THE STEAM-ENGINE. 



509 



to the corresponding point of the 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 backward and forward, through a space of 

 fourteen or fifteen inches. Instead of an index a pencil is attached to the in- 

 dicator 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 

 under the pencil, would trace upon the paper a line. If the pencil were sta- 

 tionary 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 con- 

 nexion by a light cord carried over pulleys with some part of the parallel mo- 

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

 piston ascends or descends, the whole play^f 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. 



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 attach- 

 ed 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 condensation 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 downward and stretches the spring. The pencil 

 meanwhile, with a 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 will show the pressure of the uncondensed 

 steam raising the piston in its ascent, and a comparison of the two will ex- 

 hibit the effective force on the piston. Fig. 62 represents such a diagram as 



Fig. 62. 



Ikihg'fedcba 



would be produced by this instrument. A B C is the curve traced by the 

 pencil during the descent of the piston, and C D E that during its ascent. A 



