But again, in the case of forces, the combination of the in-
clined resultant with the horizontal component would give the
other vertical component ; so here, the combination of the rotation
about the horizontal axis AB with another rotation about the
inclined axis ae, gives a rotation about an imaginary vertical axis,
viz., that passing through a, and hence the axle of the rollers does
in effect rotate about a vertical axis, just as if it were the axle of
an ordinary carriage provided with a vertical perch-pin. Such a
rotation causes the rollers to deviate on one side of the normal

direction.
The specification of Mr. Plimpton's patent, granted to A. V.
340 Elements of Mechanism.
Newton and numbered 2190 of the series for 1865, contains draw-
ings of the skate, and shows the foot stand running upon four
wheels, two on each side of the roller axle. The respective axles
are supported in frames capable of turning to a small extent
limited by stops, and are directed downwards to a point half-way
between the heel and toe of the skate. These ledges perform the
function of inclined axes. The invention is described as relating
to an improvement in attaching rollers to the foot stand of a skate,
whereby the rollers are made to turn by the rocking of the foot
stand so as to cause the skates to run in a curved line either to
the right or left.
