THE MERSEY TUNNEL. 375 
gradients of 1 in 30 are necessary in order to provide a sufficient distance 
(the average thickness being 40 feet, and the minimum 33 feet) between 
the bed of the river and the crown of the tunnel. There are four pump- 
ing engines, two on the Liverpool and two on the Birkenhead side, of the 
horizontal type known as compound differential, invented by Mr. Henry 
Davey, and constructed by Messrs. Hathorn, Davey, and Co., of Leeds. 
At Liverpool the large engine is capable of raising 288,000 gallons per 
hour, and the other 96,000 gallons, whilst at Birkenhead the large engine 
will raise 234,000 gallons, and the small one 96,000, making a total from 
both sides of 17,136,000 gallons per day. The largest quantity of water 
met with has been at Birkenhead, 180,000 gallons, and at Liverpool 
210,000 gallons per hour. 
The dimensions of these engines, and of the pumps connected with 
them, are given in the following table :-— 
Diameter Diameter 
—_— of high- of low- Stroke of |Diameter of} Stroke of 
Rene pressure pressure engine pumps pumps 
eylinder cylinder 
eo ae aa 
in. in. ft. | in. ft. 
: 1 f 33 60 10 30 10 
alpha Ut Pe s0 35 Gy tthan 5 
: 33 | 60 10 | 30 8 
Birkenhead . e { | 20 | 35 6 20 5 
The chief peculiarity of the differential engine is that it is capable of 
working with a high grade of expansion, without the controlling action 
of a crank or fly-wheel. The water-column and spear-rods constitute a 
reciprocating mass performing the function of a fly-wheel, and enabling 
an eight- or a ten-fold expansion to be employed. An analysis of the 
function of the reciprocating mass appears in the abstract of a paper 
by Mr. Davey, read at the Swansea meeting, and printed in the Re- 
port of this Association. The term ‘ differential’ is applied to the en- 
gine, kecause of its peculiar valve-gearing, in which the engine-motion 
is communicated to its own steam-valves through the medium of a 
‘floating’ lever, having no fixed fulcrum, but made to move by inde- 
pendent mechanism in the direction required for opening the valves, 
whilst the engine-motion is imparted to the same lever in the direction 
for closing them. The resultant of the two antagonistic motions is that 
actually imparted. The independent motion is adjustable, and is ren- 
dered uniform, so that any increase in the velocity of the engine-motion 
causes the valves to close earlier than they otherwise would. There is, 
therefore, a peculiar element of safety in this engine. On three occasions 
the engine has suddenly lost its load, and on two of these the valve-gear 
thas saved the machinery from injury by interposing a cushion of steam, 
although the force has been sufficient to shift the engine-bed # inch on 
its pillar, and to drive the packing out of the steam-pipe joints. There 
hhas only been one serious breakdown. This occurred at Liverpool on 
the evening of March 17 last. The load was suddenly lost with the 
No. 1 pump, through the fracture of the bolts in the top length of the 
spear. The piston returned with great force into the cylinder, thereby 
breaking the cover between the high- and low-pressure cylinders. The 
valve in this case failed to save the engine, probably on account of the 
fracture occurring near the end of the stroke, and so high up in the spear, 
