28 THE HISTORY OF [SECT. i. 



first, by limiting the opening of the regulating valves which admit the steam to 

 act on the piston, and letting it continue so open during the whole length of the 

 stroke ; secondly, by letting them open fully at first, and shutting them completely 

 when the piston has proceeded only part of its stroke ; or, thirdly, by the use of 

 a throttle valve placed in the steam pipe, which, acting in the same manner as the 

 floodgate of a mill, admits no more steam than gives the desired power. 



The second of these methods of regulating the power of the engine is the best, 

 and forms the basis of Watt's Expansive Engine ; which renders available more of 

 the power of the steam than when the piston is acted upon by the whole force of 

 the steam through the entire length of the cylinder. This principle was adopted 

 in an engine erected at Soho manufactory, and some others, in 1776, and at 

 Shad well water works in 1778 ; the same principle was publicly made known in 

 1781 by Hornblower, though applied in a different manner. 



28. There yet remained another step to complete the mechanism of the double 

 engine, viz. a guide for the piston rod ; and this appears to have been first accom- 

 plished, in 1784, by the invention of the parallel motion. This is an ingenious 

 combination of levers, one point of which describes a line nearly straight, and to 

 this point the piston rod is connected, so that its rectilineal movement causes the 

 beam to vibrate. This Mr. Watt secured by patent in 1784, together with a new 

 rotative engine, in which the steam vessel was to turn upon a pivot, and be placed 

 in a dense fluid, the resistance of which to the action of the steam was to cause 

 the rotative motion ; an improved method of applying the steam engine to work 

 pumps or other alternating machinery, by making the rods balance each other; 

 a new method of applying the power of steam engines to move mills which have 

 many wheels required to move round in concert ; a simplified method of applying 

 the power of steam engines to the working of heavy hammers or stampers ; a new 

 construction and mode of opening the valves, with an improved working gear; 

 and a portable steam engine and machinery for moving wheel carriages. 



Mr. Watt obtained a patent, in 1785, for a method of constructing furnaces, in 

 which the best principles the philosophy of the period could furnish, are applied 

 to elicit the heat and consume the smoke of the fuel. He also applied to the 

 steam engine the conical pendulum as a governor, the steam gauge, condenser 

 gauge, and a useful little instrument for ascertaining the state of the steam in the 

 cylinder, called an indicator. 



29. The principal part of the theory of the action of steam, which Mr. Watt 

 has investigated on scientific principles, is the power it affords by expansion ; and 

 this is done with great clearness. He devoted a considerable portion of the latter 

 part of his life to chemical philosophy, and particularly its application to the arts. 

 As an author, he has also contributed some historical notices of his own inventions, 



