68 



Prof. J. Perry on a Steam-Engine 



line upon dark paper, when this photograph was taken ; but 

 if an instantaneous photograph had been taken of the arrange- 

 ment actually working, it would not have been very different 

 from what I here show, except that the line of light forming 

 the diagram would have been finer. 



We have only been able to run this toy engine at a maxi- 

 mum speed of about 900 revolutions per minute, and indeed 

 at this speed it was rather dangerous to run it, as, through 

 want of balance, it set the floor and everything in the room 

 a-shaking ; but even at this high speed there was no evidence 

 on the diagram of any waviness due to the natural vibrations 

 of the disk of the indicator. There was, however, a musical 

 note in evidence of the existence of such vibrations. 



Mr. Holland has used a tapering wooden box as if it were 

 a camera for photographing the diagram and its atmo- 

 spheric line, and he tells me that it is quite easy to take 

 such photographs. He is, however, accustomed to such work, 

 and I do not think that an ordinary engine-driver would care 

 to try the photographic method. I here exhibit some of Mr. 

 Holland's photographs. This one, fig. 4, required an ex- 

 posure of one minute, the light being that of an ordinary 



Fie. 4. 



Fig. 5. 



oil-lamp. This one, fig. 5, required an exposure of ten 

 seconds, the light being obtained by the burning of a piece 

 of magnesium strip behind the hole. 



I have, however, used Mr. Holland's box with sheets of 

 tracing-paper instead of photographic plates, and the indicator- 



