APPLICATION OF MECHANICAL PRINCIPLES, ETC. 101 



tongues for ironing at the end, and spade handles for 

 the easy grasp of the hand. 



The axle-trees of wagons must he made not only- 

 strong in the middle, or at centre of pressure, hut also 

 at the entrance of the hub ; because the wheels, when 

 thrown sideways in a rut, or on a sideling road, operate 

 as levers at that point, a and h {Fig. 86), show the 

 manner in which the axles of carts may be rendered 

 lighter without lessening the strength, a being the 

 common form, and b the improved one. 



Fig. 86. 



Sometimes several forces act at once on different 

 parts. For example, the spokes of wagon- wheels re- 

 quire strength at the hub for stiffening the wheel ; they 

 must be strong m the middle to prevent bending, and 

 large enough at the outer ends, where they are soonest 

 weakened by decay. Hence there should be nearly a 

 uniform taper, slightly larger at the middle, and with 

 an enlargement at the outer end, as c {Fig. 86). 



A very useful rule in practice, in giving strength to 

 structures, is this : the strength of every square beam 

 or stick to support a weight increases exactly as the 

 width increases, and also exactly as the square of the 

 depth increases. For example, a stick of timber eight 



