After a few successful though noisy trials, an alteration was made in the endless chain and in the guide-roller near the pump bottom. An amount of slack in the chain caused the balls to knock on passing this roller before entering the pump bottom. A chain having long links or bars of iron of uniform length, from ball to ball, jointed together by cross-pins, was substituted for the short link chain, and passed over a revolving hollow square frame at the bottom of the pump, in place of the curved roller-guide in the drawing. Each of the four sides of this square hollow frame was of the same length as the jointed link, and the balls lay in the hollow of the frame without touching it, contact being only on the links. The balls were thus guided directly into the bottom of the pump on their upward course with a rigid chain, and the swing and knocking was avoided. This pump was in principle the traditional rag-and-chain pump of a hundred years before; yet no trace of its use is met with during Trevithick's life in Cornwall. The early pump had rag balls, in keeping with the mechanical ignorance of the time, and suitable to man's power. Trevithick's pump with iron balls raised "7200 gallons of water 10 feet high in a minute with 1-1/2 lb. of coal,"[140] retaining all the original simplicity of the earlier rag-pump, having uniform circular motion and constant stream, without the use of a single valve. The engine and pump are thus described by him:-- [Footnote 140: See letter, vol. ii., p. 332.] "The first engine that will be finished here for Holland will be a 36-inch cylinder and a 36-inch water-pump, to lift water about 8 feet high. On the crank-shaft there is a rag-head of 8 feet diameter, going 8 feet per second, with balls of 3 feet diameter passing through the water-pump, which will lift about 100 tons of water per minute. It is in an iron boat, 14 feet wide, 25 feet long, 6 feet high, so as to be portable and pass from one spot to another without loss of time. This will drain 18 inches deep of water (the annual produce on the surface of each acre of land) in about twenty minutes; to drain each acre with about a bushel of coal costing 6_d._ per year. The engine is high pressure and condensing."[141] [Footnote 141: See Trevithick's letter, vol. ii., p. 315.]