Page:Edinburgh Review Volume 59.djvu/311

1834. by the disengagement of the carrying claw before mentioned, and by its consequent recoil, urged by the spiral spring with which it is connected; but it may, and does, frequently happen, that though the process of addition may not have rendered a carriage necessary, one carriage may itself produce the necessity for another. This is a contingency not provided against in the mechanism as we have described it: the case would occur in the scheme represented in Fig. 1, if the figure under the index of C2 were 4 instead of 3. The addition of the number 5 at the index of C3 would, in this, ease, in the first quarter of a turn, bring 9 to the index of C2: this would obviously render no carriage necessary, and of course no preparation would be made for one by the mechanism—that is to say, the carrying claw of the wheel D2 would not be detached. Meanwhile a carriage upon C2 has been rendered necessary by the addition made in the first quarter of a turn to B2. This carriage takes place in the ordinary way, and would cause the dial C2, in the second quarter of a turn, to advance from 9 to 0: this would make the necessary preparation for a carriage from C2 to D2. But unless some special arrangement was made for the purpose, that carriage would not take place during the second quarter of a turn. This peculiar contingency is provided against by an arrangement of singular mechanical beauty, and which, at the same time, answers another purpose— that of equalizing the resistance opposed to the moving power by the carrying mechanism. The fingers placed on the axes of the several dials in the row D2, do not act at the same instant on the carrying claws adjacent to them; but they are so placed, that their action may be distributed throughout the second quarter of a turn in regular succession. Thus the finger on the axis of the dial A2 first encounters the claw upon B2, and drives it through one tooth immediately forwards; the finger on the axis of B2 encounters the claw upon C2 and drives it through one tooth; the action of the finger on C2 on the claw on D2 next succeeds, and so on. Thus, while the finger on B2 acts on C2, and causes the division from 9 to 0 to pass under the index, the thumb on C2 at the same instant acts on the trigger, and detaches the carrying claw on D2, which is forthwith encountered by the carrying finger on C2, and, driven forward one tooth. The dial D2 accordingly moves forward one division, and 5 is brought under the index. This arrangement is beautifully effected by placing the several fingers, which act upon the carrying claws, spirally on their axes, so that they come into action in regular succession.

We have stated that, at the commencement of each revolution of the moving axis, the bolts which drive the dials of the first, third, and fifth rows, are shot. The process of shooting these