FRICTION OF CARRIAGES

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lead to essential service in the formation ofthe road; and, perhaps, be of great utilityin ascertaining the proper force of carriages.It is, however, to be regretted that no experi-ments have yet been made to ascertain therequisite information.

In constructing wheel-carriages, with a viewof diminishing their friction, it would appearthat we should make the diameter of thewheels as large, and the axles as small aspossible; but there is a limit to this, inde-pendent of the inconvenience, in loading andunloading carriages, so much elevated, bybeing placed upon very high wheels; theaxles must be made sufficiently strong tosupport the weight placed upon them, andthat strength is, in a great measure, regulatedby the size of the wheels, for the larger thewheels the greater the leverage to twist andbreak the axles; but, as the tendency to breakthe axles increases only in the direct propor-tion of the diameter of the wheels, while thestrength of the axles increases in the cube ofthe diameter, it will, in all cases, be of advan-tage to make the wheels as large as circum-stances will permit.

In all rubbing surfaces there must be acertain proportion between the pressure, orweight of the body, and the extent or area