Perkins' Steam Engine. 61 



piston, fig. 2, in moving from 6 to e, gives a power of 25lbs., 

 being the last of the expansion which ends at 1 OOlbs. per inch. 

 The piston, fig. 2, in moving from/ to 8, being the beginning 

 of the stroke, gives a power of 1 OOlbs.; thus a power of 

 125lbs. will be acting on the piston 1, fig. 1, while moving 

 from b to d, giving a power of 475lbs. to which add 125, 

 will show a power of 600lbs. at this part of the stroke. The 

 piston 1 , fig. 1 , now descends from d to e, being the last quar- 

 ter of the stroke, giving 125lbs. of power to act with the pis- 

 ton 1 , fig. 2, while moving from 8 to A, giving a power of 

 600lbs., add to this the 125lbs. and it will give a power of 

 725lbs. at this part of the stroke. The piston 1 , fig. 1 , now 

 begins its stroke of 400lbs. per inch at /, and continues to g, 

 with the same power, while piston 1 , fig. 2, moves from h to 

 12 giving a power of 300lbs. to be added to the 400lbs., ob- 

 tained at the first quarter of the stroke of the piston 1 , fig. 1 , 

 at f and g, producing at this part of the stroke, 700lbs. of 

 power. The piston 1 , fig. 1 , now moves from g to i, giving 

 a power of 475, while the piston 1, fig. 2, moves from 12 to & 

 and a to 2, giving a power of 125, which add to 475, gives 

 a power of 600 at this part of the stroke. The piston 1 , fig. 

 1 , now moves from i to 1 , being the last quarter of the stroke, 

 giving a power of 125lbs., while the piston, fig. 2, moves from 

 2 to c, producing a power of 600 ; to which add 1 25lbs. will 

 make 725lbs. at this part of the stroke. 



By this arrangement, it will be seen, that a compensation 

 is obtained, giving a more equable power than that which is 

 produced by the single engine, whether high or low pressure, 

 since it is well known, that at two points of the revolution of 

 the crank, the power ceases, during at least one-twelfth of 

 the time, which is the reason that so large a fly wheel is ne- 

 cessary. It is particularly applicable to steam boats, and 

 may be used to great advantage in the double pump, as well 

 as the balance-bob lifting pump, used in Cornwall for mining 

 purposes, by the use of proper gearing. The present single 

 stroke expansive engines, used in Cornwall for pumping, are 

 preferred to all others, on account of their economy, al- 

 though they are very limited as to the extent of the expansive 

 principle, for want of compensation, as nearly the same 

 power is wanted to finish the stroke of the pump, as to be- 

 gin it. 



The variation of the velocity of the piston, occasioned by 

 the compound motion of the crank, and connecting rod, is 



