The sights serve three important functions. They improve the vision of the gunner and lay the gun in elevation and direction. The simplest sight is the one over the line of metal which lays for direction only, the second is the tangent sight mounted on a range arc centered on the axis of rotation in elevation usually having a deflection scale to correct for drift and to lead the moving target. This unit lays for range and direction. The last sight is the telescopic or panoramic sight which is mounted on a range arc and lays for direction only, it is the unit for indirect fire, laying for direction, and markedly improving the vision of the gunner. The latest model of the American panoramic sight is superior to any in existence. For precision in indirect fire, practically all carriages are equipped with some form of range quadrant, containing a means of setting off the angle of site and the range angle. All instruments are equipped to compensate for difference in wheel level. The British carriage automatically corrects for deviation, simplifying the firing date by that element. WHEELS. The height of the wheels affects the draft, weight, clearance, and stability of the carriage both as to road stability—i. e., low center of gravity; and firing stability—i. e., the overturning movement about the end of the trail. It is now believed that entirely too much stress has been laid on road clearance. Reducing the height of wheel reduces the weight and road clearance, lowers the center of gravity and increases the firing stability; but it also reduces the angle of gun elevation unless the trail is shortened. Firing stability in general is increased by adding to the weight of the gun, lengthening the recoil, slinging the gun as low as possible and lengthening the trail. The less the height of the wheel exceeds four feet the better, despite the poor draft feature, which is compensated for to some extent by a reduction in weight and turning radius. AXLES. Axles are straight or of the offset type. The straight axle is stronger for its weight. The drop axle allows the center of gravity to be lowered. TRAILS. Most modern trails are of the sectional built-up type. Some, however, are of tubular and telescopic. The most variable portion of the trail is the spade. It consists of two parts, the spade proper and the float. The former prevents recoil, the latter the burying of the trail. The spades proper are of three types: the fixed, as in the French 75-mm; semi-fixed, as in the 155-mm howitzer; and driven, as in the Deport and American 1916 75-mm. Each has advantages and disadvantages. The driven spade is considered essential for the split trail carriage, as the latter has no means of seating itself; and should one spade take, and not the other the carriage might be damaged when the gun is fired at an extreme traverse.