THE STEAM ENGINE. 



descending stroke, we should then be able to declare the actual 

 pressure of the steam at every point of the stroke. But it is 

 evident that such an observation would not be practicable. A 

 method, however, was contrived by Mr. Southern, an assistant of 

 Messrs, Boulton and Watt, by which this is perfectly effected. A 

 square piece of paper, or card, is stretched upon a board, which 

 slides in grooves formed in a frame. This frame is placed in a 

 vertical position near the indicator, so that the paper may be 

 moved in a horizontal direction backwards and forwards, through 

 a space of fourteen or fifteen inches. Instead of an index, a 

 pencil is attached to the indicator of the piston-rod : this pencil is 

 lightly pressed by a spring against the paper above mentioned, 

 and as the paper is moved in a horizontal direction, the pencil 

 would trace upon it a line. If the pencil were stationary, this 

 line would be straight and horizontal, but if the pencil were 

 subject to a vertical motion, the line traced on the paper moved 

 under the pencil horizontally would be a curve, the form of which 

 would depend on the vertical motion of the pencil. The board 

 thus supporting the paper is put into connection by a light cord 

 carried over pulleys with some part of the parallel motion, by 

 which it is alternately moved to the right and to the left, As the 

 piston ascends or descends, the whole play of the board in the 

 horizontal direction will therefore represent the length of the 

 stroke, and every fractional part of that play will correspond to a 

 proportional part of the stroke of the steam piston. 



53. The apparatus being thus arranged, let us suppose the steam 

 piston at the top of the cylinder commencing its descent. As it 

 descends, the pencil attached to the indicator piston-rod, varies its 

 height according to the varying pressure of the steam in the 

 cylinder. At the same time the paper is moved uniformly under 

 the pencil, and a curved line is traced upon it from right to left. 

 "When the piston has reached the bottom of the cylinder, the 

 upper exhausting valve is opened, and the steam drawn off to the 

 condenser. The indicator piston being immediately relieved from 

 a part of the pressure acting upon it, descends, and with it the 

 pencil also descends ; but at the same time the steam piston has 

 begun to ascend, and the paper to return from left to right under 

 the pencil. While the steam piston continues to ascend, the con- 

 densation becomes more and more perfect, and the vacuum in the 

 cylinder, and therefore also in the indicator, being gradually 

 increased in power, the atmospheric pressure above the indicator 

 piston presses it downwards and stretches the spring. The pencil 

 meanwhile, with the paper moving under it from right to left, 

 traces a second curve. As the former curve showed the actual 

 pressure of the steam impelling the piston in its descent, this latter 

 46 



