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 Ibs. on a circular inch, consider it 16 Ibs. ; and 16 Ibs. added to 1T5 Ibs., the 

 pressure of the atmosphere, will give 27'5 Ibs. 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 Ibs. 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 Ibs. 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. 



