SECT, viii.] MANAGEMENT OF THE STEAM ENGINE. 265 



main pipes, the pressure in the air vessel increases also ; and this pressure being 

 also communicated by the small pipe to the regulating cylinder, causes its loaded 

 piston to rise, and the motion is communicated to the top or steam valve by 

 means of spindles and gearing to the regulating screw, so as to regulate the 

 steam; or, on the other hand, if the engine works too slow, the pressure in the 

 air vessel diminishes, and the loaded piston descends and opens the valve. 



In order to prevent the motion of the piston being too great, the load is divided 

 into links like a chain, and, as the piston rises, more links are raised, consequently 

 the load increases ; and also as it descends, the links, by resting on the ground, 

 diminish the load. A spring might be applied to produce a similar effect. 



This plan, however, as it did not regulate the pressure of steam in the 

 boiler, and from the friction did not act promptly, has been some time abandoned. 



555. In some cases the further improvement has been adopted, of using this 

 method to ^adjust the tappets which shut off the steam. For this object, the 

 motion of the small piston is communicated to a wheel which turns a pair of 

 bevelled wheels, the one of which is on the square part of a screw rod attached to 

 the plug tree ; and whenever the motion is too rapid, the rod is turned, and moves 

 the tappet so as to cut off the steam sooner, and the reverse. The square part of 

 the rod slides in the wheel upon it without change, except when that wheel is 

 moved by the regulator piston. 



556. OF THE CATARACT. The power of an engine for raising water may 

 also be regulated by increasing or diminishing the interval between its strokes ; 

 this is done by causing the tappets to disengage a loaded piston, which descends 

 in a small air vessel, expelling the air from it by a pipe, which can be regulated 

 by a cock at pleasure ; the valves are not free to open till this piston be at the 

 end of its stroke. The air vessel is a cylinder of from five to six inches diameter, 

 and twenty inches in length, open at the top with a valve opening inwards at the 

 bottom, that it may ascend without unnecessary resistance. It is provided with 

 a pipe from the bottom, of sufficient diameter to allow the air to escape when the 

 engine is at full speed, which has a cock to regulate the time of discharge. It is 

 also fitted with an air-tight piston, the rod of which is connected with the 

 apparatus which opens the valves. Two air vessels are required for a double 



engine. 



OF THE METHODS OF ASCERTAINING THE STATE AND EFFECTIVE POWER 



OF A STEAM ENGINE. 



557. Certain instruments have been invented which are of great use in 

 ascertaining the state of an engine ; and these ought to be kept in good order, 



2 L 



