WHEELS.—CASKS.

207

The fellies of carriage-wheels, rn, m, m, fig. 18, formalso an annular surface of revolution. At the centreof the wheel is a solid piece, called the nave, ABCD.The spokes, or radii, at equal distances from one another,connect the nave, which is also a surface of revolution, withthe ring formed by the fellies. The latter, consisting ofportions equal to one another, are covered by the tire, aflat band or hoop of iron, which extends over the wholecircumference of the wheel, and covering the joinings ofthe fellies, is nailed to them.

Some wheels have all the spokes in the same plane,rRRr, fig. 18, and on them the tire is at right angles tothis plane, and forms a cylinder.

In other wheels the spokes, Ss, S.s, are placed in thedirection of so many edges of a right lined circular cone.The tire being in them at right angles to the directionof the spokes, forms a conical surface. These are conicalwheels.

When we come to examine the mechanical propertiesof wheels, we shall compare the advantages and disadvan-tages of these two species of surfaces of revolution formoving burdens from place to place.

Casks may be considered as surfaces of revolution re-markable for the simplicity of their structure. They aremade of thin planks (staves), narrower towards the endsthan at the middle, and joined at their sides, in such amanner that being forcibly bent, and kept bent by meansof parallel hoops, AB, ab, cd, CD, fig. 19, a surface ofrevolution is produced, of which these circles are the pa-rallels, and the joinings of the staves the meridians.

To close these surfaces of revolution, a circular planeis formed of other narrow and thin planks (the heading),the edge or circumference of which is fashioned into a co-nical shape, so as to fit into a circular notch or groove,made across the whole breadth of the inner face of eachstave.

After the staves are reduced to a convenient thickness,the cooper makes their ends narrow, by pushing them