THE STEAM-ENGINE. 



If this explanation be comprehended, it will not be difficult to apply it to the 

 actual case of the governor. If a sudden increase of the energy of the moving 

 power, or a diminution of the load, should give the machine an increased ve- 

 locity, then the increased speed of the balls of the governor will give them an 

 increased centrifugal force, which for the moment will be greater than the ten- 

 dency of their gravitation to make them fall toward the vertical axis. This 

 centrifugal force, therefore, prevailing, the balls will recede from the axis ; but 

 as they recede, their gravitation toward the vertical axis will, as has been al- 

 ready explained, be increased, and will become equal to the centrifugal force 

 produced by the increased velocity, provided that velocity do not exceed a 

 certain limit. When the balls, by diverging, get such increased gravitation as 

 to balance the centrifugal force, then they will continue to revolve at a fixed 

 distance from the vertical axis. When this happens, the time of the revolu- 

 tion must be nearly the same as it was before their increased divergence ; in 

 other words, the proportion of the moving power to the load will be so restored 

 by the action of the levers of the governor on the throttle-valve that the machine 

 will move at its former velocity, or nearly so. 



The principle on which the governor acts, as just explained, necessarily 

 supposes temporary disarrangements of the speed. In fact, the governor, strictly 

 speaking, does not maintain a uniform velocity, but restores it after it has been 

 disturbed. When a sudden change of motion of the engine takes place, the 

 governor, being immediately affected, will cause a corresponding alteration in 

 the throttle-valve ; and this will not merely correct the change of motion, but it 

 will, as it were, overdo it, and will cause a derangement of speed of the oppo- 

 site kind. Thus if the speed be suddenly increased to an undue amount, then I 

 the governor being affected will first close the throttle-valve too much, so as to 

 reduce the speed below the proper limit. This second error will again affect 

 the governor in the contrary way, and the speed will again be increased rather 

 too much. In this way a succession of alterations of effect will ensue until 

 the governor settles down into that position in which it will maintain the en- 

 gine at the proper speed. 



To prevent the inconvenience which would attend any excess of such varia- 

 tions, the governor is made to act with great delicacy on the throttle-valve, so 

 that even a considerable change in the divergence of the balls shall not pro- 

 duce too much alteration in the opening of that valve : the steam in the boiler 

 should have at least two pounds per square inch pressure more than is gener- 

 ally required in the cylinder. This excess is necessary to afford scope for 

 that extent of variation of the power which it is the duty of the throttle-valve 

 to regulate. 



The governor is usually so adjusted as to make thirty-six revolutions per 

 minute, when in uniform motion ; but if the motion is increased to the rate of 

 thirty-nine revolutions, the balls will fly to the utmost extent allowed them, be- 

 ing the limitation of the grooves in which their rods move ; and if, on the other 

 hand, the speed be diminished to thirty-four revolutions per minute, they will 

 collapse to the lowest extent of their play. The duty of the governor, there- 

 fore, is to correct smaller casual derangements of the velocity ; but if any per- 

 manent change to a considerable extent be made either in the load driven by 

 the machine or in the moving power supplied to it from the boiler, then a per- 

 manent change is necessary to be made in the connexion between the governor 

 and the throttle-valve, so as to render the governor capable of regulating those 

 smaller changes to which the speed of the machine is liable. 



Having thus explained the principal mechanical contrivances provided by 

 Watt for the maintenance and regulation of the rotatory motion to be produced 

 by his double-acting steam-engine, let us now consider the machine as a whole, 



