528 
Trials of Traclion-Eiigines at IVolverhampton. 
improper to resist that pressure, or even a tithe of that pressure, 
than the flat chambers. But Hancock had this happy idea: 
he raised bosses (hemispheres) all over the surface of his cham- 
bers, and the summits of the bosses of one chamber bore upon 
the summits of the bosses of the neighbouring chambers through- 
out the boiler, and thus each chamber, by means of its bosses, 
served as the abutment for the chamber on either side of it, and 
they served as the abutments for it. The final pressure, that of 
the outside chambers, was taken by two thick wrought-iron plates 
against the flat surfaces of which the bosses of the outer chambers 
bore. Outside the plates were girders which projected beyond 
the ends of the chambers so as to admit of tie-bolts passing from 
the girders of one plate to the girders of the other, thus holding 
together the two plates containing between them the chambers 
of which the boiler was composed. There were also two other 
bolts — one low down and the other high up — which passed 
through the whole of the chambers. Washers were introduced, 
so as to leave an annular space round the bolts, and this annular 
space formed at the bottom the water connection, and at the top 
the steam connection. The boilers thus constructed were made 
of iron not more than one-eighth of an inch thick, and they were 
absolutely safe at 100 lbs. They were placed immediately over 
a fire which played up between the chambers, the heat being 
compelled to pass up among the tortuous channels left between 
the bosses. These boilers were most rapid generators of steam, 
and they also were very free from " priming." This arose, how- 
ever, very greatly from the fact of Hancock having soon disco- 
covered that the best way to prevent a boiler from " priming " 
is to keep the steam at a high pressure in it, and it was with this 
object that he used an extremely small steam-pipe to supply his 
engines. 
The engines of Hancock's steam-coaches were two-cylinder 
direct-acting inverted, communicating motion to the crank-shaft, 
which was at the bottom. On this crank-shaft there was a pulley 
shaped to take an ordinary chain, which chain worked another 
pulley on the axle of the driving-wheels. The chain-pulley upon 
the crank-shaft was provided with a clutch, so that the engine 
was thrown out of gear with the driving-wheels when it was 
required to run either for pumping or for blowing the fire, which 
was urged by a fan-blast. The driving-wheels were loose upon 
the axle, and were worked by clutches having a large amount of 
clearance. By this means, when easy curves were to be made 
on the road, one wheel was enabled to overrun the other to allow 
for the difference of travel between the wheel on the inner and 
that on the outer side of the circle. To make a very sharp turn, 
the clutch of the wheel that was intended to be on the inner side 
