iding valves and steam ports, and those
with conical valves and seats, of which the latter kind are the best. The
former kind have for the most part consisted of a circular valve and face,
with radial apertures, the valve resembling the outstretched wings of a
butterfly, and being made to revolve on its central pivot by connecting
links between its outer edges, or by its central spindle. In some of
Stephenson's engines the regulator consists of a slide valve covering a
port on the top of the valve chests. A rod passes from this valve through
the smoke box below the boiler, and by means of a lever parallel to the
starting lever, is brought up to the engineer's reach. Cocks were at first
used as regulators, but were given up, as they were found liable to stick
fast. A gridiron slide valve has been used by Stephenson, which consists of
a perforated square moving upon a face with an equal number of holes. This
plan of a valve gives, with a small movement, a large area of opening. In
Bury's engines a sort of conical plug is used, which is withdrawn by
turning the handle in front of the fire box: a spiral grove of a very large
pitch is made in the valve spindle, in which fits a pin fixed to the
boiler, and by turning the spindle an end motion is given to it, which
either shuts or opens the steam passage according to the direction in which
it is turned. The best regulator would probably be a valve of the
equilibrium description, such as is used in the Cornish engine: there would
be no friction in such a regulator, and it could be opened or shut with a
small amount of force. Such valves, indeed, are now sometimes employed for
regulators in locomotives.

CHAPTER VIII.

CONSTRUCTIVE DETAILS OF ENGINES.

PUMPING ENGINES.

422. _Q._--Will you explain the course of procedure in the erection of a
pumping engine, such as Boulton and Watt introduced into Cornwall?

_A._--The best instructions on this subject are those of Mr. Watt himself,
which are as follows:--Having fixed on the proper situation of the pump in
the pit, from its centre measure out the distance to the centre of the
cylinder, from which set off all the other dimensions of the house,
including the thickness of the walls, and dig out the whole of the included
ground to the depth of the bottom of the cellar, so that the bottom of the
cylinder may stand on a level with the natural ground of the place, or
lower, if convenient, for the less the height of the hou