390 THE MECHANISM OF BICYCLES. 
saddle. It |is easy to understand 
that if the saddle is placed so low 
that the leg is still bent when the 
pedal is at its furthest position from 
the saddle, high knee action will be 
observed at the highest position of 
the pedal. But also the forward 
or back position has a great influence 
that will be understood by referring 
to the diagram, which shows the 
position of the knees in the two 
cases, first when the saddle is placed 
immediately over the back centre 
position of the pedal and secondly 
when placed six inches further back. 
To compare the action, I have drawn 
the pedal path of the second case 
six inches further to the front so 
as to leave the saddles on the 
same vertical lines. It will be seen 
that the angular movement of the 
upper part of the leg is 53° in the 
first case and 68° (or 15° more) 
in the second. With the forward 
position also the vertical jolting 
due to the back wheel going over 
a stone is diminished. When sink¬ 
ing the heel there is still less knee 
action. 
The driving mechanism which I 
propose to speak chiefly about this 
evening is the method by which the 
power applied to the pedals by the foot 
Diagram 66 is utilized in driving the machine 
along the road. 
The method usually employed consists in keying to the same shaft 
as the pedals a sprocket wheel, a smaller sprocket wheel being 
attached to the hub of the hind or driving wheel, and the two sprocket 
wheels are connected by a sprocket chain. If the two sprocket wheels 
were the same size, then for one revolution of the pedals the machine 
would move a distance equal to the circumference of the driving wheel, 
but with the usual sized wheel, of 28 inches, this movement would be 
too small, so it is customary to “ gear ” the machine in such a way, 
that while the pedal revolves once the driving wheel revolves twice, 
three times, four times, or any fraction of these numbers, according 
to whatever leverage is most suitable for the rider. If the wheel (of 
28 inches) revolves twice, it is said to be geared to 56 since the 
distance that the machine would travel, for one revolution of the 
pedals, would be equal to the circumference of a wheel 56 inches in 
diameter. This way of estimating the gear was used when first the 
