Portable AyricuUural Steam Engines at Newcastle. 697 
with that of Cardiff. Table V., on page 726, gives the com- 
parative results. 
Taking the second line, the figures of which are strictly com- 
parable, it will be seen that the prize simple-engine at Newcastle 
consumed 2 • 678 lbs. of coal per brake horse-power against 2 * 79 
used by the prize engine at Cardiff, a gain of 4 per cent., while 
the prize compound-engine consumed only 1 • 902 lbs. of coal per 
brake horse-power, a gain of 32 per cent., or nearly one-third. 
Whence comes the important improvement which these trials 
have so conclusively established ? It is not easy to imagine that 
Messrs. Davey Paxman's simple engine was designed any better 
or was more accurately fitted than Messrs. Clayton and Shuttle- 
worth's engine, which carried off the first prize at Cardiff ; nor can 
it be supposed that the handling of the engine at Newcastle was 
better than that at the earlier trial. The cause, therefore, of the 
success achieved must be sought chiefly in the thermo-dynamic 
principles involved. The Royal Agricultural Society, in the 
supposed interests of public safety, had at Cardiff, and previously, 
restricted the steam pressure at which engines were allowed to 
"work ; but as design, materials, and workmanship improved, the 
restrictions were relaxed, and on the introduction of the com- 
pound system were practically removed altogether. 
It is a fundamental principle of all heat engines that the 
quantity of work done depends on the amount of heat which is 
converted into work ; steam, air, or gas of any kind, is merely 
the agent by which the conversion takes place, and can do no 
work without falling in temperature or yielding up heat. 
Hence it follows that the greater the fall of temperature during 
the working of a heat engine the greater the proportion of 
work got out of the agent. Supposing it were possible to get 
all the heat out of a perfect gas, a temperature would be 
reached which is known as " Absolute Zero," and is minus 
460° on the Fahrenheit scale. " Absolute temperature " means 
the temperature measured from absolute zero ; hence, to con- 
vert our ordinary temperatures into absolute, it is only neces- 
sary to add 460° to them. Now the doctrine of Carnot asserts 
that the proportion of work which it is possible to obtain 
from the heat depends upon the ratio which the fall of tem- 
perature in the working substance bears to the original absolute 
temperature ; in other words it depends upon what propor- 
tion of all the heat from absolute zero can in practice be 
wrested from the working agent. Obviously there are two 
ways in which this proportion may be increased : the one, the 
not allowing the agent to escape till it has been cooled low 
down, so as to yield up more heat : the other, to start at a 
higher temperature. This is true as well of the generation of 
