ON THE INDICATOR. 167 



V 1 1 I. O N THE INDICATOR. 



BY JOSEPH GLYNN, ESQ., C.E., F.R.S., &c. 



ALTHOUGH the construction and application of the indicator has been described in such a 

 manner that most persons conversant with the mechanical contrivances of the time, and 

 with the steam engine as now manufactured, have a general idea of its form and use, yet it 

 seems to have been regarded rather as an ingenious invention, among many others, of the 

 late Mr. Watt fitted more for the philosophical lecturer than for the practical engineer ; as 

 a curious instrument by means of which the action of the steam in the cylinder, and that of 

 the vacuum or atmospheric pressure upon the piston, might be delineated, and its varying 

 force represented by a curve, as it was at one time the fashion to represent every thing ; even 

 the arguments and statements of the political economist were drawn out in curved lines and 

 figured in a diagram. 



But the indicator shows not only the relative action of the vacuum and pressure of the 

 steam upon the piston of an engine, but their absolute force and effect ; it shows how much 

 of that force is taken to overcome the friction of the machine, and produce the change of 

 motion in its parts, and how much is available for useful purposes ; it exhibits, if we may so 

 say, the disposable force of the steam engine, and the perfection or imperfection of its con- 

 struction or condition at the time of making the trials. The indicator, in its most simple and 

 best form, is shown in the engraving (see PL xvi). The cylinder, or tube of the instrument, 

 should be truly bored, and the piston ground into it, so that the workmanship should be as far 

 as possible perfectly accurate. The piston should fit the tube without packing, so that it may 

 be made air-tight by pouring olive oil upon it when it is in use. Its area should be exactly 

 lj HJ 1^9 lf> or two square inches, and a loop or an eye should be fixed in the under side of 

 it, so that by means of a cord and weights attached to it, the extension of the spiral spring 

 with each ft. upon the square inch may be proved, and a scale of extension formed, up to 

 14lbs., ruling parallel lines for each B5. from the line of rest, which may be marked with a 

 cipher. The compression of the spring from the line of rest may also be tried, by attaching 

 the cord to the loop at the top of the piston rod which holds the tracing pencil, and hanging 

 the weights over a pulley. To insure accurate observations, it is best to prove the spring of 

 the indicator every time the instrument is used, and form the scales for the occasion : it is 

 almost needless to say that it should be cleaned and oiled afresh for each trial ; but unless this 

 be done, it may cause great discrepancy in the results, and much time may be wasted in trying 

 to reconcile apparent differences arising only from a little stiffened oil. These observations 



