relatively instead of through a horizontal line of short range. If now the resistance to the wheels a and b be the same, the pinion h (acting in precisely the same way as the cross-piece h of fig. 3) will by its teeth at f and g impart equal degree of rotation to both, but if wheel b be on the inner side of a curve or meets with obstruction greater than that to a, then the pinion h will turn on its pin k, and allow one wheel to move faster than the other. That is say, the pinion h will impart the same push to either wheel a, b, and if one of these moves more easily than the other from any cause, it accommo-
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Fig. 4—A Differential Gear
dates that wheel by itself turning and allowing the wheel to move more rapidly than the other. This accommodating action of the gear is important for free turning of corners, but it has its disadvantage, in that if one wheel, while the vehicle is on the straight, meets with more obstruction than the other, the gear allows it to be obstructed, and tends to push the other wheel round against the action of the steering gear. Correspondingly, if one wheel is on a greater thickness of greasy mud or a more slimy bit than the other wheel, it has more freedom of, and help to, rotation than the other, and skidding or sideslipping on the greasy road results and is aggravated. Hence
