ood
expansive action can be obtained by giving a suitable adjustment to it,
without employing an expansion valve at all. Diagrams taken from engines
worked in this manner show a very excellent result, and most of the modern
engines trust for their expansive working to the link motion and the
throttle valve.

[1] In 1838 I patented an arrangement of expansion valve, consisting of two
movable plates set upon the ordinary slide valve, and which might be drawn
together or asunder by means of a right and left handed screw passing
through both plates. The valve spindle was hollow, and a prolongation of
the screw passed up through it, and was armed on the top with a small
wheel, by means of which the plates might be adjusted while the engine was
at work. In 1839 I fitted an expansion valve in a steam vessel, consisting
of two plates, connected by a rod, and moved by tappets up against the
steam edges of the valve. In another steam vessel I fitted the same species
of valve, but the motion was not derived from tappets, but from a moving
part of the engine, though at the moderate speed at which these engines
worked I found tappets to operate well and make little noise. In 1837 I
employed, as an expansion valve, a rectangular throttle valve, accurately
fitting a bored out seat, in which it might be made to revolve, though it
did not revolve in working. This valve was moved by a pin in a pinion,
making two revolutions for every revolution of the engine, and the
configuration of the seat determined the amount of the expansion. In 1855 I
have again used expansion valves of this construction in engines making one
hundred revolutions per minute, and with perfectly satisfactory results.--
J.B.

CHAPTER IV.

MODES OF ESTIMATING THE POWER AND PERFORMANCE OF ENGINES AND BOILERS.

HORSES POWER.

209. _Q._--What do you understand by a horse power?

_A._--An amount of mechanical force that will raise 33,000 lbs. one foot
high in a minute. This standard was adopted by Mr. Watt, as the average
force exerted by the strongest London horses; the object of his
investigation being to enable him to determine the relation between the
power of a certain size of engine and the power of a horse, so that when it
was desired to supersede the use of horses by the erection of an engine, he
might, from the number of horses employed, determine the size of engine
that would be suitable for the work.

210. _Q._--Then when we talk of an engine o