagram 2 ; the area of the diagram is very much more than half 
of that of Fig. 2 ; as a matter of fact, it is about "83 of that 
area, and thus a power of "83 has been obtained (by using the 
steam expansively) for half the fuel that was required in the case 
of Fig. 2. 
As a further illustration take Fig. 4, which is the diagram that 
would be made if the steam were cut off when the piston had 
moved one-fourth of the stroke. In this instance only one-fourth 
the steam would be required that was required for Fig. 2 ; but 
the total area of the diagram is about "54 of that of Fig. 2, so 
that "54 of the work is got for one-fourth of the steam. 
The diagrams given here are theoretical ones, which very 
