160 Steam-Engine—Physies, yc. 
sible in the steam, which thereupon acquires expansive force to 
an unlimited degree: that this force increases geometrically; 
or, that every accession of about 30° of heat, nearly doubles its 
power at those stages of progression ; that when the pressure at 
4 high temperature is taken off, or the steam allowed to flow, 
there is an instantaneous and rapid production of steam ; a fact 
which proves there can be no necessity of a large space for the 
steam to form in above the water, provided it be sufficient te 
prevent water from issuing with the’ steam, and, therefore, that 
boilers of a small cylindrical form are best. 
It may be a@ fair question, why Mr. Watt did not further 
employ this principle of expansive force? We may readily 
conceive of several motives to the contrary. Watt and Bolton’s 
engines were in great demand; they gave entire satisfaction, 
and the work they performed saved’ so much labour as to afford 
the purchase at a high price. The public had gained immensely 
by this better form of the engine, and Mr. Watt enjoyed the 
benefits of the patent he had obtained ; and, at a later period, 
this preference was increased by an accident which happened 
to Trevethick’s engine, though caused by gross mismanage 
ment, that would have been equally fatal to any other. 
From an investigation, by a committee of parliament, into 
the causes of the several fatal explosions of steam-engine boil- 
ers within a few years, published in Tillock’s Magazine, vol. 
1. it appears that in every instance the accident was fairly 
attributable to neglect or mismanagement. Many competent 
persons were summoned to give their opinions; and throug? 
the contrariety of their testimony, the prevalent opinion 4)” 
pears to have been; that cast-iron boilers cannot be safes 
as many engines of high steam as of low are now used in Enz’ 
land, but that the high are much the most economical iD fuel 
and cost; that they are more safe, if properly constructed; it 
being argued by some, that boilers fer steam of 100 pounds sg 
the inch, are easily made of strength to sustain 500 poundss 
this excess being much greater than in those constructed for 
low steam, makes them comparatively the safest, as the salety 
valves are less liable to be accidentally prevented from 1 
ing the steam. 
