LoDDER. — On Compound Engines. 149 



then the same quantity of coal to evaporate the same number of pounds of 

 water (same as in the first example) this will give a consumption of 8-9, or say 

 9 cwt. per hour, with an increase of power equal to 143 horse-power more 

 than in the preceding example, and the consumption would be 1*2 lbs. per 

 indicated horse-power per hour. 



Taking this then as a purely comparative statement, it shows an increase 

 of power and at the same time nearly 50 per cent, saving of fuel. It must 

 not be taken to mean that these calculations which leave out many sources of 

 loss of heat and force, are likely to be attained in practice in any altered 

 engines, but the least result of 1 '2 lbs. per indicated horse-power per hour has 

 been surpassed by compound engines. 



It has been stated by some that equally good results could be obtained 

 with using high -pressure steam in single cylinder engines, and cutting of at a 

 fractional portion of the stroke. There are objections to this plan ; for instance, 

 in expanding the steam say 6 to 1, as in the other cases, the terminal pressure 

 would be very great, and totally lost as far as exerting any power is con- 

 cerned, unless it was a very long stroke, and this for screw-engines is imprac- 

 ticable ; besides, the vacuum would not be nearly so good, and there would be 

 more loss by condensation than with compound engines. 



The compound engine uses the steam down to its very lowest pressure, 

 and none is lost, except a little by condensation, and this can be reduced to 

 about 1*5 lbs. 



From what has been advanced it will be seen that there can be little 

 doubt of the superiority of the compound engine in point of economy over the 

 old system with low pressure steam and jet condenser. There is not such a 

 low consumption per indicated horse-power with the " Star of the South's " 

 engines as is stated to be got on the trial trips at home from some of the large 

 boats, but the surest test is, when knowing the consumption and. speed of a 

 certain vessel with the old system to compare the obtained results after 

 conversion, as has been done in the " Star of the South's " case, and a saving 

 in fuel proven, of 42*1 per cent., after six months running, with no diminution 

 in speed, but an increase of one per cent., as shown by the above tables; and 

 no doubt even a better result would be obtained with new compound engines 

 than by converting old. 



If we consider the two theoretical examples given we find a very small 

 consumption per indicated horse-power. There is no doubt but a very great 

 saving could be effected in a vessel of the class selected. 



We now come to some of the disadvantages of using surface condensers 

 and high-pressure steam ; and first, with regard to priming, it is one of the 

 phenomena of ebullition, and occurs more or less in all boilers using surface 

 condensers, whether with high or low pressure steam, irrespective of the kind 



