e area of the eduction pipe,
especially when the engine is worked expansively, and with a considerable
pressure of steam. In the case of ordinary condensing engines, however,
working with the usual pressure of from 4 to 8 lbs. above the atmosphere,
the area of the steam pipe is not less than a circular inch per horse
power. In such engines the diameter of the steam pipe may be found by the
following rule: divide the number of nominal horse power by 0.8 and extract
the square root of the quotient, which will be the internal diameter of the
steam pipe.

319. _Q._--Will you explain by what process of computation these
proportions are arrived at?

_A._--The size of the steam pipe is so regulated that there will be no
material disparity of pressure between the cylinder and boiler; and in
fixing the size of the eduction passage the same object is kept in view.
When the diameter of the cylinder and the velocity with which the piston
travels are known, it is easy to tell what the velocity of the steam in the
steam pipe will be; for if the area of the cylinder be 25 times greater
than that of the steam pipe, the steam in the steam pipe must travel 25
times faster than the piston, and the difference of pressure requisite to
produce this velocity of the steam can easily be ascertained, by finding
what height a column of steam must be to give that velocity, and what the
weight or pressure is of such a column. In practice, however, this
proportion is always exceeded from the condensation of steam in the pipe.

320. _Q._--If the relation you have mentioned subsist between the area of
the steam passages and the velocity of the piston, then the passages must
be larger when the piston travels very rapidly?

_A._--And they are so made. The area of the ports of locomotive engines is
usually so proportioned as to be from 1/10th to 1/8th the area of the
cylinder--in some cases even as much as 1/6th; and in all high speed
engines the ports should be very large, and the valve should have a good
deal of travel so as to open the port very quickly. The area of port which
it appears advisable to give to modern engines of every description, is
expressed by the following rule:--multiply the area of the cylinder in
square inches by the speed of the piston in feet per minute, and divide the
product by 4,000; the quotient is the area of each cylinder port in square
inches. This rule gives rather more than a square inch of port per nominal
horse pow