320 



REPORT — 1841. 



the apertures through which the piston-rod passes, which piece is intended to 

 be rigid, and to serve merely as an attachment to the elastic arms of the spring, 

 having been constructed (by a mistake of the workman) greatly too thin, and 

 the apertures in it of too large a diameter. These causes seem not only to 

 have affected the limits of the deflection of the spring, but (in some degree) 

 its elasticity. 



Springs of different degrees of elasticity are required, 

 that the same, or nearly the same, deflection and play of the 

 pistons of the indicator may be obtained when it is applied 

 to engines working at different steam pressures. A move- 

 able link, represented in the accompanying figure, has, how- 

 ever, been applied to obtain the same deflection from the 

 same pair of springs under a comparatively small variation 

 in the pressure to which they are subjected. The two per- 

 manent links which unite the two springs of any system 

 are removed and replaced by two such moveable links, 

 which are clamped upon the springs by means of the screws 

 shown in the figure, at such distances from their extre- 

 mities, as may, by shortening the arms of the spring, I'educe 

 the deflection due to a given strain to the required limit. 

 If this removal of the link from the extremity of the spring 

 be, however, carried beyond two, or at the utmost three 

 inches, the increased rigidity of the shortened arm and other 

 causes, will begin to affect the elastic properties of the 

 springs towards the limits of their deflection. 



Friction op the Pistons, Integrating Wheel, &c. 



It will be observed that the entire amount of that friction F which affects 

 the registration of the instrument is due to two causes, of which the first is 

 that constant friction of the pistons which results from the precautions ne- 

 cessary to prevent the escape of the steam, and the second that which is 

 due to the pressure of the cone, which is of brass, upon the integrating 

 wheel acting in the direction of the axis of the cone, and produced by the 

 pressure of the spring upon the extremity of its spindle. 



By this last pressure friction is produced, — 1st, at the point of contact of 

 the integrating wheel with the cone, and 2ndly, at the point of contact of the 

 integrating wheel with the piston-rod upon which it turns, and which serves 

 it as an axis. 



Every precaution has been taken to diminish the friction which results 

 from these several causes. The pistons have been made hollow, as shown in 

 the accompanying figure. Each may be conceived to 

 be formed of a thin brass cylinder or tube, accurately 

 fitting to the cylinder of the indicator, when heated to the 

 temperature of the steam, and crossed in its centre by a 

 ligid diaphragm or plate, about one-fourth of an inch 

 thick, in which the piston-rod is fixed. That part of the 

 surface of the piston which immediately surrounds this solid plate is hollowed 

 into a groove, so that no rigid portion of the mass of the piston is brought 

 in contact with the interior of tlie cylinder of the indicator, and thus the pos- 

 sibility of the surfaces binding upon one another by the expansion of one of 

 them is avoided, and any uncertain influence on the results from this cause 

 guarded against. To draw off the water of condensation the diaphragm of 



