460 



THE STEAM-ENGINE. 



This simple and effectual expedient of producing a continued rotatory mo- 

 tion by a crank was abandoned by Watt, as already explained, by reason of a 

 patent having been obtained upon information of his experiments surreptitious- 

 ly procured. To avoid litigation, he therefore substituted for the crank the sun- 

 and-planet wheel already described ; but at the expiration of the patent, which 

 restricted the use of the crank, the sun-and-planet wheel was discontin y*i in 

 Watt's engine, and the crank restored. 



Whether the crank or the sun-and-planet wheel be used, there is still a dif- 

 ficulty in the maintenance of a regular motion of rotation. In the various po- 

 sitions which the crank and connecting-rod assume throughout a complete 

 revolution, there are two in which the moving power loses all influence in im- 

 pelling the crank. These positions are those which the crank assumes when 

 the piston is at the top and bottom of the cylinder, and is just about to change 

 the direction of its motion. When the piston is at the bottom of the cylinder, 

 the pivot I, fig. 19, by which the connecting-rod H I is attached to the end of 

 the crank, is immediately over the axle K of the crank, and under the pivot H, 

 which joins the upper end of the connecting-rod with the beam. In fact, in 

 this position the connecting-rod and crank are in the same straight line, ex- 

 tending from the end of the beam to the axle of the crank. The steam, on 

 entering the cylinder below the piston, and pressing it upward, would produce 

 a corresponding downward force on the connecting rod at H, which would be 

 continued along the connecting-rod and crank to the axle K. It is evident 

 thut such a force could have no tendency to turn the crank round, but would 

 expend its whole energy in pressing the axle K downward. 



The other position in which the power loses its effect upon the crank is 

 when the piston is at the top of the cylinder. In this case, the working end 

 of the beam will be at the lowest point of its play, and the crank-pin I will be 

 immediately below the axle K ; so that K will be placed immediately between 

 H and I. When the steam presses on the top of the piston, it will expend its 

 force in drawing the end H of the connecting-rod upward, by which the crank- 

 pin I will likewise be drawn upward. It is evident that this force can have no 

 effect in turning the crank round, but will expend its whole energy in pro- 

 ducing an upward strain on the axle K. 



If the crank were absolutely at rest in either of the positions above de- 

 scribed, it is apparent that the engine could not be put in motion by the 

 steam ; but if the engine has been previously in motion, then the mass of 

 matter forming the crank, and the axle on which the crank is formed, having 

 already had a motion of rotation, will have a tendency to preserve the mo- 

 mentum it has received, and this tendency will be sufficient to throw the 

 crank K I out of either of those critical positions which have been described. 

 Having once escaped these dead points, then the connecting-rod forming an 

 angle, however obtuse or acute, with the crank, the pressure or pull upon the 

 former will have a tendency to produce rotation in the latter. As the crank 

 revolves, however, the influence of the connecting-rod upon it will vary accord- 

 ing to the angle formed by the connecting-rod and crank. When that angle is 

 a right angle, then the effect of the connecting-rod on the crank is greatest, 

 since the force upon it has the advantage of the whole leverage of the crank ; 

 but according as the angle formed by the crank and connecting-rod becomes 

 more or less acute or obtuse in the successive attitudes which they assume in 

 the revolution of the crank, the influence of the connecting-rod over the crank 

 varies, changing from nothing at the two dead points already described, to ihe 

 full effect produced in the two positions where they are at right angles. In 

 consequence of this varying leverage, by which the force with which the con- 

 necting-rod is driven by the steam is transmitted to the axle on which the crank 



