SECT. viii. J MANAGEMENT OF THE STEAM ENGINE. 261 



or suddenly set on, if the moving power were to remain the same, an alteration 

 of the velocity must take place ; it must move faster or slower. This change of 

 velocity would in most cases be very hurtful to the work, and cause considerable 

 loss ; besides, there is always a velocity at which a machine will act with greater 

 advantage than at any other ; therefore the change of velocity arising from the 

 above cause, is in all cases a disadvantage, and in all delicate operations ex- 

 ceedingly injurious. In a cotton mill, for example, where the power moved the 

 spindles with a given speed, if so much of the work were at once thrown off as 

 to increase the velocity in a considerable degree, a loss of work would immediately 

 take place, and an increase of waste from the breaking of the threads ; on the 

 other hand, there would be much loss of the time of the attendants, if the machinery 

 moved too slow. 



An equally bad effect is observed in raising water, and other species of work. 



544. THE THROTTLE VALVE. The power of a steam engine is usually regu- 

 lated by increasing or diminishing the steam passage, and this is generally per- 

 formed by admitting the steam into the cylinder, more or less freely, by means 

 of what is called a throttle valve : this valve is formed of a circular plate of metal, 

 a, Fig. 1. Plate vm. having a spindle fixed across its diameter. The plate is 

 accurately fitted to an aperture in a metal ring of some thickness, through which 

 the spindle is fitted steam-tight, and the ring is fixed between the flanches of that 

 joint of the steam pipe which is next to the cylinder. A square part is formed 

 on one end of the spindle, to receive an arm or lever b, by which the valve may 

 be turned in either direction. 



545. For many purposes engines are thus regulated by hand at the pleasure 

 of the attendant ; but where a regular velocity is required, means must be applied 

 to open and shut it, without any attention on the part of the person who has the 

 care of the engine. For this purpose Mr. Watt, after trying various methods, 

 fixed upon the conical pendulum, which he called a governor. (See art. 550.) 



An axis valve of this kind has much advantage over a valve of any other form 

 for a circular pipe, because it contracts the aperture without being difficult to move, 

 or presenting more than the necessary obstruction ; but it is by no means an 

 economical mode of varying the power of the steam engine. 



546. To regulate by working more or less by expansion. This may be done by 

 adjusting the motion of the steam valves, so that they may be closed at an earlier 

 or later period of the stroke, according as the engine has less or more work upon it. 

 This method is confined chiefly to regulating by hand, (see art. 481. and Plate ix.) 

 The self-acting regulator in use applies with good effect only to valve engines, as 

 neither the common slide nor cock can be adjusted otherwise than to close the 

 passage to the condenser. (See art. 448. and 456.) 



