WHEELS. 375 



riage, it is necessary again to speak of the wheels ; and 

 although I have done so before, I did not fully describe 

 their construction. Carriage-wheels, in their manu- 

 facture, require the greatest ingenuity ; they must have 

 a degree of elasticity sufficient to bear the strains that 

 they are subjected to when in motion. The simplest 

 shape in which wheels for carriages could be formed 

 would be that in which the spokes would stand exactly 

 at right angles with the axle, and would form a flat or 

 plane figure when the wheel is viewed edgeways, but 

 such a wheel would be ill adapted to meet the lateral 

 shocks to which a carriage-wheel is exposed. The 

 more common form is that called the dished wheel, in 

 which the centre or nave is made to fall back a little 

 from the plane of the felloes, so that the face of the 

 wheel is not flat but slightly concave ; the elasticity of 

 this form is a very great recommendation. It possesses, 

 also, this advantage, that if the axle be slightly bent 

 downwards towards its extremity so as to bring the 

 spokes of the lower half of the wheel into a nearly 

 vertical position which will enable them to bear the 

 greatest possible weight, the upper half of the wheel 

 will have such an inclination outward as to leave more 

 room for the body of the carriage, and to throw 

 particles of dirt caught up in its revolution away from 

 it. Very strong wheels are occasionally made in a 

 double dished form, or with the spokes alternately 

 inclining outward and inward from the felloe, so that 

 the centre or nave of the wheel forms the base of 

 a pyramid of which the felloe forms the apex ; but 

 such wheels are deficient in elasticity and consequently 

 will not bear much concussion. The dished form of 

 wheel, together with the bending of the axle, involves 

 some increase of actual friction, as it cannot possibly 

 roll in a straight line without a degree of rubbing 



