IM- PROVEMENTS IN STEAM ENGINES
Year: 1859

page_text: LETTERS PATENT to William Marshall, of Leith Walk, in the County
of Mid Lothian, North Britain, Engineer, for the Invention of "IM-
PROVEMENTS IN STEAM ENGINES."
Sealed the 14th June 1859, and dated the 26th April 1859.
PROVISIONAL SPECIFICATION left by the said William Marshall at the
Office of the Commissioners of Patents, with his Petition, on the 26th
April 1859.
I, WILLIAM MARSHALL, of Leith Walk, in the County of Mid Lothian,
North Britain, Engineer, do hereby declare the nature of the said Invention
of "IMPROVEMENTS IN STEAM ENGINES," to be as follows, that is to say:-
This Invention relates to the arrangement and construction of steam
engines of what are known as Woolf's or the double cylinder expansive class.
In these engines the high-pressure or smaller cylinder is placed inside the low-
pressure or larger cylinder, which latter is arranged to work on the "annular".
principle introduced some years ago by Mr. Maudslay, of London. Such

page_text: engines are suited for all general purposes, but as adapted for marine purposes,
as for a screw steamer, for example, they consist in each case of a pair of large
inverted low-pressure steam cylinders, supported upon suitable framing upon
a bottom sole plate. The new arrangement may indeed be judiciously carried
out on the general basis of the arrangement of "Improvements in Steam
Engines," for which I have already obtained a grant of British Letters Patent,
bearing date on or about the Nineteenth day of November, One thousand eight
hundred and fifty-eight, No. 2625. Each of these large low-pressure cylinders
is fitted with a top and bottom end cover, both of which are checked out
internally to receive the two ends of the small high-pressure cylinder, which is
set exactly concentric with the larger cylinder, and entirely within it. The
large cylinder is turned out truly on its interior, and the small one is both
turned and bored. In this way, when the small cylinder is fitted in its place,
the central solid piston of the smaller cylinder works within that cylinder
in the ordinary way, whilst an annular piston fitted with packing both inside
and out works in the annular space comprehended between the exterior of the
small cylinder and the interior of the large one. The smaller piston is fitted
with one central piston rod working downwards through a central stuffing box
in the bottom cylinder cover, whilst the larger annular piston is fitted with
two or more piston rods, which also work through suitable stuffing boxes in
the bottom cover of the cylinder, all the piston rods being connected to one
crosshead, from which a connecting rod passes down to the crank shaft
beneath. Or, instead of this arrangement, the high and low pressure pistons
may be arranged to work independently and in reverse directions by using
separate connecting rods and cranks for each. The steam and exhaust valves
for both cylinders may be fitted up in various ways, but it is preferred to use
a single valve in each case. The valve has both a central cup or recess, and
an intermediate thoroughfare or through passage between this cup and the
extreme back of the valve shell. This valve works upon a vertical face on
the cylinder with five parts in it. The two end ports, as they are alternately
uncovered by the slide valve, admit the high-pressure steam to the two ends
of the small high-pressure steam cylinder. The exhaust high-pressure steam
escapes from this cylinder by the long intermediate thoroughfare in the body
of the valve, and passes thence by two other ports in the cylinder face into
the two ends of the larger low-pressure cylinder; and the final exhaust from
the low-pressure cylinder takes place through the central cup of the slide
valve down a central cylinder face part to the condenser. Instead of this
arrangement, separate valves may be used, but this single compound valve is

page_text: preferred. Engines of this kind set with their cylinders on the sole plate will
answer also for paddle-wheel steamers, and they may further be variously
arranged for stationary purposes. The arrangement secures peculiar com-
pactness, and as the annular steam cylinders act as steam jackets to the high-
pressure cylinders, they will work most economically.
The essence of this Invention is the disposition of the smaller high-pressure
cylinders within the larger low-pressure cylinders, but the reverse arrangement
may also be adopted.