SECT, vii.] STEAM ENGINES. 207 



and that which formed the seat in the common valve moves instead of the plate, 

 and should obviously slide in a steam-tight case. 



SLIDING VALVES. 



443. The sluice is the oldest form of this valve, but its advantages for any 

 other than rough work in wood do not appear to have been understood : indeed 

 it was not to be expected that metallic surfaces would slide on each other so closelv 

 as to be tight and durable, unless very truly worked and of a hard metal. 



Mr. Watt endeavoured to employ them at first, but did not succeed, and it was 

 not till about thirty years ago, when more accurate methods of workmanship were 

 introduced, that the slide valve appeared. 



444. BRAMAH'S SLIDE VALVE. This slide valve is extensively used for pipes 

 of water works, breweries, gas works, and various other purposes, and is ex- 

 ceedingly well adapted for steam passages. It consists of a box with a slider at 

 right angles to the passage, moved by a rod passing through a stuffing box. 



The slider is ground to fit accurately against the circumference of the passage 

 with one surface, and is held close by a spring ; it is moved by a handle for small 

 apertures, and for larger ones by a rack and pinion. 



445. The first idea of employing slides for more than one aperture appears 

 to have been to the air pump by Lavoisier or some of his associates, on which 

 Dr. Robison has remarked, that a sliding plate performs the office of four cocks 

 in a very beautiful and simple manner ; he adds, however, " that the best workmen 

 in London thought they would be difficult to execute." The same principle was 

 applied to the steam engine by Murray in 1799, a sliding box answering the 

 purpose of opening and closing four steam passages, to use Dr. Robison's words, 

 " in a beautiful and simple manner." 1 



446. MURRAY'S SLIDE. The apertures all terminate in a steam-tight case, 

 and within this a smaller box slides up and down, so as alternately to open and 

 close the passages. A section of it is shown in Plate vi. Fig. 5. The sliding part 

 is moved by the rod o passing through a stuffing box. The steam from the boiler 

 enters at S, and, when the slide is down, passes through a to the top of the 

 cylinder, while the passage b c from the bottom of the cylinder to the condenser 

 is open through the interior of the slide ; in like manner when the slide is up, 

 the passage b for the steam to the bottom of the cylinder is open, and the passage 

 a c from the top to the condenser is open. 



A small reciprocating motion is obviously sufficient for the motion of the slide ; 



1 Art. Pneumatics. Robison's Mech. Phil. 



