212 GREENHOUSE CONSTRUCTION AND HEATING. 
farther, but one, placed preferably on the flow pipe, is 
sufficient, and if a second one is employed it should be 
placed in the cross-pipe, as shown at 8B, in order to force 
the current to pass round the longer way when required. 
Valves of several kinds are employed in hot-water fitting. 
The simplest and least expensive form is the ordinary 
throttle-valve (Fig. 153), which consists of a metal disc 
fitting the bore of the pipe exactly, and turning on a spindle 
passing through a stuffing-box at the top with a cross-handle 
outside. Being very simple in construction, and only 
roughly fitted, this form of valve is comparatively inex- 
pensive, and though not by any means ‘‘tight” it is quite 
sufficient to check the current of hot water which, in most 
Fia. 153. 
cases, is all that is required. An “improved” throttle-valve 
is now made, in which the disc and spindle are removable 
for repairs, etc., without disturbing the pipes at all; this is 
also more carefully fitted than the ordinary throttle-valve, 
Where a really “‘ tight” valve is required, either a ‘‘screw- 
down” or a ‘“‘sluice’”’ valve must be employed. In the 
former there is a circular opening in a diaphragm dividing 
the interior of a bulb-like enlargement on the short length of 
piping which forms the foundation of the valve. This 
opening is bored quite true, and is closed by a circular 
brass or gun-metal disc, turned to fit the opening exactly, 
and fitted on a spindle with a screw, stuffing-box, and a 
small wheel outside, as shown in Fig. 154. When screwed 
