472 



THE STEAM-ENGINE. 



be attended with the danger of straining and breaking the moveable parts of 

 the mechanism. 



To secure, therefore, the necessary accuracy of the joints, Watt contrived 

 that every joint in the engine should admit of the size of the socket being ex- 

 actly adapted to the size of the pin, so as always to make a good fitting by 

 closing the socket upon the pin, when any looseness would be produced by 

 wear. With this view, all the joints were fitted with sockets made of brass 

 or gun-metal, capable of adjustment. Each socket was composed of two 

 pieces, accurately fitted into a cell or groove, in which one of the brasses can 

 be moved toward the other by means of a wedge or screw. Each brass has 

 in it a semi-cylindrical cavity, and the two cavities being opposed to each 

 other, form a socket for the joint-pin. One of the two brasses can always be 

 tightened round that pin, so as to enclose it tight between the two semi-cylin- 

 drical cavities, and to prevent any looseness taking place. The brasses, and 

 other parts of such a joint, are represented in fig. 25. These joints still con- 

 tinue to be used in the engines as now constructed 



Fig. 23. 



The motion of the working beam, and the pump-rods which it drives, and 

 of the connecting rod, ought, if the whole were constructed with perfect pre- 

 cision, to take place in the same or parallel vertical planes ; but this supposes 

 a perfection of execution which could hardly have been expected in the early 

 manufacture of such engines, whatever may have been attained by improve- 

 ments which have been since made. In the details of construction, Watt 

 saw that there would be a liability to lateral strain, owing to the planes of the 

 different motions not being truly vertical and truly parallel, and that if a 

 provision were not made for such laternal motion, the machinery would be 

 subject to constant strain in its joints and rapid wear. He provided against 

 this by constructing the main joints by which the great working lever was 

 connected with the pistons and connecting rod, so as to form universal joints, 

 giving freedom of motion laterally as well as vertically. 



The great lever, or working beam, was so called from being originally made 

 from a beam of oak. It is now, however, universally constructed of cast-iron. 

 The connecting rod is also made of cast-iron, and attached to the beam and to 

 the crank by axles or pivots. 



The mechanism by which the four valves are opened and closed, is subject 

 to considerable variation in different engines. They have been described 

 above as being opened and closed simultaneously by a single lever. Some- 

 times, however, they are opened alternately in pairs by two distinct levers 

 driven by two pins attached to the air-pump rod. One pin strikes the lever, 

 which opens and closes the upper steam-valve, and lower exhausting-valve : 

 the other strikes that which opens and closes the lower steam-valve and upper 

 exhausting-valve. 



Since the date of the earlier double-acting engines, constructed by Boulton 

 and Watt, a great variety of mechanical expedients have been practised for 

 working the valves, by which the steam is admitted to and withdrawn from the 

 cylinder. We shall here describe a few of these methods : 



