Let us illustrate with a common experiment in physics. Suspend a number of ivory balls by cords of equal length in such a manner that each is in contact with its fellow and all are in a straight line. When the balls are properly adjusted a straight line should connect their centers. Hold one end ball firmly in the hand or with an instrument which renders it absolutely fixed. Then strike sharply with a light hammer. The balls will all remain stationary except the one on the opposite end which will fly off to a distance exactly measurable according to the force of the blow. How does this occur? A shock is transmitted through the molecules of the ivory until it reaches the end ball, which is not held back by another. Here the transmitted force is expended in molar motion, the ball leaping away from its fellows as if it had been hung alone and had been struck with the same force. It is well known that by placing an elbow firmly against a man’s jaw and then sharply striking the closed fist with the other hand, open, a very heavy blow can be given; yet the forearm, through which the shock is transmitted, does not move. Now ivory is very like human bone. Further, it has been demonstrated that the law illustrated by the above experiment is equally applicable to the movement of vertebrae. The pushing or thrusting movement _may_ move a specific vertebra, but it is probable that the chief factor in so doing is the element of transmitted shock contained in the movement and delivered at the instant of release of the hand from the spine at the end of the movement. On the other hand it is obvious that a pushing or thrusting movement may move several vertebrae in addition to the one directly in contact with the adjusting hand, in consequence of the way in which the spinal segments are closely bound together. If a steady strain is used, in which muscles and ligaments have time to act, one of three results may occur: (a) the specific adjustment; (b) the movement of several vertebrae at one time, which does not constitute an adjustment; (c) the giving way of the spine at its weakest point, which may be some distance from the point of contact with the adjusting hand, the ligaments and muscles having communicated and diffused the strain throughout a large area. In the latter contingency the result is usually a new subluxation or the increase of an old one, instead of an adjustment. The Rapid Movement Thus _Speed_ becomes an important factor in correct adjustment.