the force with which the piston is urged, it is necessary to refer to 

 both the barometer and the steam gauge. This double computa- 

 tion may be obviated by making one gauge serve both purposes. 

 If the end c of the steam gauge (fig. 7), instead of communi- 

 cating with the atmosphere, were continued to the condenser, we 

 should have the pressure of the steam acting upon the mercury in 

 the tube B A, and the pressure of the uncondensed vapour which 

 resists the piston acting on the mercury in the tube B c. Hence 

 the difference of the levels of the mercury in the tubes would at 

 once indicate the difference between the force of the steam and 

 that of the uncondensed vapour, which is the effective force with 

 which the piston is urged. 



50. Perfect as these expedients must appear, they have been 

 deemed insufficient as indicators of an element so important as the 

 economy of steam power. If, during the motion of the piston 

 from end to end of the cylinder, the steam really acted upon it 

 with an uniform force, and if the reaction against it were also 

 uniform, then the steam and barometer gauges would give an exact 

 measure of the effective power. But many causes co-operate in 

 preventing such uniformity of action and reaction. 



In the first place, the end of the cylinder from which the piston 

 moves is never left in free communication with the boiler through 

 the entire stroke. In all cases the steam is shut off by closing 

 the steam valve before the stroke is completed, and if the engine 

 works by expansion, which most engines do,, the steam is shut off 

 after a certain part of the stroke such as three-fourths, two- 

 thirds, a half, and sometimes even a third, or a fourth has been 

 made. In all such cases, the pressure on the piston after the 

 steam has been shut off becomes less and less, as the steam in the 

 cylinder expands by the advance of the piston. 



Neither is the reaction uniform; for the condensation of the 

 steam in the condenser is not absolutely instantaneous, though 

 very rapid, but still less is the removal of the air and gases, which 

 are fixed in the water injected to produce the condensation, 

 instantaneous. The action of the air pump is gradual, and con- 

 sequently the reaction on the piston, considerable at first, becomes 

 gradually less and less towards the end of the stroke. 



Now it is clear that, under these circumstances, the effective 

 power of the piston, being always measured by the excess of the 

 impelling force over the reaction, must vary continually from the 

 beginning to the end of the stroke ; and as the total effective force 

 must consist of the aggregate of this varying action, it would seem 

 to be a problem of the greatest practical difficulty to ascertain it. 



51. Nevertheless, the inexhaustible resources of the genius of 

 Watt, which surmounted so many other difficulties, did not shrink 



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