THE STEAM-ENGINE. 149 



still get an effect of 31 without condensation, or with con- 

 densation 44, and the gain per cent, therefore is 42. 



No. 9 is a very similar Diagram, showing you the effect 

 of using 120 Ibs. steam with condensation, but without 

 expansion, and also where you expand that steam twice 4 

 times, 8 times, 16 times, and 32 times, the net useful effects 

 being 118, 199, 277, 352, 418|, 468-f, and the ratio 1, 1-68, 

 2-34, 3, 3-54, 3-89. 



One sees, therefore, that for economy, condensation is a 

 most necessary thing. More especially when the pressures 

 are low, condensation really becomes a duty. There are 

 various means of effecting it. There is the old injection 

 condenser invented by Watt, where the stream of water goes 

 into a chamber and the condensed steam and the liberated air 

 are withdrawn by means of an air-pump. This requires a 

 constant supply of cold water, and that cannot always be 

 obtained in sufficient quantities. Then we have to resort to 

 means of recooling. One of the commonest is, as we know, 

 a cooling pond, but that requires a considerable amount 

 of space. Another one is a pile of brushwood, down which 

 the water is allowed to trickle. In both those cases the loss 

 by evaporation equals the feed water. Another method was 

 that pursued by Howard to be used with his quicksilver boiler. 

 He passed the injection water through pipes which were 

 outside the hull of the vessel, and which were acted upon 

 by the sea-water, and in that way he recooled the injection 

 water and used the same water over and over again. It 

 was a necessity for him to have pure water, because with 

 the boiler I have described a deposit upon the surface above 

 the amalgam if at all serious would very rapidly have caused 

 the boiler to burn out. Then we have the surface condensers, 

 of which we found a model yesterday in Mr. Watt's case, 

 This condenser is largely, in fact almost universally employed 

 now-a-days in marine engines. The proportions of it vary 

 very much according to the rapidity with which the water 

 is driven through, and according also to the temperature at 

 which the water is expected to be found. A steam-boat 

 intended to navigate the Eed Sea should be provided with a 

 much larger surface in her condenser than one intended for 

 the Atlantic. There is a very elaborate paper by a French 

 engineer, an abstract of which has been translated and will 

 be found in the Proceedings of the Civil Engineers, which 



