pensating gear, its object being to drive the road-wheels so that though both are turned by the same rotating source, they may turn at different speeds when turning a corner; the two wheels then describe parts of two circles of different sizes, the one wheel advancing perhaps five or six feet to one foot of the other. The differential gear will be explained with reference to the live hind axle. If both wheels were driven at the same speed on a spindle without differential motion, then one or both wheels would have to skid and rub over the ground for the whole of the difference of five or six feet to one. Railway and tramway wheels do this, but the curves they traverse are, except on tramways, always much larger, and the difference between the curvature followed by the two wheels is small. On tramways, however, it is the cause of very great strains and wear, and is a very unmechanical and barbaric form of simplicity.
So far as driving is concerned the automobilist has only to remember with regard to the fixed hind axle that the axle should be kept well oiled, and that careful examination should frequently be given to all nuts and pins.
The Differential Gear.—The live hind axle used in so many of the more recent forms of light car is a very different thing, and needs more attention, as part of its structure is the differential gear.
The differential gear acts on the principle of the action of the pair-horse whippletree and equalising bar, the gear acting continuously in a rotating circle while the whippletrees act only through a small range rectilinearly.
The gear may be explained by reference to the diagrams figs. 3 and 4, which are merely for explanatory purposes.
In fig. 3 wheels a and b are supposed to be upon an axle in two parts, c and d, united by a pin at e loose in both parts of the axle. Fixed on both parts are arms k and g, connected by a loosely-fitting cross-piece h, supposed to be pulled at k by a rod j. It will be readily understood that if the carriage of which a and b are the driving wheels be set to move in a straight course and the road resistance to both wheels be the
