454 MOLESWORTH’S POCKET-BOOK
Expansion of Steam—continued.
The pressure of the atmosphere is to be included in calcu-
l iting the expansion; it must therefore be deducted from the
results in high-pressure engines. In condensing engines a
deduction must be made lor imperfect vacuum; this will
amount to about 2* lbs. per square inch in well-proportioned
engines.
Expansion of Compound Engine?.
A = Area of the large cylinder.
a — „ >• small ditto.
I, = Length of stroke. . .
I — Distance travelled by piston before steam is cut off in
small cylinder.
A L
E = Rate of expansion = — •
The mean pressure of steam in lbs. per square inch may be
taken as if it acted upon the large cylinder only, and the
steam were cut off at —th of the stroke.
a __ A.
The best proportion of a to A = — V or a-----------— •
Bicst Proportion of Capacity of Cylinders and Point
for Cutting off Steam.
Best Point for Cutting Relative Capacity
No. of Times Steam of Small to Large
is to be Expanded. off Steam. Cylinder.
4 times. 5 „ 7 * 50 L I * 45 L ■ 41 L • 38 L • 35 L ■ 50 A • 45 A • 41 A • 38 A • 35 A
10 „ 12 „ •32 L •29L •32 A •29 A