63 Mr Russell on the Fallacies of the 



he consistent with such decrees of correctness as ordinary work^ 

 men can accomplish, and with such care as ordinary attendants 

 can be trusted to bestow ; also, that the progress of disrepair 

 shall be so gradual as to give timely warning of the necessary- 

 readjustment. Just such apiece of mechanism i.y the crank. It 

 is at the top and bottom of the stroke, or in the line of the can-' 

 tres, as it is technically called, that the opening and shutting 

 of the valves should take place ; and it is just at this point that 

 pressure on the piston can produce no effect on the crank ; but 

 suppose the valves not to open with absolute precision, suppose 

 them to open and shut too soon or too late, then will the error 

 at that part of the circuit be of comparatively small importance, 

 because, just then, the motion of the piston is so slight, that, 

 through an arc of twenty degrees, it does not describe one hun- 

 dredth part of a stroke, and the effect of any error in that space 

 will not affect the crank by more than one hundredth part of 

 its amount ; any error of adjustment is therefore diminished in 

 effect to one hundredth part of what would be produced, were 

 the motion of the piston to be uniform in portions correspond- 

 ing to the arc of description, as would be the case in any other 

 species of rotatory conversion. 



3d, In like manner, errors arising out of construction, ma- 

 nagement, or wear, are diminished one hundred-fold by trans- 

 mission through the crank. It has been to me matter of fre- 

 quent astonishment, that although I have seen at the mouths of 

 coal-pits, small mines, and quarries, mere remnants of engines, 

 frail rusty old fragments of iron and wood, working so loose as 

 scarcely to remain upright upon their basements, they were still 

 working within 30 per cent, of their full power. 



4th, To all these circumstances, I may add, that the constitu- 

 tion of the crank is one reason why an engine may be constructed 

 of erroneous weight, and of the most unwieldy dimensions, with- 

 out being thereby much injured in its working, because the crank 

 acquires so slow a motion at the commencement and termina- 

 tion of the stroke, that it equally slowly communicates motion 

 to all the parts of the machine, and in hke manner receives from 

 them the impetus which they give out in the act of being again 

 slowly brought to rest towards the end of the stroke. The im- 

 petus, therefore, given to the reciprocating parts of the machine 

 is lent not lost. 



