436 THE STEAM-ENGINE. 



principle of the expansive action of steam, which subsequently proved to be 

 of such importance in the performance of steam-engines. Watt was, howev- 

 er, so much engrossed at that time, and subsequently, by the difficulties he had 

 to encounter in the construction of his engines, that he did not attempt to bring 

 this principle into operation. It was not until after he had organized that part 

 of the establishment at Soho which was appropriated to the manufacture of 

 steam-engines, that he proceeded to apply the expansive principle. Since 

 the date of the patent which he took out for this (1782) was subsequent to 

 the application of the same principle by another engineer, named Hornblower, 

 it is right to state that the claim of Mr. Watt to this important step in the im- 

 provement of the steam-engine, is established by a letter addressed by him to 

 Dr. Small, of Birmingham, dated Glasgow, May, 1769 : 



" I mentioned to you a method of still doubling the effect of the steam, and 

 that tolerably easy, by using the power of steam rushing into a vacuum, at 

 present lost. This would do little more than double the effect, but it would 

 too much enlarge the vessels to use it all : it is peculiarly applicable to wheel- 

 engines, and may supply the want of a condenser, where the force of steam 

 only is used ; for open one of the steam-valves, and admit steam until one 

 fourth of the distance between it and the next valve is filled with steam, then 

 shut the valve, and the steam will continue to expand, and to press round the 

 wheel, with a diminishing power, ending in one fourth of its first exertion. 

 The sum of the series you will find greater than one half, though only one 

 fourth of steam was used. The power will indeed be unequal, but this can be 

 remedied by a fly, or by several other means." 



In 1776, the engine, which had been then recently erected at Soho, was 

 adapted to act upon the principle of expansion. When the piston had been 

 pressed down in the cylinder for a certain portion of the stroke, the fur- 

 ther supply of steam from the boiler was cut off, by closing the upper steam- 

 valve, and the remainder of the stroke was accomplished by the expansive 

 power of the steam which had already been introduced into the cylinder. 



To make this method of applying the force of steam intelligible, some pre- 

 vious explanation of mechanical principles will be necessary. 



If a body which offers a certain resistance be urged by a certain moving 

 force, the motion which it will receive will depend on the relation between 

 the energy of the moving force and the amount of the resistance opposed to it. 

 If the moving force be precisely equal to the resistance, the motion which the 

 body will receive will be perfectly uniform. 



If the energy of the moving force be greater than the resistance, then its 

 surplus or excess above the amount of resistance will be expended in impart- 

 ing momentum to the mass of the body moved, and the latter will consequently 

 continually acquire augmented speed. The motion of the body will therefore 

 be in this case accelerated. 



If die energy of the moving force be less in amount than the resistance, 

 then all that portion of the resistance which exceeds the amount of the moving 

 force will be expended in depriving the mass of the body of momentum, and 

 the body will therefore be moved with continually diminished speed until it be 

 brought to rest. 



Whenever, therefore, a uniform motion is produced in a body, it may be 

 taken as an indication of the equality of the moving force to the resistance ; 

 and, on the other hand, according as the speed of the body is augmented or 

 diminished, it may be inferred that the energy of the moving force has been 

 greater or less than the resistance. 



It is an error to suppose that rest is the only condition possible for a body 

 to assume when under the operation of two or more mechanical forces which 



