170 History of' Mechanical Inventions and 



charged steam is rushing from the safety valve, or any other aperture^ it 

 may be known by its perfect invisibihty, even in the coldest day, nor can 

 it be seen at any distance from the valve or cock ; it is, however, conden- 

 sable, as may be seen by holding any cold substance in its range. 



2. On the Economy of using highly Elastic Steam expansively, S^c. By Jacob 

 Perkins, Esq. 



The diagram. Figs. 9 and 10, will show the economy of using steam ex- 

 pansively, and also the method of compensating for the inequality of the 

 pressure on the piston, which, if steam of 400lbs. to the square inch is 

 used, and stopped off at the quarter stroke, will end its stroke at lOOlbs. 

 per inch. The diagram will also show that the motion of the piston is 

 constantly varying, while that of the crank is uniform in its motion. * 



From repeated experiments and much reflection, 1 am led to believe 

 that there is great economy in using very high steam, and that expansive- 

 ly: that the high'er you can practically use the steam the sooner you may 

 put it off. The diagram shows the gain in cutting off the steam at quarter 

 stroke. Let the piston, which is represented by the line k 1 a, descend to 

 ibj being one quarter of the stroke, with a constant pressure of iOOlbs. per 

 square inch. At this point let the steam be cut off and expand to double 

 its volume ; when it arrives at h c, it will be exerting a pressure of SOOlbs. 

 per inch, producing a mean of SOOlbs. per inch through the quarter stroke. 

 Let the steam again expand to double its volume, and the piston will finish 

 its stroke at fe, at lOOlbs. per inch, giving a mean of 150lbs. per inch 

 for each quarter, which add to the other two quarters, 400. 300. 150. 150. 

 and the whole sum will be 1000 (t?) giving an average pressure of 250 

 per square inch. It will be seen that, when the stroke is completed, the 

 cylinder will be filled with steam at a pressure of lOOlbs. per inch, which 

 will be the same in quantity as though the steam had begun with a pres- 

 sure of lOOlbs. per inch, and continued all the stroke at that pressure. By 

 using the same quantity of steam expansively, beginning at 400lbs. there 

 is a gain of 150 per cent. If the steam is used at GOOlbs. per inch, and 

 cut off at one-eighth of the stroke, !sJ25 per cent will be the gain. To 

 compensate for the unequal pressure of the steam on the piston two cy- 

 linders should be used, particularly for steam-boats and pumping, where 

 the fly should be dispensed with. With the following arrangement it will 

 be seen, while one of the pistons is at its greatest power, the other is act- 

 ing with a diminished power. 



The piston 1, Fig. 9, in descending from a to b, moves in the same time 



to 120 feet ; and the new high pressure safety engine, now coming before the pub- 

 lic, would require a column more than four times as high as St Paul's cross to ba- 

 lance the steam. 



• This diagram does not pretend to mathematical accuracy ; the object is mere- 

 ly to explain to the practical mechanic, in a suiBciently clear and concise manner, 

 the principle of the advantage gained by using steam expansively. 



■f- If the steam had continued the whole length of the stroke at 4001bs. per square 

 inch, the sum would have been 16001bs., consuming four times the steam with the 

 addition of only 60 per cent, to the power. 



