33 APPLIED SCIENCE 



where these forces are given for each position. When the speed 

 is increased to 4 revolutions per second, these forces are mul- 

 tiplied by sixteen, and their effect is then much greater than 

 that of the weight of the parts and suffices completely to change 

 the character of the effort curve. The negative portion of the 

 curve lasts for nearly half the revolution of the crank, so that 

 for nearly half of each revolution the fly-wheel would have to 

 pull the engine round. This is true both for B 3 and B 4 for the 

 curves without and with friction and is simply due to the fact 

 that during the first half of each stroke the reciprocating masses 

 are being positively accelerated. The positive ordinates during 

 the period when the engine is driving of course exceed those 

 during which the engine is being driven ; so that for curve B 3 

 the balance of positive area ought to be 3,216 inch Ibs. It 

 actually is, on the drawing, 3,256, the excess being due to 

 small errors in drawing and computation. The area of B 4 is 

 only 1,744 inch Ibs. 



The efficiency, therefore, has sunk to O536 ; almost half 

 the power of the engine is taken up in driving itself ; the pres- 

 sures on the joints caused by resistance to acceleration have at 

 this high speed greatly increased the loss due to friction. The 

 inequality between back and front strokes is also very great, 

 the area of the front branch being 953 inch Ibs., that of the 

 back branch only 791. The loss due to friction sinks, however, 

 almost to nothing at one point of the front stroke, very near the 

 place where, in Example A 4 , it was a maximum. This may 

 serve as a warning against hasty generalisations. In curve B 4 

 there are sensible sudden changes of efficiency at points and 

 12, due to a sudden change in the position of the points where 

 the elements bear 011 one another. 



36. Example G Y , Fig. 48. Example C was selected with 

 the object of ascertaining how far the efficiency is affected by 

 using the steam expansively instead of admitting it throughout 

 the stroke. There is a very general idea that the sudden 

 shock, as it is called, of admitting steam at a much higher 

 pressure for a short time at the beginning of the stroke must 

 diminish the efficiency of an engine. This is not so in the 

 present example. An imaginary indicator diagram was selected 

 for Example C, drawn on the supposition that lhe .steam was 



