he velocity of the piston in feet per minute, and divide the
product by 6,000; the quotient is the number of nominal horses power. In
using this rule, however, it is necessary to adopt the speed of piston
prescribed by Mr. Watt, which varies with the length of the stroke. The
speed of piston with a 2 feet stroke is, according to his system, 160 per
minute; with a 2 ft. 6 in. stroke, 170; 3 ft., 180; 3 ft. 6 in., 189; 4
ft., 200; 5 ft., 215; 6 ft., 228; 7 ft., 245; 8 ft., 256 ft.

214. _Q._--Does not the speed of the piston increase with the length of the
stroke?

_A._--It does: the speed of the piston varies nearly as the cube root of
the length of the stroke.

215. _Q._--And may not therefore some multiple of the cube root of the
length of the stroke be substituted for the velocity of the piston in
determining the nominal power?

_A._--The substitution is quite practicable, and will accomplish some
simplification, as the speed of piston proper for the different lengths of
stroke cannot always be remembered. The rule for the nominal power of
condensing engines when thus arranged, will be as follows: multiply the
square of the diameter of the cylinder in inches by the cube root of the
stroke in feet, and divide the product by 47; the quotient is the number of
nominal horses power of the engine, supposing it to be of the ordinary
condensing description. This rule assumes the existence of a uniform
effective pressure upon the piston of 7 lbs. per square inch; Mr. Watt
estimated the effective pressure upon the piston of his 4 horse power
engines at 6-8 lbs. per square inch, and the pressure increased slightly
with the power, and became 6.94 lbs. per square inch in engines of 100
horse power; but it appears to be more convenient to take a uniform
pressure of 7 lbs. for all powers. Small engines, indeed, are somewhat less
effective in proportion than large ones, but the difference can be made up
by slightly increasing the pressure in the boiler; and small boilers will
bear such an increase without inconvenience.

216. _Q._--How do you ascertain the power of high pressure engines?

_A._--The actual power is readily ascertained by the indicator, by the same
process by which the actual power of low pressure engines is ascertained.
The friction of a locomotive engine when unloaded is found by experiment to
be about 1 lb. per square inch on the surface of the pistons, and the
additional friction caused by any additional resis