60 



MECHANICS. 



scent of the piston, it will retain, by 

 virtue of its inertia, during the suspen- 

 sion of the power in the ascent ; and 

 this force will drive the machinery, or 

 act against the resistance, whatever it 

 be, in the intervals of the intermission 

 of the power. 

 Even where both the impelling power 



and the resistance are uniform, the one 

 may not be uniformly transmitted to the 

 other. Suppose that piston P, work- 

 ing in a cylinder C, is connected with 

 the end B of a beam, which works on 

 the centre A. The other end, B', of the 

 beam, is connected by a rod, B' D, with 

 a crank F G H I K, which is turned 



Fig. 87. 



round the axis E F I K. The piston 

 being supposed to be pressed both up- 

 wards and downwards by steam, sup- 

 pose it at the top of the cylinder, as in 

 fig. 87. The end B of the beam will 

 then be in the highest position, and 

 the end B' in the lowest, and there- 

 fore, the crank G H in the lowest posi- 

 tion. In this position, let us suppose 

 the pressure of steam to urge the piston 

 P downwards. It is evident, that the 

 piston and beam have no power what- 

 ever in turning the crank. The piston 

 being pressed downwards, draws the 

 connecting rod, B'D, directly upwards. 

 But since, in the present position, the 

 rods B'D, E F, F G, G H, H I, and I 

 K, are all in the same vertical plane, 

 the upward force of B I) is resisted by 

 the pivots E K, of the axis of the crank, 

 and there is no leverage to enable the 

 force of B'D to turn the crank. The 

 machine is, therefore, placed in that 

 mechanical dilemma in which the im- 

 pelling power loses all influence in 

 moving it, But if we suppose the crank 

 moved a little out of this position, the 

 rod B'D and crank cease to be in the 

 same vertical plane, and the rod ac- 

 quires a small leverage on the crank, 

 by which it turns it. This leverage con- 

 tinues to increase until the plane of the 



crank F G H I becomes perpendicular 

 to the connecting rod, as represented in 

 fig. 88. In this case, the power of the 

 connecting rod over the crank is at its 

 maximum, and when the crank is fur- 

 ther raised-, the leverage constantly 



Fig. 89. 

 Fig. 88. 



diminishes, until the piston reaches the 

 bottom of the cylinder, and the crank 

 attains its highest position, as in fig. 

 89. Here, again, the impelling power 

 loses all influence on the crank. Sup- 

 pose the pressure of steam to urge thQ 



