Indicator for High Speeds. 69 



diagram is now an object which is very easily observed and 

 traced. 



I exhibit a few drawings made on tracing-paper in this 

 way by Mr. Holland. First, we have a few taken at about 

 the same time ; these taught us that we had better make con- 

 siderable changes in the valve of our engine. We made 

 these changes, and here are some others taken subsequently. 

 The speed and other necessary information are given on each 

 sheet of paper, so that the effect of change of speed on our 

 toy engine may be noted. 



With an ordinary indicator the instrument must be stopped 

 in its action, and a sheet of paper taken from the drum, before 



the diagram can be looked at. But here the diagram is 



... . 



visible all the time. To the student or improver of steam- 

 engines it is very instructive to keep looking at the diagram 

 whilst altering steam-pressure, or speed, or load of the engine, 

 and it is an amusement of which one does not very soon 

 become tired. Even with this toy engine of mine I have 

 already observed such changes in the shape of the diagram 

 as have thrown a perfectly new light upon the condensation 

 phenomena occurring inside a steam-engine cylinder. 



The first form of this new indicator is lying on the table. 

 You will see that to give it the piston motion in miniature 

 required considerable force to be exerted, a fault which I 

 have corrected since. Now the very first time the instru- 

 ment was tried a very curious phenomenon was observed, 

 namely, that the indicator-diagram was not one continuous 

 line of light, but a series of " blobs " or spots, connected by 

 a much fainter line. I easily saw that these indicated a 

 vibration going on in the rods which gave to the box ,the 

 motion of the piston in miniature. I found that the cause of 

 this was due to the bad fitting or " backlash" in a lever 

 which my students had been in the habit of using with ordi- 

 nary indicators on the same engine. With an ordinary in- 

 dicator there was no possibility of observing from the diagram 

 that such a fault existed, and was probably greatly accen- 

 tuated by the use of a cord instead of our rod ; here it was 

 very evident. Indeed, although only to a small extent, it 

 will be observed in the photographs that there are regular 

 changes of intensity of light in every diagram, and that this 

 vibration of the piston-motion mechanism has not been 

 altogether done away with even in our more carefully fitted 

 stiff rods. 



Added, June 5th, 1891. 

 In the discussion which followed the reading of the paper 



