VARIOUS EXAMPLES. 



79 



be given if the bar is supported at the middle, with a 

 weight at each end, or with the weight at the middle, 

 supported at each end, as c. This form, therefore, is a 

 proper one for many parts of imj^lements, as the bars of 

 whiffle-trees, the rounds of ladders, string-pieces of bridges, 

 and any cross-beams for supporting weights. The proper 

 form for rake-teeth and fence-posts, the pressure being 

 nearly alike on all parts, is nearly that of a long wedge, 

 or with a straight and uniform taper. Therefore a fence- 

 post of equal size throughout contains nearly twice as 

 much timber as is needed for strength only. 



The form of these parts must, however, be modified to 

 suit cii'cum stances ; as whiflle-trees must be large enough 



^-^^^r^^^^^ 



^Ki^;^ 



at the ends to receive the iron hooks, wagon-tongues for 

 ironing at the end, and spade handles for the easy grasp 

 of the hand. 



The axle-trees of wagons must be made not only strong 

 in the middle, or at centre of pressure, but also at the en- 

 trance of the hub; because the wheels, when thrown side- 

 wise in a rut, or on a sideling road, operate as levers at 

 that point, a and h (fig. 94), show the manner in which 

 the axles of carts may be rendered lighter without lessen- 

 ing the strength, a being the common form, and b the im- 

 proved one. 



