40 WATT. 



Isaac Newton, in the ' Principia,' that if a circle moves 

 upon another of twice its diameter, each point describes 

 a straight line. This is precisely the principle of the 

 parallel motion. Three beams are made to revolve 

 round different centres, two of these being moveable in 

 the arm of the engine, and one fixed without it. These 

 three are connected together and with the piston-rods 

 of the cylinder and the pumps, which their revolutions 

 cause to describe accurately straight lines. 



A fifth invention is the Float, which, placed on the 

 surface of the water in the boiler, descends until the 

 water is so low as ito require a supply ; it then opens 

 a valve which lets in the quantity wanted; for, as soon as 

 it rises to a certain height, the valve is shut by the float. 



The most refined contrivance of the whole may now 

 be mentioned, in the sixth and last place, the adaptation 

 of the Governor, previously used in wind and water 

 mills. It is evident that the velocity of the working- 

 may be increased beyond what is required or con- 

 venient without the safety-valve giving any indication 

 of the excess, and also that the warning given by this 

 valve does little more than point out the risk without 

 providing the remedy or preventive. The governor 

 is a far more subtle invention. Two balls are fixed 

 to the end X)f arms which are connected with the 

 engine by a moveable socket ; this can play up and 

 down a vertical rod revolving by a band on the 

 axis or spindle of the fly-wheel, and it revolves, there- 

 fore, with the velocity of that spindle. The arms are 

 perfectly moveable on their centres, which are fixed in 

 the socket and on opposite sides of it. Their centrifu- 

 gal force, therefore, makes them diverge, more or less, 

 in proportion as the rotatory motion of the spindle and 

 consequently the velocity of the engine increases; 

 their divergence pushes the collar up the spindle, its 

 axis, and as it rises, it closes, by means of cranks, a 

 valve called the "throttle-valve," in the pipe which 

 conveys the steam from the boiler to the cylinder, and 



