Page:Walker (1888) The Severn Tunnel.djvu/67

20 The bucket-pump is simply the ordinary lift-pump used all over the country, with a bottom valve fixed as near as possible down to the level from which the water is to be pumped, and with large rods running down the centre of the pump attached to the bucket-valve which works up and down the length called the working barrel, the bucket-valve opening as the pump-rods descend, and allowing the water to pass to the upper side of the bucket; while the valves of the bucket close when the engine makes the upward stroke, and the valve at the bottom opening allows the water to enter, and to follow the bucket in its ascent.

The plunger-pump is a force-pump;the pump-barrel, or rising-main, has no rods inside it, but near the bottom is connected by a casting called the ‘H-piece,’ with a separate closed cast-iron cylinder known as the ‘pole case,’ There are two valves as in the bucket-pump, but they are both fixtures. The top and bottom of the pole-case are closed, but the plunger at the lower end of the pump-rods works through a stuffing-box on the top of the pole-case, and when the rods are drawn upwards the bottom valve opens while the top one is held closed by the column of water above it, and the atmospheric pressure drives the water through the bottom valve into the pole-case. On the return-stroke of the engine the pump-rods descending close the bottom valve and force the water through the