Polytechnic Association. 843 



with increase of weight only up to a certain point ; which point is 

 dependent on the initial pressure of the steam in the cylinder, on the 

 point of the stroke at which cut-off is made, and on the velocity of 

 revolution. A high-speed, heavy-piston engine depends, like every 

 other reciprocating engine, upon the inertia of its fly-wheel or other 

 rotating parts for the facility with which it passes the centers ; but 

 it taxes these regulators less than the ordinary reciprocating engine, 

 because of the brevity of the intervals during which the effective 

 force intermits. 



There must be, at the beginning of the stroke, a large excess of 

 steam pressure above that which is required to overcome' the inertia 

 of the piston and impart to it the necessary acceleration ; otherwise 

 the advantages derivable from this construction and mode of working 

 will not be fully secured ; and the irregularity of working of the 

 engine may even be exaggerated. 



The counterpoise weight, employed to balance the strain exerted 

 toward the close of the stroke by the heavy piston upon the crank, 

 ought not to exceed one-half the weight of the reciprocating mass to 

 which it is opposed : otherwise, immediately after the passage of the 

 centers, the strain on the main shaft in the direction opposite the 

 crank will be excessive, and may be injurious. 



F. A. P. BAENAED, 

 Chairman of Judges > Group 1, Depa?"tme?it V, 1870. 



Mr. C. E. Emery — After so admirable and elaborate a discussion, 

 in so scientific a manner, it seems useless to add anything further. 

 Unfortunately, however, those who build steam-engines do not have 

 the leisure to make such abstruse calculations. Facts of this charac- 

 ter must be reached in some simple manner, until the exact results are 

 published, as I hope these will be. I will mention that in most engines, 

 practically, the mean pressure is substantially the same as that given 

 by the parabolic theory. This occurs for two reasons. First, there 

 must be a terminal clearance, which diminishes the pressure some- 

 what ; and, second, most engines use more steam than the theory 

 would give, absorbing heat from the cylinder toward the end of the 

 stroke, to evaporate the water, and make more steam. In ordinary 

 practice, therefore, the rule by the parabolic system is sufficiently 

 accurate. 



I suppose Mr. Porter would hardly admit, as this paper intimates, 

 that he put the weight into his piston without calculations on the sub- 

 ject. Those who heard the discussions here a year ago will not con- 



