All About Engines

Engines for Ships 291 the main valve. At the same time, it admits steam through f to the right-hand end of the main valve, causing it to be forced to the left against the cushion formed by the exhaust steam. The port c is now open to exhaust, and the port d is open to steam. The same action takes place on the down stroke. Under certain conditions the piston will not com- plete its stroke by the expansion of the steam from three-quarter stroke—for example, if the pump is cold on starting, much of the steam will be condensed and the piston will come to rest. This difficulty is provided against by the by-passes 1 and j in Fig. 162, which can be operated by hand. What excites admiration about this pump is the amount of thought which must have been necessary in order to design the valves and ports by which the distribution of the steam is so delicately controlled. The sea-going engineer, however, would say that what he admired most was its efficiency and reliability. He is less interested in pretty devices than in results. And, after all, this is the main test. Although the steam used by the main engines and auxiliary apparatus is condensed and returned to the boilers, there is a small loss of from 4 per cent, to 8 per cent, daily, which has to be made up; and to carry this amount of fresh water would be impos- sible. For suppose a 10,000 horse-power vessel to evaporate 12 lb. of water per horse-power per hour, the total evaporation per hour will be 120,000 lb., and for twenty-four hours it will be 2,880,000 lb.