Agricultural Steam-engines at Newcastle. 
675 
engine boilers tried at Newcastle had more than ample steaming 
capacity. 
A brief word must, finally, be said as to the rationale of high 
pressures, and their influence upon economy in the compound 
engine. It has already been shown that there is a fixed, although 
not regularly progressive, relation between the pressure and the 
temperature of steam. For example, steam at a pressure of 
1 lb. per square inch has a temperature of 102°, while an 
increase of 25° in this temperature only raises the pressure 
by a single pound. On the other hand, a rise of 25° in the 
temperature of steam at a pressure of (say) 260 lbs. is accom- 
panied by an increase of 80 lbs. in its pressure. Evidently, 
therefore, it is better policy on the part of the steam-engineer 
to spend his units of heat in the higher, rather than the lower 
ranges of temperature, the interest on capital, so to speak, being 
so much greater in one quarter than in the other. 
But the advantage has corresponding drawbacks. The diffi- 
culty of the steam-engineer in regard to heat is, how, having got 
it, to keep it. Among such higher tensions as are now 
available in practice, a drop of ten degrees in temperature 
means a loss of 20 lbs. pressure, while a similar drop in the 
tensions in vogue only a few years ago involved a loss of only 
8 lbs. pressure ; so that condensation and re-evaporation are 
enemies, more than twice as dangerous now as formerly. It is 
of no use laying out heat-units prudently, and then squandering 
the proceeds in riotous condensation. 
Happily, the Compound engine is no prodigal. Heat-energy, 
in the form of steam, is better trusted to the keeping of its 
cylinders, because their range of temperature is very much 
shorter than that of the cylinder in a Simple engine. To 
increase steam-pressure, and cut off still earlier in the Simple 
engine, with a view of rivalling the economy which follows upon 
compounding, only aggravates the evils from which this type of 
engine already suffers. A point may, indeed, be reached where 
all the economy derivable from greater expansion in a single 
cylinder is more than lost by the increased condensation and 
re-evaporation which follows upon its use. It is even probable 
that makers of compound portables will find it best to fix the 
point of cut-off in the small cylinder, for, this being well chosen, 
little further fuel economy will be gained from the use of a 
variable automatic expansion gear. Lastly, a very early cut-off 
in a single cylinder, using steam of high tension, means very 
unequal impulsion of the crank, and great inequalities in the 
strains thrown upon all the parts of the engine. With a fairly 
long admission, the shocks so caused are of little moment, 
VOL. XXIII.— S. S. 2 T 
