The force of the steam thus impressed upon the pistons is communicated by the piston rods Y, the cross beads of which move in guides to the connecting rods B, which are attached to the crank pins of the working axle C•, so that, as the pistor., rods are driven backwards and forwards in the cylinders, the working axle is made to revolve. As this axle is the instrument by which the impelling force is conveyed to the load, and as it has to support a great portion of the weight of the engine, it is constructed with great strength and precision. Its length is 64 feet, and its diameter 5 inches. At the centre part it is cylindrical, and is increased to 54 inches, where the cranks are formed. The sides of the cranks are four inches thick ; and the crank pins, which are truly cylindrical, arc 5 inches in diameter and 3 in length. Upon the parts which are 74 inches long, the great driving wheels D are firmly fastened, so as to be prevented from turning or shaking upon the axle. Brasses are fixed on the out side of the engine, which rest upon these projections G of the axle ; and upon these brasses the weight of the engine is sup ported.
The strength and accuracy of construe tion necessary for these axles render them expensive ; they cost almost $200 each. They are seldom broken, but sometimes bent when the engine escapes from the rails.
The method by which the slides are made to govern the admission and escape of the steam to and from the cylinders, is nearly the same as in the steam-engine used for the general purposes of manu facture ; and for a general description of the method see STEAM-ENGINE. Mean while it may be here briefly stated, that this is effected by two circular plates call ed eccentrics, on the great working axle. These eccentrics are circular plates or rings, formed upon or attached to the axle so as to revolve in their own plane, form ing, in effect, a part of the axle itself; but they arc so placed that their centres do not coincide with the centre of the axle, and, consequently, as they revolve with the axle, their centres are alternately thrown backwards and forwards, as they pass on the one side or the other of the axle. These circular plates are surround ed by rings, within which they revolve, but which do not revolve with them. These rings are alternately thrown back ward and forward by the play of the ec centrics ; and to these rings are attached rods e e, which communicate motion to the arms which drive the rods of the slides. Thus the alternate motions of the eccentrics backward and forward pro ceeding from the working axle, produce a corresponding backward and forward motion in the slides, and thereby govern the admission and escape of the steam to and from the cylinders. When it is re quired to reverse the motion of the en gine, or to make it move backwards, the motion of the slides, and therefore the' positions of the eccentrics on the work ing axle, must be the contrary of that ne cessary to produce a progressive motion. Sometimes this is effected by shifting the position of the eccentrics on the working axle ; but more commonly it is effected by a second pair of eccentrics, first placed on the axle in a position contrary to the others. When the engine is driven back
ward, the eccentrics are thrown out of gear, and the other eccentrics are brought into action.
As all the moving parts of the engine require to be constantly lubricated with oil, to diminish the friction and keep them cool, oil cups for this purpose are fixed upon them. In sonic engines these oil cups are attached separately to all the moving parts ; in others they are placed near each other in a row on the side of the boiler, and communicate by small tubes with the several parts to be lubricated.
The tender is a carriage attached be hind the engine, and close to it, carrying coke for the supply of the furnace, and a tank containing water for the boiler. The feed for the boiler is conducted through a curved pipe proceeding from the tank and carried first downwards, and after wards in a horizontal direction, as repre sented at K, under the boiler. It com municates with a forcing pump, which is worked by an arm driven by the cross head of the steam piston. By this pump water is constantly forced into the boiler, so long as the pump is kept in communi cation with the tank ; but this communi cation may be opened and cut off by a cock 1, governed by the engineer. As the feed of the boiler by the introduction of cold water checks the activity of the evaporation, it is the custom not to feed the evaporation, regularly and constantly, but to throw on the feed when the work on the engine is light and the consumption of steam small, and to shut it off when much steam is required. The circum stances of a railway naturally suggest this. When the engine is ascending an incline, all the steam which the boiler is capable of producing is required, and therefore the activity of the boiler is stimulated by shutting off the feed ; but in descending an incline less power is required, and the feed is put on.
Until within the last few years, loco motive engines were supported on only four wheels. It is now, however, the general practice to place them on six, the driving wheels being in the middle. To give greater security to the position of the engine between the rails, it is usual to construct flanges on the tires of all the six wheels. Mr. Stevenson, however, has been in the practice of constructing the driving wheels without flanges, and with tires truly cylindrical, depending on the flanges of the two pairs of smaller wheels to maintain the engine between the rails. The wheels of the engine are constructed in this manner. The driving wheels D are fixed on the cranked axle C, and are constructed with cylindrical tires without flanges. They are 5 feet in diameter. The wheels L are 3 feet 6 in. in diameter, and have conical tires with flanges. They arc placed immediately behind the smoke-box. The wheels M are precisely similar to L, and are placed immediately behind the fire-box.