THE LOCOMOTIVE. 



the end of the cylinder nearest to c. After this the piston begins 

 to move from c towards the more remote end of the cylinder, and 

 the joint r assumes successively the positions shown in F, G, and 

 H, the crank making first an acute angle, then a right angle, and, 

 in fine, an obtuse angle with the connecting-rod, until the piston 

 has arrived at the more remote end of the cylinder, when the 

 points c, r, and p, receive the position shown in A. 



7. It must be observed, that in the positions shown in A and E, 

 the connecting-rod being parallel to the crank, can have no power 

 to turn it; that in passing from the position A to the position c,. 

 the rod being less and less oblique to the crank, has a continually 

 increasing power to turn it, until at c, being at right angles 

 to it, it has full power upon it. After passing the position c, the 

 rod becoming more and more oblique to c, has less and less power 

 upon it, until arriving at the position E, it is parallel with it, and 

 loses all power over it. 



The two positions shown in A and E, in which the piston is at 

 one end or the other of the cylinder, and in which the piston loses 

 all power to move the crank, are called the DEAD POINTS. 



8. After passing the position E, when the piston, having changed 

 the direction of its motion begins to return to the other end of the 

 cylinder, the rod again forms an acute angle with the crank, and 

 acts upon it, but with disadvantage, as shown in F. 



The angle formed by the rod and the crank increasing, becomes 

 at length a right angle, as in G, when the rod acts with full effect 

 on the crank. 



After this, the angle between the rod and the crank becomes 

 obtuse, as in H, and the action is again disadvantageous, and 

 more and more so as the angle becomes more and more obtuse, 

 until at length the rod and crank return to the position repre- 

 sented in A. 



Since the action of the piston upon the wheel is, therefore, 

 unequal, having its greatest efficiency at the points shown in c 

 and G, and ceasing altogether in the positions A and E, a single 

 piston would give to the engine an unequal progressive motion. It 

 would advance by starts, being impelled with most effect when 

 the piston has the positions c and G, and moving only in virtue of 

 the velocity already imparted to it when the piston is at the dead 

 points A and E. The motion would be alternately fast and slow, 

 according to the varying position of the connecting-rod and crank. 



9. This inequality is effaced, and an uniform motion obtained 

 by using two cylinders driving different cranks or different wheels, 

 and so arranging them, that when either is at its dead points, the 

 other is in its positions of greatest efficiency. This is accomplished 

 simply by placing the two cranks at right angles to each other, or 



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