206 
OF THE PARTS OF 
[sect. VII. 
lanced by the reaction of the sides. With less taper the valve has a tendency to 
set fast: with greater it occupies more space. When the conical valve exceeds five 
or six inches in diameter, it requires great power to open it against the pressure of 
the steam, and is therefore inconvenient. Mr. Watt applied a piston to the stem 
of the valve, fitted to a cylinder of the same diameter as the valve, on the opposite 
side of the passage; and the steam acting on the valve and piston equally, the 
difficulty of raising it was much reduced. 
When the valve is to be self-acting, that is, to move as soon as its narrower 
surface is exposed to a given pressure, then the weight of the valve must be equal 
to the square of the diameter multiplied by the pressure in lbs. on a circular inch. 
440. A valve is sometimes made with the seat a portion of a sphere, and the 
valve either a portion or a complete sphere to fit it. This species, under the name 
of a cup valve, has been strongly recommended for safety valves ; and by suspend 
ing the weight below the valve, it is expected in a steam vessel to be constantly in 
motion, so as to prevent sticking. See U, Plate xvn. Fig. 1. In other respects 
the cup valve seems to be inferior to the conical valve. 
441. Hornblower’s valve. A common valve must often have to be opened 
against a pressure depending on its surface : to avoid this, a valve on a different 
principle was invented by Hornblower. This valve, Fig. 4. Plate vi. is inclosed in 
a box, and consists of a short cylinder resting on two conical seats, one on the ex 
terior of the cylinder, and the other an interior seat at the bottom of it. The valve 
is raised or depressed by the usual methods applied to the cross bar at the top, and 
it is guided by the rod which slides in a socket in the lower seat. If there be 
strong steam on the upper side of the valve, and light vapour below, the pressure 
tending to keep the valve close is exerted only on the horizontal areas of the two 
seats, instead of being distributed over the whole surface of the valve. 1 
This reduction of pressing surface is obviously considerable in large valves. The 
principal passage for the steam is very direct; and at the lower seat, the steam in 
its passage going chiefly down through the body of the valve, it is interrupted only 
by the cross bar at the top. 
442. Improved form for Hornblower’s valve. The obvious difficulty of the 
valve is to make it fit steam-tight on two seats; but if we make the outside of the 
cylinder to slide in a stuffing box, or in an elastic packing of metal (see V, Fig. 1. 
Plate vi.) that difficulty is removed, and the largest valves may be made with no 
other resistance to being opened, than the pressure on the seat, and the friction 
of the surface of the cylinder. It is simply the common conical valve inverted, 
1 Professor Robison saw the theoretical advantages of this construction : but why has the account 
he gave of it been omitted in the reprint of his works ?