The conditions under which it is impracticable are (I) when the piece has either an extremely large or an extremely small cross section, and (2) when its cross section varies materially in different parts of its length.
We saw enthusiasm engendered in a wide cross section of pupils.
Teeth of Bevel- Wheels.The acting surfaces of the teeth of bevel-wheels are of the conical kind, generated by the motion of a line passing through the common apex of the pitch-cones, while its extremity is carried round the outlines of the cross section of the teeth made by a sphere described about that apex.
Pieces which vary materially in cross section from point to point in their length cannot well be made by rolling, because the cross section of the piece as it emerges from the rolls is necessarily that of the aperture between the rolls from which it is emerging, and this aperture is naturally of constant size because the rolls are cylindrical.
In this case the thrust at the lower end B must exceed the thrust at A, the upper end, by the weight of the prism of liquid; so that, denoting the cross section of the prism by a ft.
Of course, by making the rolls eccentric, and by varying, the depth and shape of the different parts of a given groove cut in their surface, the cross section of the piece made in this groove may vary somewhat from point to point.