156 LECTURES TO SCIENCE TEACHERS. 



the loss by the imperfection of the outline of the figure is 

 extremely small. There is also the expansion used in the 

 locomotive by the link motion. I perhaps ought not to dwell 

 upon that now, for if time should permit 1 may say a few 

 words about the locomotive engine. 



The objection to all expansion in single cylinder engines is 

 the variation in the load and strain throughout the stroke, 

 and the necessity of making the cylinders of great size, and 

 also the proportioning of parts so as to bear the initial strain, 

 although the average pressure is small when the expansion is 

 high. To diminish those difficulties double cylinder engines 

 (the Hornblower or Wolfe engines) were invented, and are 

 now largely employed. Of these there have been several 

 constructions. A common one with beam-engines is to put 

 the high-pressure cylinder nearer to the centre of the beam 

 than is the position in which the low-pressure one is placed, 

 and by the shorter stroke thus obtained and by a diminished 

 diameter of bore the desired proportion of contents of the 

 two cylinders is obtained. We have had also cylinders placed 

 side by side at the end of the beam ; we have had the 

 high-pressure cylinder placed within the low pressure, and 

 their piston rods coupled up to one cross-head. In this case the 

 internal cylinder should always be steam-jacketted, but I do 

 not look upon this arrangement as a very happy one. 



We have also had the cylinders placed end on by putting 

 the high-pressure cylinder below the low-pressure and letting 

 the piston-rod come through. Great difficulties have been 

 experienced in arrangements of this kind by having in- 

 accessible stuffing-boxes, and so on, but these have been 

 cured in later constructions. Then we have had McNaught's 

 system, which possesses great advantages for the con- 

 version of non-expansive old-fashioned beam-engines into 

 expansive engines by the application of a second cylinder. 

 McNaught puts his expansive cylinder at that end of the 

 engine which is between the beam centre and the crank shaft, 

 and thereby instead of taking the strains of the two cylinders 

 through the beam-gudgeons and the bolts, the strain of the 

 high-pressure cylinder is brought on the crank side ; in that 

 way an engine which would not stand the application of the 

 high-pressure cylinder put in the ordinary position, will 

 support that application very well. These are the modes in 

 which compound cylinders have been employed. There may 



