Indicator for High Speeds. 



65 



maximum steam-pressures of about 30 lb. to the square inch 

 above the atmosphere. It was my intention to use corru- 

 gated disks, but as these could not be obtained cheaply, ex- 

 cept in quantity, I have hitherto used plane disks, and found 

 them quite satisfactory for such sizes of diagram as I have 

 hitherto dealt with. I am, however, making arrangements 

 for obtaining corrugated disks for use in the indicator. 



Fio-. 1. 



Now when this box E is put in communication with the 

 cylinder of a steam-engine in the usual way, that is through 

 the pipe A and the indicator-cock, the disk yields more or 

 less as the pressure is greater or less. To magnify this yield- 

 ing I fix upon the disk, about halfway between its centre 

 and circumference, a small mirror B such as is used in elec- 

 trical laboratories. This mirror has a light frame of metal, 

 and can be screwed to or unscrewed from any of these disks 

 quite readily. I let a beam of light from an ordinary oil- 

 lamp fall upon the mirror, and it is reflected and falls upon a 

 sheet of white paper which it illuminates at a small spot. 

 Now the yielding of the disk under fluid pressure is evidenced 

 by the movement of the spot of light on the paper. For 

 example, if steam of 10, 20 ; or 301b. pressure per square 

 inch (above the atmosphere) is admitted to this box with its 

 present disk, and the spot of light is on a sheet of paper 

 about 4 feet away from the mirror, the spot will be seen to 

 travel one, two, or three inches from its original position. 

 If there is a partial vacuum inside the box, the spot travels in 

 the opposite direction. It will be seen, therefore, that I have 



Phil. Mag. S. 5. Vol. 32. No. 194. July 1891. F 



