OP STEAM ENGINES. 



173 





In the annexed section, the pistonl? passes through 

 an air-right stuffing box ; and the steam is entering 

 beneath it by the four-way cock c. If we now sup- 

 pose the piston at the top of the cylinder, a new 

 arrangement of the communicating pipe then takes 

 place ; as the steam which was beneath escapes, 

 while a fresh supply enters above, and an alternate 

 upward and downward motion of the piston is thus 

 produced. 



The lever and balance-ball safety valve employed 

 in all the early boilers appear but little calculated for those en- 

 gines in which high pressure steam is employed, as the engine- 

 man, in an over anxious zeal for the full performance of the machi- 

 nery confided to his care, has been frequently known to increase 

 the internal pressure of a large boiler many thousand pounds be- 

 yond the resistance to which it was originally proved. To prevent 

 a recurrence of those accidents, which first drew the attention of 

 the legislature to this important part of the engine, it appears 

 advisable to inclose the safety valve in an iron case, of which a 

 section is annexed. 



The va!ve B in this case rests upon a 

 conical seat in the boiler A, and is fur- 

 1 with a series of small moveable 

 plates lettered r. which are employed to 

 increase or diminish the entire weight of 

 the safety-valve, the whole being covered 

 by the box D ; and as this is pierced with a 



number of small holes, the steam readily escapes when the ex- 

 pansive force exceeds the resistance offered by the loaded valve. 

 In Mr. Woolfs engine, high pressure steam is also employed; 

 but in a cylinder, or rather a pair of cylinders of unequal size. 

 These are so constructed that the steam, after it has performed 

 its office in the first cylinder, is allowed to expand into one of 

 larger dimensions, where it produces a vacuum by condensation. 

 The steam entering above the small piston while a vacuum is 

 formed beneath the large one. and vice versa. 



The valve of this engine may, however, be best understood, 

 by an examination of its effective force, when applied to the rais- 

 ing of water. A double cylinder expansion engine, was con- 

 structed for Wheal Vor mine in 1815, furnished with a large 

 cylinder of 53 inches in diameter, and making a nine-feet stroke , 

 the small cylinder being about one fifth of the size of that which 

 was employed for the expansion of the steam. The engine was 



LECT. 

 V 



