ON A CONSTANT INDICATOR FOR STEAM-ENGINES. 311 



spring S T, which spring is connected by means of links at its extremities with 

 a second similar and equal spring Q R, supported at its centre upon a solid 

 projection P, from the cast-iron frame of the instrument. The pressure of the 

 piston-rod upon the lower spring borne at the extremities of the upper spring, 

 whose centre is fixed, is thus made to separate the two springs from one another, 

 and the separation produced is, by a well-known law of deflection, directly 

 proportional to the pressure sustained, so long as the deflections are small. The 

 limits within which this law of deflection obtains, are greatly extended by the 

 peculiar form given to these springs, first suggested (it is believed) by M. 

 Morin. One surface of each spring is plane, and the opposite surface of that 

 well-known parabolic form by which an equal strength is given to every por- 

 tion of the length of the spring. The spring being thus tapered from its centre 

 to its extremities without impairing its strength, its deflection is distributed 

 more uniformly throughout its length*, and being thus made less (for a given 

 separation of the springs) at every point, the elastic limits are nowhere so 

 soon passed. 



By this connexion of the piston-rod with the springs, its position is made 

 to vary directly as the eflTective pressure upon its extremities, or as the effec- 

 tive pressure upon four square inches of the piston of the engine, so that 

 every additional pound in that pressure will cause the piston-rod to alter its 

 position by the same additional distance in the direction of its length. 



The pulley or wheel I K (which, from the peculiar functions assigned to 

 it in this machine, we propose to call the integrating wheel,) turns upon the 

 piston-rod as its axis, and traverses with it at the same time in the direction 

 of its length, being prevented from moving on it in that direction by means 

 of two shoulders fixed by adjustment screws. 



The arms of this wheel are pierced by apertures, through which pass three 

 rods united at their extremities (as shown in the figure), so as to form the 

 rigid frame G H, which frame turns also upon the piston-rod as its axis, but 

 does not traverse with it in the direction of its length ; so that the wheel I K 

 is made by its motion with the piston-rod to traverse the rods of the frame 

 longitudinally, whilst it is made to sweep the frame round with it by any 

 motion of rotation which may at the same time be communicated to it about 

 its axis. It receives such a motion of rotation from the cone K L, which is 

 so placed that its side may be accurately parallel to the piston-rod, and which 

 is kept continually pressed against the wheel at K by means of a spiral 

 spring inclosed in a tube at M, and acting continually against the extremity 

 of the spindle on which the cone turns. 



A system of bevel wheels U, Y, X, communicates to this spindle, and with 

 it to the cone, the rotation of the pulley N, which pulley is driven by a cord 

 carrying a weight at one extremity, and passing by the other extremity (over 

 directing pulleys) to the piston of the engine, or to some point which moves 

 precisely as the piston of the engine does, but through a less space. The circum- 

 ference of the pulley N being thus made to move precisely as the piston of the 

 engine, the angle described by the cone, during any exceedingly small period 

 of time, is made to be exactly proportional to the space described during that 

 time by the piston of the engine. Now let it be observed, that the circum- 

 ference of the integrating wheel I K partaking of the motion of that portion of 

 the surface of the cone with which it is at any instant in contact, the number 



* An absolute uniformiti/ of deflection throughout the length of the spring is not obtained 

 by the construction of M. Morin, but only a uniformity of strength ; the two objects are not 

 compatible. It would, perhaps, be more expedient for the use here given to the springs, 

 that, by a different but equally simple law ef the variation of their thickness, they should be 

 made to bend uniformly throughout. 



