LOCOMOTIVE ENGINE. 
425 
The priming is much increased when the water in the boiler is not clean or pure; 
and an engine that does not prime perceptibly with good water may prime very much 
if supplied with impure or hard water. When priming, the water is thrown out in a 
shower from the chimney at each blast, and the power of the engine is very much 
impaired, as the violence of the ebullition of the water is thereby greatly increased. 
The prevention of the priming was found to be a very great difficulty in the first 
locomotives, and several contrivances were made use of for the purpose; such as 
making the steam pass through a plate pierced with holes before entering the steam 
pipe, or dividing the dome by a plate over which the steam had to pass to the steam 
pipe. But it has been observed that the priming has been gradually diminished by 
increasing the steam room or space in the boiler occupied only by steam; for this 
renders the generating of the steam more uniform, as the abstraction of the successive 
cylinders full of steam is less felt and causes less agitation when the total quantity 
of steam is greater. In the first locomotives very little steam room was afforded, but 
in this engine the steam room is generally about 44 cubic feet, and there is no per 
ceptible priming under ordinary circumstances. 
Regulator.—In the box a (Plates XC. and XCII.,) is placed the 
regulator e, extending across the box so that all the steam must pass 
through it before it can enter the steam pipe S ; and by means of this 
the steam is either shut off or allowed to enter the steam pipe in greater 
or less quantities. The regulator is shewn in figs. 11, 12, and 13, 
to a scale of double the size of the engravings, or 1| inch to a foot. 
The box is made of cast iron half an inch thick, and has a plate, A, 
extending across it, with two openings BB, of nearly a quadrant each; the diameter of 
the opening is inches, and that of the solid part in the centre separating them, 2 
inches. The brass plate C, placed against this plate, fits 
exactly the space between the openings B B, and if turned 
round and placed over the openings would project beyond or 
overlap them three quarters of an inch on each side, as they 
are three quarters of an inch smaller than a complete quadrant. 
The spindle D passes through a boss in the centre of the plate 
C, turning at its end in a socket in the plate A, but without 
touching the bottom of the socket, and a small pin is fixed 
into the spindle fitting into a notch, E, in the centre boss, 
to enable the spindle to turn round the regulator plate C, 
and yet allow it to be withdrawn a little without draw 
ing the regulator plate away from the other plate. The 
plates are held together by the pressure of the steam; and by 
3 h