418 
STEPHENSON’S PATENT 
and in summer is invisible only when the hygrometer is high; its appearance is, 
indeed, a tolerably correct indication of the hygrometric state of the atmosphere. 
When the damper is lowered, the steam instantly becomes visible from the want 
of hot air to absorb it, and it then issues from the chimney in dense white 
volumes. 
Smoke-box Doors.—A large door, 11, (Plates XC. and XCII.,) is made in the 
front plate of the smoke-box, for the purpose of affording access to the cylinders 
and the tubes ; there is a ledge fixed inside round the opening, against which the 
door is closely pressed by four finger nuts put upon screws fixed in the smoke-box 
plate, and passing through projecting lugs upon the door. There is also a small 
door, u, near the bottom of the smoke-box, for the purpose of clearing out the 
cinders and ashes that collect in it: both doors have to fit closely, that no air may 
enter at them to impair the draught. 
Feed Pumps.—The boiler is supplied with water by the two feed pumps K K, 
which are worked by the engine; their construction will be explained afterwards. 
One of them is sufficient to supply the necessary quantity of water for the boiler, 
and the other is thrown out of action; but two are furnished in the engine, in 
order that if one should fail from any accident, the other may take its place without 
any delay being caused. On forcing the water into the boiler, it checks the generation 
of steam by its coldness, and the effect of the engine is also diminished in consequence 
of the power required to work the pump ; for this reason the action of the feed-pump 
is generally suspended when the engine is ascending an inclination, and requires its 
greatest power, and the supply of water is made up by working both pumps on a 
succeeding level or descent:—this is an additional advantage in having duplicate 
pumps. A small cock, called the pet cock, is fixed in the pipe leading to the boiler from 
each pump, and a long handle, v, is fixed to it, extending to within reach of the engine- 
man, standing behind the engine; these cocks are opened occasionally to ascertain 
whether the pumps are working properly, when a stream of water should be forced 
out at each stroke *. 
* The pet cock was invented by Mr. George Stephenson, to obviate what was for some time an 
incurable defect in the feed pumps of locomotive engines,—the pumps could not be made to keep in 
action, as they were fixed close to the boiler, and hot water entered from the leaking of the valves, 
causing them to be filled with steam instead of water at each stroke; thus preventing them from 
forcing any water into the boiler, which could, therefore, be used only for a short time, whilst the 
water in it would last, the boiler having then to be emptied of steam and refilled. The cause of this 
stoppage of the action was at length discovered, and the pet cock applied, by opening which the steam 
in the pump was let out, and the action renewed; the name was given to it playfully, from its petting 
or coaxing, as it were, the pump to do its duty. It is not now required for this purpose, as the pump 
is separated from the boiler by a long tube with a valve at each end, and the hot water cannot get into it.