The speed with which the carriers are ejected from the receiving tube through the opening J and down the chute Q is regulated by the valve L, which can be opened or closed by a hand-wheel, _p_. Before the wheel and receiving tube can be rotated, the needles must be withdrawn from the receiving tube, and this is accomplished by a small cylinder and piston, _q_, shown in Fig. 40. The needles and their encasement are attached to a cross-head, _r_, on the end of a hollow piston-rod, _s_. When air is admitted to the top of the piston in the rotating cylinder O, Fig. 39, it is also admitted through the pipe _t_, Fig. 38, to the cylinder and upper side of the piston _q_, Fig. 40. This moves the piston _q_ down against the force of a spring, _u_, and withdraws the needles from the receiving tube. This takes place after the needles have served their purpose and before the wheel is rotated. The piston _q_ has much less inertia than the wheel, therefore it moves much quicker. When the wheel begins to rotate it closes a valve, _v_, in the pipe _t_, Fig. 38, confining the air in the cylinder _q_, and preventing the needles from being raised by the spring _u_ before the wheel returns to its normal position. If by any accident the needles should be raised, no serious harm would result, for their ends would simply bear against the face of the wheel. If this took place constantly, grooves might be worn in the face of the wheel; for this reason the valve _v_ is provided. In order to facilitate the inspection of the needles and electric contact springs W, they are contained in a cylindrical brass case, _w_, that is held in place beneath the receiving tube by two bolts. By removing the nuts from these bolts the entire mechanism can be removed, examined, and cleaned. It also gives easy access to the receiving tube. The receiving tube is long enough to receive two carriers, if it should ever happen that two arrive at the same time. [Illustration: FIG. 41. DIAGRAM OF CONTACT-DISKS AND NEEDLES.]