Page:Walker (1888) The Severn Tunnel.djvu/69

22 When it has reached the top, the weight of the pump-rods (assisted, if necessary, by weights placed on the inner end of the beam) makes the down- stroke without using steam.

In the 75-inch engine, working a 35-inch bucket-pump, the weight of the wrought-iron beam was nearly 23 tons, and the weight of the pump-rods was nearly 12 tons. The engine could run eleven strokes per minute, raising 376 gallons of water 173 feet high at each stroke. Steam was admitted to the cylinder at 45 lbs. pressure. The vacuum was about 13½ lbs. Steam was cut off at less than one-third of the stroke. The engine worked best at about eight strokes per minute, giving 3,000 gallons raised in that time.

The Bull-engine takes steam only below the piston, the pump-rods being attached to and working in a direct line from the piston-rod. Steam being admitted below the piston, the pump-rods are raised by a direct lift. Their weight, which must be greater than the column of water, makes the return-stroke without steam. The Bull-engine is the most compact-looking, but does not work so economically as the beam. The heavy beam itself, being once set in motion by steam, acts as an accumulator, allowing steam to be used more expansively than is possible in the Bull-engine.

In the 50-inch Bulls, steam was admitted at 45 lbs. pressure, and cut off at one-third of the stroke. The pump-rods weighed about 5½ tons, and the