. 
effective intensity of pressure of 2 lbs. per square inch, as in examples A and B. 
: The work done by the steam per stroke is, therefore, as before, 3212 inch lbs. 
_ The speed of the engine is taken, as for curve A, at one revolution per second. 
{ Curves C; and C, give the effects with and without friction; the area of C; is 
3223 inch lbs., showing a very small error of execution; the area of C, is 2640 ; 
the ratio of these values gives 0°819 as the efficiency of the engine worked in 
_ this way. This efficiency is actually a little higher than that of curve A, which 
_ is repeated in this figure to allow it to be more readily compared with C. 
A similar result is to be observed in curve D, fig. 49, Plate XX VIIL., con- 
structed from the same data, but at the high speed of 4 revolutions per 
second. The area of D; is 3270 inch lbs., that of D, 1965 inch lbs., giving an 
efficiency of 0:°601—a value sensibly higher than that derived from curve B 
when the steam was admitted throughout the stroke. The errors due to 
_ imperfect drawing have generally the result of slightly increasing the effort, and 
the error in curve D, for this particular drawing (3270 over 3216) is nearly 1:7 
per cent. The liability to error is much increased when, as here, a large 
portion of the area is negative. If this percentage were reckoned on the 
arithmetical sum of the areas, instead of on their difference, it would be insignifi- 
cant. Notwithstanding this inevitable imperfection, there is every reason to 
expect that the errors in curves D, and D, resemble one another ; and we have 
the less reason to suspect the accuracy of the conclusion, because we can see 
that since the tendency of resistance to acceleration during the beginning of 
each stroke is to diminish the effort, while that of a large initial pressure is to 
increase it, the two tendencies counteract one another without causing pressure 
~ on the main-bearings or crank-pin. Thus, in curves B,; and B, we found that 
the loss due to friction at positions 5 and 18 was much reduced from this cause. 
~ Incurves C; and C, we have this useful result of the mass of the moving parts 
without the reversal of the stresses which brings down the efficiency in B,; and 
'B, Similarly, in curves D, and D,, the effect of the high initial pressure is to 
‘prevent the negative parts of the curve from falling so low as in example B. 
‘The extremely low efficiency of A, B,, and C, illustrates the evil effect of 
‘using a large and heavy engine running with small mean pressures of steam. 
We not unfrequently see large engines ordered for factories, mines, or water- 
works to allow for subsequent extension, or for large variations in the work 
required at different times. The above cases show how very serious the loss 
may be when an engine is habitually worked much below its power. The 
cases are no doubt extreme, but they show the tendency of the practice. 
_ § 37. Example E, figs. 50 and 51, Plates X XIX. and XX X.—The four cases 
hitherto analysed are not, strictly speaking, practical cases: they were chosen 
r as to bring into prominence the effects of friction and high speed, separ- 
TO THE DETERMINATION OF THE EFFICIENCY OF MACHINERY. 707 
























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