CALCULATING THE STRENGTH OF PARTS. 81 



Again, the strength of a round stick increases as the cube 

 of the diameter increases ; that is, a round piece of wood 

 three inches in diameter is eight times as strong as one an 

 inch and a lialf in diameter, and twenty-seven times as 

 strong as one an inch i:i diameter. This rule shows that 

 a fork handle an inch and a half in diameter at the middle 

 is as much stronger than one an inch and a quarter in 

 diameter, as seven is greater than four. Now this rule 

 would enable the farmer to ascertain this without break- 

 ing half a dozen fork handles in trying the experiment, 

 and it would enable the manufacturer to know, -without 



Fig. 95. 



the labor of trying many experiments, that if he makes a 

 fork handle an inch and a half at the middle, tapering a 

 quarter of an inch toward the ends, it will enable the 

 workman to lift with it nearly twice as much hay as with 

 one an inch and a quarter only through its whole length. 

 A mode of adding strength to light bars of wood, by 

 means of braces, is shown in fig. 95, representing light 

 whiffle-trees, stiffened by iron rods in a simple manner. 

 The same method is sometimes adopted to advantage in 

 making light fruit ladders, and for other purposes. 



CHAPTER VI. 



FRICTION. 



The subject of friction has been postponed, or merely 



alluded to, to prevent the confusion of considering too 



many things at once. As it has an important influence on 



the action of machines, it is worthy of careful investigation. 



4* 



