APPLIED MECHANICS. 



>ER Axn purroir. 



jacitt, on the opposite side of which are throe openings, 

 : and lower, called the jwrtj, communicating by 

 - or passages with the upper ami lower ends of the 

 ;Jer respectively ; and tl. "ne, or exhaust 



channel, communicating by a cavity E with an ojn-nin-,' 

 at the tide, by which steam can escape. These throe 

 openings are partially covered by a hollowed plate of 

 metal, the l>-i'.i'U, which, as its name implies, can be 

 made to slide up or down by means of a rod C passing 

 i tho jacket The hollowed part of the 

 D-alide is made to embrace the middle exhaust-passage 

 and he ports, so as to let (team escape from 



the cylinder, wliile it leaves the other port open for the 

 ingren of steam to the cylinder ; and as for every ascent 

 and descent of the piston in the cylinder, a correspond- 

 ing ascent and descent of the slide is effected by means 

 of apparatus connected with the moving parts of the 

 engine, the complete successive alternation of the steam 

 is maintained without the expenditure of more power 

 than in necessary to overcome the friction of the slide 

 i in which the ports are situated. The 

 face of the slide, and the surface on which it rubs, are 

 Hindu very true and smooth in the first place ; and when 

 they are not subjected to undue wear by the ingress of 

 dirt or grit, their contact remains steam-tight for a long 

 period. The slide we have described is called the short 

 D-slide, and is generally used in locomotives and engines 

 which hare not long cylinders. But when the cylinder 

 is of considerable length, the passages from the ports to 

 the ends of the cylinder are also long ; and having, at 

 every stroke or alternation of the piston, to bo filled with 

 steam, which is ineffective in producing power, consider- 

 able loss is occasioned from this waste of steam in merely 

 tilling the passages. To obviate this defect, the slide is 

 sometimes lengthened, so that the passages from the 

 ports to the cylinder are proportionally shortened. It is 

 sometimes found convenient fur the construction to make 

 this kind of slide of a hollow cylindrical form, fitting 

 into a cylindrical jacket at each end, and being smaller 

 in diameter at the middle. The steam entering at B 

 (Fig. 107) fills the cavity surrounding the slide, and gets 

 Fig. 167. 



to the cylinder by the upper or lower port as the 

 slide is moved upwards or downwards, while the steam 

 passes from either of these ports into the cavities at the 

 UI-IKT or lower ends which communicate through the 

 tabular body of the slide, and from one of which the 

 **te-pil>e ( ' ' lie steam which has done its work. 



e might mention m.-my other varieties of slides; 

 but all being constructed on similar principles to those 



' hay* described, we need not discuss them in 

 " will now proceed to describe the practical construc- 



tion of a cylinder and piston suitable for a non-condens- 

 ing engine. 



CYLINDER AMI PISTON. Referring to V\ :*. KM 

 and 100, which give a general view of tho cylinder and 

 slide, we have to inquire into tho proportions 1 of the 

 parts, and the details of their construction, in such a 

 manner as to be economical, durable, and . !li< i.-nt. 



Tho cylinder is made of cast-iron, bored in a suitable 

 lathe, so that the interior is aa nearly as possible perfectly 

 cylindrical. The covers are also of oast-iron, having a 

 projecting part turned to fit into the ends of the cylinder, 

 to which they are secured by bolts and nuts. The 

 months of the cylinder are generally bored somewhat 

 larger than the rest, so that if, after some years' wear, it 

 become necessary to bore out the cylinder afresh, I 



y making its diameter a little larger, the same 

 covers may still fit it. The flanges or projecting rims of 

 the cylinder, and the faces of the covers which lie against 

 them, are turned very true ; and if well smoothed, require 

 only a little thin flour-paste, or white and red lead paint, 

 spread over them, to render the joints impervious to 

 steam when the bolts are screwed tightly up. The 

 length of the cylinder is determined by the length of 

 stroke or movement of the piston : it should be such as 

 to allow from Jth to an inch clearance between the 

 piston and the cover at each end of the stroke. Theo- 

 retically, the less clearance the better, because any space 

 left between the piston and cover at the end of the stroke, 

 has to be filled at every stroke with steam uselessly. 

 But, practically, it may be difficult to determine the 

 length of stroke with perfect accuracy ; the wear of the 

 machinery may alter it a little, and water or dirt may 

 get into the cylinder, so that it is necessary to allow a 

 little space, such as we have named, for clearance. When 

 the steam first enters a cold cylinder it becomes con- 

 densed into water ; and not uufrequently, during the 

 working of the engine, the boiler primes or boils over, 

 sending water along with the steam into the cylinder. 

 But as water is practically incompressible, its presence 

 between the piston and the cylinder-covers at each end 

 of the stroke, would be quite as detrimental as the 

 presence of a mass of iron or any hard material, 

 unless an exit were provided for it ; for the sudden 

 approach of the piston would be arrested by the 

 water, and either the cover would be forced off, or 

 tho parts connecting the piston to the rest of the 

 machinery would give way. Accordingly, cylinders 

 aro often fitted with relief-valvessmall safety-- 

 valves loaded by weights or springs, and communi- 

 cating with each end of the cylinder so that when- 

 ever, by the presence of water, the pressure becomes 

 increased to a dangerous extent, the valve is opened, 

 and permits the water to escape. In the absence 

 of relief-valves, pet-coda or small stop-cooks are 

 fitted for the same purpose. On first admitting 

 steam to the cylinder, these are left open to permit 

 the escape of the water arising from condensation ; 

 and during the working of the engine, they ir.ay 

 generally be left a little open, especially when the 

 presence of water is manifested by a sharp blow, 

 heard when the piston strikes upon it. The thick- 

 ness of the cylinder depends on its diameter, and 

 tho pressure to which it is subjected ; and the 

 strength of tho flanges, covers, and bolts and nuts, 

 must be determined on the same ground. For such 

 details it is difficult to give precise rules, as expe- 

 rience and study of well-proportioned works can 

 alone give the power of determining them. 



The piston is constructed in various ways, one of 

 which, being simple and effective, wo will describe : 

 The body of the piston consists of a dims and boss A 

 (Figs. 168 and 109), the outer edge of tho disc fitting 

 the cylinder, and the boss having a central conical hole, 

 in which the piston-rod B is secured by means of a key, 

 or thin bar of iron slightly tapered in width, driven 

 through a slot or opening in the boss and rod, so as to 

 tighten the conical end of the rod in the corresponding 

 conical hole. To tho body of the piston is secured by 

 screws, a cover C fitting the cylinder, and leaving between 



