SECT, vii.] STEAM ENGINES. 227 



the rods will be placed with most advantage under the rods, as on the axis Z, 

 Plate viii. Fig. 1 and 2. 



483. To apply the same principle to a reciprocating engine ; let A B, Fig. 4, 5. 

 Plate ix. be the plug tree, with the curve C D to act on the roller at C, which, 

 as soon as the plug tree descends to C, begins to cause the roller frame to slide, 

 and turn the axis E, so as to depress the slide rod by the arm F. The steam will 

 be shut off by H I on the descending stroke, and by K L in an ascending one. 



484. To regulate the period of cutting off the stroke, the portions containing 

 the curves I H and K L may be made in two parts, to slide side by side by means 

 of a screw ; and if the rod having the screw upon it slides in a wheel acted upon 

 by either a governor or other regulator, the engine will regulate itself. (See 

 art. 554.) 



485. In all cases an axis to be alternately moved in opposite directions should 

 be balanced, and the stress of all heavy parts -should be relieved by counter- 

 balancing them by weights acting on levers. The hand gear should be a power 

 proportioned to the force required to move the slides, cocks, or valves. (See art. 

 449.) 



PISTON GUIDES. 



486. The motion of the piston rod should be in a straight line in the direction 

 of its length, and when the point it acts upon describes the part of a circle, the 

 construction must be such that each may be confined to its proper motion, and 

 yet the piston rod must produce the circular motion with as little oblique action 

 as possible. 



The most simple method is to confine the piston rod to its direction by means 

 of a guide or guides, and to let it act on the part which moves in a circular direc- 

 tion by means of a connecting rod. To reduce the friction of the guides, rollers 

 may be added. A very simple combination of this kind is shown in Plate xv. 

 Fig. 1. A wheel or roller F is fixed on the piston rod D, and is confined to a 

 vertical motion by the guides G G, and the motion is transmitted to the crank I 

 by a connecting rod H H. When the fly is of sufficient power, the whole loss of 

 force in this combination is simply the friction produced by oblique action, and 



employment of the expanding force of steam in the engines of most makers, we infer that, Boulton 

 and Watt's excepted, very few have availed themselves of this great source of economy. The 

 proprietors of engines are too anxious about the power that an engine of a given sized cylinder 

 possesses, forgetting that if an engine work with a minimum quantity, of fuel, it must have a larger 

 cylinder to do the same work. In estimating the comparative economy of engines, nominal power 

 should not be considered, but the effect produced by each pound of fuel. 



