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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[JANVAnY, 



rourts on ordinary application?, when a correspondence may be 

 made with him at any distance. 



As a means of railway administration the electric telefjraph has 

 proved its elKcacy, and it is impossible to conduct single lines pro- 

 perly without it. Already the convenience to passeiijirers has been 

 very preat, and that to the companies cannot be undervalued. It 

 extends the supervision of the central authorities, and allows the 

 most effective action to take place on every emergency, whether of 

 accident or otherwise. Lately, some half-dozen gentlemen were on 

 business at a minor station on the Eastern C'ounties line, and being 

 desirous of proceeding early to Cambridge, they made application 

 to stop the next train, which would otherwise have passeil the sta- 

 tion. The message was passed to the superintendent at Shoreditch, 

 leave granted, and within half an hour the gentlemen were on their 

 way to Cambridge, where it was of great importance they should 

 arrive early. A lesser case, which happened on the South Eastern 

 a month or two ago, nun- be worth notice. An old «dman proceed- 

 ing from Minster to Tunbridge, or some intermediate station, after 

 paying her second-class fare, in her hurry left her money on the 

 counter. On arriving at Canterbury she found out her loss and 

 wished to return to Alinster, but the superintendent persuaded her 

 to go on, in the hope that she might be able to learn something of 

 it at Ashford. On her arrival there she was told that the money 

 iiad been found on the counter at Minster to the amount she de- 

 scribed, and at the next station the sum was handed to her ; but 

 though glad to receive the money, she could not repress her fears 

 that tlie railway officers to whom she was indebted had dealings 

 with the powers of evil. In the United States it is said that a 

 marriage was contracted by railway between two parties hundreds 

 of miles apart. Under the law of Scotland a telegraph marriage 

 might, we believe, legally take place. Telegraph clerks are some- 

 times however able to help themselves, and a case lately occurred 

 of a superintendent, having to convey to a branch bank notice of 

 stoppage, drawing out his own balance before he delivered the no- 

 tice. 



In the progress of such an invention, and in its greater economy, 

 its aj)plication must be very extensive. In the last session a tele- 

 graph was worked between the House of Commons and the com- 

 mittee rooms, and it is evident that it can be usefully employed in 

 large offices and factories, where in time tlie telegraph wire will be 

 as extended as the l)ellwire. The greater the extension the greater 

 the prospect of improvement and economy to the public, and we 

 can only wish, though we scarcely hope, that a system so valuable 

 will be received in a favourable spirit on the part of the public, 

 and meet with a greater degree of encouragement than is usually 

 afforded to new inventions. 



ON THE LAP AND LEAD OF THE SLIDE VALVE. 



The slide valve is that part of a steam engine which causes the 

 motion of the piston to be reciprocating. It is made to slide upon 

 a smooth surface, called the cylinder face, in which there are three 

 openings to as many pipes or passages : two for the admission 

 of steam to the cylinder, above and below the piston, alternately ; 

 while the use of the third is to convey away the waste steam. The 

 first two are, therefore, termed the induction or steam ports, and 

 the remaining one the eduction or exhaustion port. 



The slide is enclosed in a steam-tight case, called the slide- 

 jacket ; and motion is communicated to it by means of a rod work- 

 ing through a stuffing-box. 



The steam from the boiler first enters the jacket, and thence 

 passes into the cylinder, thriuigh either steam port, according to 

 the position of the slide, which is so C(Mitrived that steam cannot 

 pass from the jacket to the cylinder through botli steam ports at 

 the same time, or through the eduction port at any time. 



Case 1. — When a Slide has neither Lead ou Lap. 



Fig. 1 represents the cylinder face for a " Murray slide" without 

 lap ; a and b being the induction ports, and c the eduction. 



Figs. 2, 3, and t, are similar sec^tions of the nosle, showing the 

 slide in its central and two extreme positions. It occupies the 

 mid-position, fig. 2, when the piston is at either extremity of its 

 stroke ; the extreme position, fig. 3, when the piston is at half- 

 stroke in its descent ; and that shown in fig. -t, when the piston is 

 at half-stroke in its ascent. 



When a slide has no lap, the width of its facing, at/and^ (fig. 



2), equals that of the steam ports; the lap being any additional 

 width whereby those ports are overlapped. 



r 



Fis, 1. Fig. 2. Fig. 3. Fie. 4. 



That the waste steam may have unobstructed egress, the exhaus- 

 tion port c must be made of no less widtli tiian the steam ports ; 

 and, for the same reason, the bars d and e should corres])ond with 

 the slide face at / and g. The three ports, together with the bars 

 between and beyond them, are therefore drawn of equal width ; 

 the total length of the slide being equal to the distance between 

 the steam sides of the steam ports. 



The distance through which the slide moves, in passing from one 

 extreme position to the other, is called its travel ; which, in this 

 case, equals twice the port. 



When the motion of a slide is produced by means of an eccen- 

 tric, keyed to the crank shaft and revolving with it, the relative 

 positions of the piston and slide depend upon the relative positions 

 of the crank and eccentric. 



Demonstration. 



Diagram 1. 



Let ab, diagram 1, represent the 

 crank ; then b being the crank- 

 pin, and a the centre of motion, 

 the larger circle represents the 

 orbit of the crank, and its diame- 

 ter b c the stroke of the piston. 

 Supposing the cylinder to be an 

 upright one, having the crank- 

 shaft immediately aboie or below 

 it, the connection between the 

 piston-rod and crank being merely 

 a connecting-rod, i\ithout the in- 

 tervention of a beam, it is evident 

 r that when the position of the crank 



is a b, the piston will be at the top of the cylinder, and at the bot- 

 tom when its position is a c. The relative positions of the crank 

 and piston, at any point of the stroke between the t«o extremes, 

 depend upon the length of the connecting-rod : for the present, 

 however, let us suppose the connecting-rod to be of infinite length, 

 and therefore always acting upon the crank in parallel lines, so 

 that when the crank is at d, e will be the apparent position of the 

 piston, and / the same when the crank is at </ ; the piston being 

 represented by the sine of the arc described by the crank from 

 either of the points b and c, in the direction of the arr()W. 



The diameter h i, of the inner circle of the diagram, represents 

 the travel of the slide, and its radius the eccentricity of tlie eccen- 

 tric; or, regarding the eccentric as a crank, the radius may be said 

 to represent that crank, as ab represents the main crank. The 

 travel of a slide, without lap, being equal to twice the port, the 

 two steam ports are represented by the spaces a h and a ?, but 

 transposed, a i being the passage to the top of the cylinder, and a h 

 that to the bottom. 



Supposing the piston to be at b (the top of the cylinder), the 

 position of the slide will be that shown in fig. 2, the direction of 

 its motion being downward, so that the jxirt a (same figure), or a i 

 in the diagram, may be gradually opened for the admission of steam 

 above the piston, iintilthe piston has arrived at half-stroke, when 

 it will be fully open, as shown in fig. 3. The direction of the 

 slide's motion is then reversed, so that when the piston has com- 

 pleted its descent, the port /i, figs. 1 to 4, or a h in the diagram, 

 will begin to open for the admission of steam beneath it, and ex- 

 naustion will commence from above it through the port n, or a i, 

 and exhaustion port c, the slide being again brought into its central 

 position, fig. 2. 



Now the slide being at half-stroke, when the piston is at either 

 extremity of its stroke, if we make ab the position of the crank, 

 ak will be that of the eccentric; and the axis of the crank being 



