66 Prof. J. Perry on a Steam-Engine 



used <a, reflected beam of light as if it were a rigid pointer 

 four feet long. By using a lens, by using magnesium light 

 instead of a common oil-lamp, and by taking certain pre- 

 cautions which are quite obvious to the members of this 

 Society, I could throw a well-defined spot upon a screen 

 forty feet away from a mirror upon a small corrugated disk, 

 whose motion would be with great exactness proportional to 

 the pressure, for motions of as much as five or six feet. 



The end of the arm F receives a miniature motion of the 

 piston of the engine by stiff rods, and this causes the spot to 

 move at right angles to its former motion ; and when both 

 motions are being given the spot travels round on the screen, 

 its position at any instant indicating the pressure of the steam 

 and the position of the piston in its stroke. 



Now, although I have never heard of such a method being 

 used, I feel that many people must have thought of using it. 

 I myself thought of it many years ago ; but the making of a 

 photographic record seemed to me to introduce great difficulty, 

 so I never tried it*. What I have now discovered is this, that 

 a photographic method of recording is quite unnecessary. 

 In fact, even at speeds of 60 revolutions per minute the image 

 of the spot remains on the retina sufficiently long to enable a 

 man to draw upon the screen the path of the spot. He first 

 turns the indicator-cock, so that there is atmospheric pressure 

 under the disk. The spot now travels in a straight line, and 

 this is the atmospheric line. When I wish to check the scale 

 I do what I should like to see done with all indicators, I let 

 steam at boiler-pressure underneath the disk and mark out 

 another straight line parallel to the atmospheric line. I now 

 let the box communicate with the cylinder, and I draw the 

 actual diagram. A very little practice will enable anyone to 

 draw the diagram quite accurately, even when the engine 

 makes only 60 revolutions per minute. But at such speeds 

 as 150 revolutions per minute, the diagram is quite con- 

 tinuous as a thin line of light on the paper, and the most un- 

 skilled person need not make errors of as much as one per 

 cent, in drawing a pencil line, which remains quite visible in 

 the middle of the thin line of illumination. 



Using a common oil-lamp, a diagram about six inches long 

 and four inches broad, formed of a band of light one tenth of 

 an inch broad, is quite visible even in a well illuminated 

 room. If the room is darkened the diagram becomes quite 



* Prof. E. H. Smith has, since the reading of the paper, called my 

 attention to an indicator described in 'Engineering' of July 10th, 1885, 

 by Messrs. Clarke and Low, in which a reflected beam of light is used 

 for magnification. 



