896 
THE MECHANISM OF BICYCLES. 
The crank on the ‘ off 3 side of the machine is double ended, having 
the pedal at one end, and, in addition, at each end a groove about three 
inches in length, about f inch wide, and open at the end. The pedal 
on the near side is keyed to a spindle joining it with the “ double 
crank.” The sprocket wheel, which rotates on a fixed spider wheel 
provided with six rollers, has on it three arms, at angles of 120° with 
each other, each fitted with a roller and each of these rollers in turn 
comes into contact with the grooves on the double crank. In this 
way each pedal in its downward half revolution turns the sprocket 
wheel through an angle of 120°, aud it follows that the pedal rotates 
three times while the sprocket wheel revolves twice. 
Turnbull’s . 
The result of this arrangement is that instead of the simple lever of 
the crank and large sprocket wheel we have a compound lever—the 
two levers being the crank and the distance between the end of the 
arm and the centre of the sprocket wheel, the two counterlevers 
being the distance from the crank spindle to the point the roller 
is in contact with the groove and the radius of the sprocket wheel. 
From what has already been said it is impossible to obtain extra 
power except at the loss of speed and consequently I was unable to 
agree with the statement originally put forward that “ a greater lever¬ 
age on the down stroke amounting to 38°/ 0 on a 6J inch crank is 
obtained.” 
To some it may appear that extra leverage, due to the arms pro¬ 
jecting outside the sprocket wheel is gained, but on examining the 
diagrams it will be seen that the tangential component of the foot 
