36 



NOTES ON STATIONARY ENGINES. 



accumulate and not get away, and this air is exposed to 

 alternate condensation and rarefaction. The avoidanco of 

 this trouble may, in rare cases and on long lifts, lead to some 

 extra noise in the valves, unless they are narofolly designed ; 

 but on the whole, the gain is a really important one. 



Steam being raised in the boiler is expended in the engine, 

 and this brings us to what is perhaps supposed to be the 

 chief part of the subject. 



The enormous variety in engines which obtains in the 

 -world cannot be realised without some consideration. There 

 Sire horizontal, vortical direct, vertical invented, diagonal, 

 beam and other types, also high and low pressure, condensing 

 and non-condensing, simple, compound, and triple, and each 

 of these exists in innumerably modified forms. The most 

 common form is, however, the horizontal, which for large 

 powers is almost invariably compound. Thirty years ago, 

 almost all large engines were built of the old beam pattern, 

 which though a costly arrangement, lias never yet been 

 equalled for comfort in working, and for easy access all over; 

 in fact, neglecting the one element of cost, it is almost im- 

 possible to hope that any mechanical contrivance will ever 

 be invented which will be more perfect than a well-built 

 beam engine as arranged and perfected by Watt about a 

 century ago. In comparison with other types, however, the 

 engine itself is a very costly oue, and the foundations and 

 other preparations still more so ; true, some eoonomy has been 

 effected by the use of A frames under the beam centres and 

 in other ways, but these means oidy rednco the difference, 

 and do not change its character. Of the old beam engines 

 in use at present, there are comparatively few which are not 

 compounded, and usually under the system of MoNaught. 

 In this case the high pressure cylinder is placed under the 

 arm of the beam, to which the connecting rod is attached. 



