551 THE STEAM-ENGINE. 



mencement of the next stroke, so that when the impelling steam begins to pro- 

 duce the returning stroke, there is less waste steam on the other side of the pis- 

 ton to resist it. 



When the motion of the engine is very rapid, the resistance of the waste 

 steam, as it escapes from the blast-pipe to the piston, has been generally sup- 

 posed to oe very considerable, though we are not aware of any direct experi- 

 ments by which its amount has been ascertained. In the account of the loco- 

 motive engine which has been here described, supplied by Mr. Stephenson 

 for the last edition of Tredgold on the steam-engine, he states, that the average 

 resisting pressure of the waste steam throughout the stroke- is 6 Ibs. per square 

 inch, when running at the usual rate of from 25 to 28 miles an hour, and that 

 at greater velocities this negative pressure has been found to increase to more 

 than double that amount. No experiments are, however, cited from which this 

 inference has been drawn. 



It has been also thought that the pressure of steam upon the piston in the 

 cylinder, at high velocities, is considerably below the pressure of steam in the 

 boiler ; but this has riot been, so far as we are informed, ascertained by any 

 satisfactory experimental test. Mr. Stephenson likewise states, that this loss 

 of pressure, causes the negative pressure or resistance of the waste steam to 

 amount to from 30 to 40 per cent, of the positive pressure upon the piston when 

 the engine is running very fast, and that therefore the power of the engine is 

 diminished nearly one haif.f 



But it will be perceived that besides the uncertainty which attends the 

 estimate of the actual amount of pressure on the piston compared with the 

 pressure of steam in the boiler, the inference here drawn does not appear to be 

 compatible with what has been already proved respecting the mechanical effect 

 of steam. No change of pressure which may take place between the boiler 

 and the cylinder can affect the practical efficacy of the steam. As the steam 

 passes through the engine, whatever change of pressure it may be subject to, 

 it still remains common steam : and though its pressure may be diminished, 

 its volume being increased in a nearly equal proportion, its mechanical effect 

 will remain the same. The power of the engine, therefore, estimated as it 

 ought to be, by the whole mechanical effect produced, will not be altered 

 otherwise than by the effect of the increased resistance produced by the blast- 

 pipe. What that resistance is, we repeat, has not, so far as we know, been 

 ascertained by direct experiment, and there are circumstances attending it 

 which render it probable that, even at high velocities, it is less in amount than 

 Mr. Stephenson's estimate. 



The position of the eccentrics which is necessary to make the pistons drive 

 the engine forward must be directly the reverse of that which would cause 

 them to drive the engine backward. To be able, therefore, to reverse the 

 motion of the engine, it would only be necessary to be able lo reverse the 

 position of the eccentrics, which may be accomplished by either of two 

 expedients. 



First, The eccentrics may be capable of revolving on the great working 

 axle, and also of sliding upon it through a small space. Their revolution on 

 the axle may be checked by letting a pin attached to a collar fastened on the 

 axle fall into a hole on the side of the eccentric. Such a pin wi/i drive the 

 eccentric round with the axle, and the position of this pin arid the hole will 

 determine the position of the eccentric with reference to the crank. At a short 

 distance on the other side of the eccentric may be a corresponding collar with 

 a pin in the opposite position. By moving the eccentric longitudinally on the 

 axle, the former pin may be withdrawn from the hole, and the latter allowed 

 to fall into the hole on the other side. Proper mechanism may be provided 



