Portable Agricultural Steam Engines at Newcastle. 717 
which, according to Table IX., page 729, was at the rate of 
43,688 units per hour, added to 17,536 units, the excess of 
cooling arising from the high temperature of the smoke-box ; 
and thirdly, in ashes and unconsumed fuel amounting to 12,309 ' 
units, making altogether 550,643 units, leaving 12,002 units 
unaccounted for, a little more than 2 per cent. As we have 
carried the working no lower than 385^, we have nothing to 
do with what takes place below that temperature, and hence 
the simplicity of this mode of calculation. 
The maximum temperature, and consequently the highest 
duty, will be attained when only the quantity of air theo- 
retically necessary for combustion is used ; every addition of air 
lowers the temperature, and carries off heat wastefuUy among 
the hot products of combustion escaping by the chimney. It is 
obvious from the foregoing that, with a given temperature for 
the escaping products of combustion, the useful effect will 
increase as the temperature of the furnace is heightened. The 
temperature of the chimney, 385°, in this case could hardly 
have been reduced much lower, for the steam temperature was 
334, leaving a difference of 52° only. Mr. Foden used less 
air than any of the other exhibitors. This arose from the 
precaution he took of closing his chimney damper every time 
he opened his fire-door. In the peculiar way of managing 
the furnace always adopted at " Trials," namely of firing very 
often, laying on small quantities of coal at a time, the fire- 
doors are open a great deal, and much air enters in that way, 
never passing through the fuel at all. The effect of this can 
be easily seen on the thermometer in cne smoke-box, for when 
the door or the ash-pan damper is opened, the mercury falls to 
a considerable extent at once. Had the other exhibitors been 
as cautious as ^Ir. Foden, still better results might have been 
attained. 
Reverting to the Balance-Sheet and to its Credit side, it will 
be seen that items 1 to 5, involving, as a whole, an absorption of 
7 per cent, of the heat produced by the fuel, are inevitable 
losses, so that indeed they might have been dealt with by de- 
ducting their amount from the fuel, before considering the heat 
that is available for work ; but item 6, which relates to the 
j excess of air, and comprises the two heads of heating that excess, 
I ' and of displacing the atmosphere for its reception, gives a further 
amount of loss = 6"34 per cent., which is (at all events to some 
extent) preventable. In the case of this engine, No. 3125, 
we have an excess of air weighing 12*31 lbs. for each lb. of 
fuel burnt, being, therefore, practically equal to the air which is 
theoretically needed, — while in engine No. 3114 the excess was 
