260 M. Arago's Biographical Memoir of James Watt. 



stand the trial. Seven long years of litigation excited a 

 feeling of vexation which sometimes manifested itself in bit- 

 ter terms, " We have been so beset with plagiaries," he 

 remarks, " that if I had not a very distinct recollection of 

 my doing it, their impudent assertions would lead me to doubt 

 whether I was the author of any improvement on the steam- 

 engine ; and the ill-will of those we have most essentially 

 served, leads them to canvass whether such improvements have 

 not been highly prejudicial to the commonwealth.''' 



But, though much irritated. Watt did not suffer himself to be 

 discouraged. At first, his machines, like those of Newcomen, 

 were nothing more than simple pumps, in other words, simple 

 means of raising water. In a few years, however, he trans- 

 formed them into machines capable of producing all kinds of 

 movements, and of indefinite power. His first step in this new 

 career was the construction of the double-acting engine. To 

 understand the pi'inciple of this engine, we must refer to the 

 modified machine, already explained (p. 253). In it, as we have 

 seen, the cylinder is closed ; the external air is excluded ; it is 

 the pressure of steam, and not that of the atmosphere, which 

 produces the descent of the piston ; and finally, it is a simple 

 counterpoise which produces the ascending movement, for at 

 the moment the movement is effected, the steam being allowed 

 to circulate freely between the upper part and the lower of the 

 cylinder, presses the piston equally in the two opposite direc- 

 tions. Hence, it will be clearly perceived, that the modified 

 machine, like that of Newcomen, has no real force except du- 

 ring the descending stroke of the piston. 



A very simple change will remedy this serious defect, and 

 present us with the double-acting engine. In the engine known 

 under this name, as in that which we designated the Modified 

 Engine, the steam of the boiler, when required, passes in freely 

 above the piston, and depresses it without encountering any 

 obstacle, because, at the moment, the lower part of the cylin- 

 der is in direct communication with the condenser. This 

 movement once accomplished, and a certain stopcock being 

 opened, the steam issuing from the boiler now rushes under- 

 neath the piston only, and elevates it, whilst the upper steam 

 which had produced the descending movement, passes off to 



