ON A CONSTANT INDICATOR FOR STEAM-ENGINES. 313 



with it the spindle of the cone, the cone also revolves backwards, so that if 

 the integrating wheel remain in contact with the cone at any other point than 

 its apex, it too will be made to revolve backwards, and the number registered in 

 the preceding stroke will thus be dimi7iished by the number of revolutions 

 made by the integrating wheel during this stroke : but let it be observed, that 

 before the motion of the piston of the engine is reversed, the direction of the 

 pressure of the steam upon it, and therefore the direction of the pressure 

 upon the piston-rod of the indicator, is reversed, by whicii altered direction 

 of the pressure, as well as by the elasticity of the springs, the integrating 

 wheel is made to ascend to the apex of the cone, and to remain there during 

 the return-stroke, so that no number whatever is registered during that 

 stroke. 



Thus, then, the remaining number registered during any time by the in- 

 strument when applied to the double acting engine, is proportional to the 

 work done upon the piston by the steam at the alternate strokes during that 

 time. The mathematical formula by which the work is determined from this 

 number, and allowance made for the friction of the indicator, is given in a 

 subsequent part of this Report. 



In order eifectually to guard against any error which might accidentally re- 

 sult from the reversed motion of the piston, a contrivance has been intro- 

 duced in the combination of wheels Y, X, U, by which the revolution of the 

 cone may be arrested whilst that reversed motion takes place. To adapt the 

 instrument to register (if required) the work done at every stroke of the piston, 

 a four- way cock has been constructed, which may be made, by the action of 

 the engine, to control the direction of the steam passages of the indicator, in 

 such a way that the upper cylinder C shall always communicate with that 

 portion of the cylinder of the engine which is filled with steam, whilst the 

 lower cylinder D is made always to communicate with the vacuum. When 

 the indicator is thus used, the cone is made, by a simple adjustment of the 

 mechanical combination U X Y, to revolve constantly forwards, whilst the 

 motion of the piston of the engine, and therefore of the pulley N, alternates. 

 It remains now only to speak of the application of the indicator to the single 

 acting engine, or Cornish engine. 



The registration during the down or in-doors stroke of that engine is 

 already explained. During the first poi'tion of the up or out-of-doors stroke 

 the equilibrium valve is opened, and the pressure upon both the pistons of 

 the indicator thus becoming the same, the integrating wheel is brought by 

 the elasticity of the springs to its initial positron at the apex of the cone, and 

 thus, although the cone turns backwards, the wheel remains at rest and no- 

 thing is registered. When the equilibrium valve closes, the pressure of the 

 steam in the upper portion of the cylinder of the engine, and therefore in 

 the upper cylinder C of the indicator, preponderates ; the integrating wheel 

 descends from the apex of the cone and the register revolves backwards, de- 

 ducting from, or diminishing, the number before registered by precisely the 

 number of units of work which is done by the steam (jammed back into the 

 upper portion of the cylinder) against the returning piston. Now this is pre- 

 cisely the deduction which the theory of the steam-engine shows ought in this 

 case to be made, and the registration of each double stroke of the single acting 

 or Cornish engine is, by this backward movement of the integrating wheel, 

 perfected. Let it be here observed, that the indicator in the doion-sivokQ per- 

 forms the definite integration of a logarithmic function, viz. that A\hich repre- 

 sents the pressure of the steam as a function of space, described by the piston 

 during the expansion of the steam, that it then calculates the numerical 

 amount of the integral it has thus completed, and registers that amount ; 



