wheel, and steer with the other two, having one in front and the other
behind. So far then, the single driving rear-steerer and the Coventry
rotary pattern are easily understood. The evils of single driving,
minimized, it is true, to a large extent, in the Coventry rotary, have
led to the contrivance of means by which a wheel on each side may be
driven without interfering with their differential motion in turning a
corner.

Three methods are commonly used, but as only two are employed on
tricycles, I shall leave the third till I come to the special machine
for which it is necessary. The most easy to understand is the clutch,
a model of which I have on the table. If each main wheel is driven by
means of one of these, though compelled to go forward by the crankshaft,
it is yet free to go faster without restraint. By this means "double
driving" is effected in several forms of tricycle.

Differential gear, which is well understood, and of which there are
several mechanically equivalent forms, divides the applied driving
power, whether forward or backward, between the main wheels, equally if
the gear is perfect, unequally if imperfect. To understand the effect
of the two systems of driving, and of single driving, let us place on
grooves a block which offers resistance to a moving force. If we wish
to move it, and apply our force at the end of one side, it will tend to
turn round as well as move forward, and much friction will be spent on
the guides by their keeping it straight.

This is the single driver. If, instead of applying force at one side, we
push the block bodily forward by a beam moving parallel to itself, then
so long as the guides are straight no strain will be put upon them,
even though one side of the block is resisted more than the other; if,