SECT. VII.] 
STEAM ENGINES. 
239 
Of the Strength of the Parts of Steam Engines. 
496. In considering this important branch of my subject, I propose to follow 
the most simple methods I can devise, and those most readily applied in practice. 
The foundation of the inquiry must be the power of the steam in the boiler, or 
rather the greatest power it can possibly acquire without escaping at the safety 
valve. Now since there is always a risk of the safety valve not being in perfect 
order, we may in a great degree provide against such risk, by taking the load on 
the valve at double the actual load upon it. Thus, if the load on the valve be 
8 lbs. on a circular inch, consider it 16 lbs.; and 16 lbs. added to 11*5 lbs., the 
pressure of the atmosphere, will give 27'5 lbs. for the strength of the steam, or the 
pressure which must cause the machine to move backwards. 
497. In the case of steam boats, a greater degree of surplus of strength ought 
to be provided, because accidents at sea are attended with more serious conse 
quences ; and I would recommend all good machinery to be regulated by the fol 
lowing rule : it is, to add the load per circular inch on the safety valve to the 
pressure of the atmosphere, and to take double this quantity as the utmost force of 
the steam : that is, if the load on the safety valve be 8 lbs. on a circular inch, let 
this be added to 11*5, the pressure of atmosphere, the sum is 19*5, and double this 
is 39 lbs. per circular inch, for the possible pressure on the piston. 
If the parts be formed to resist this pressure, then, in the case of the machinery 
being impelled backwards by an excess of resistance, they will not be injured by it, 
except where the momentum of a heavy fly wheel renders it necessary to provide 
a resistance to impulsive force. 
498. The datum for the resistance of the material must be the strain it will 
bear without a permanent derangement of its parts ; and this strain is about one- 
third of its cohesive force. 1 
499. In respect to the effect of the friction of an engine, it ought to be added 
to the power in estimating the strength ; because when the resistance is capable of 
reversing the motion of the engine, it also must have to overcome the friction of 
the intermediate parts; but when the force of the steam is considered double its 
whole pressure, as limited by the safety valve, the friction may be neglected. 
500. The stress on any of the moving parts of a steam engine may be most 
easily found by comparing the number of revolutions or vibrations it makes for 
each double stroke of the piston: the stress is inversely as the number of revo 
lutions or vibrations multiplied by the diameter of the circle, or the chord of the 
arc described by the point where the force acts : thus if a wheel 4 feet in diameter 
makes three revolutions while the piston makes one stroke, and the length of the 
1 See Practical Essay on the Strength of Cast Iron, &c. sect. v. Second edition.