Fig. 48.
The clock will be wanted to go for a week without winding, and as people may forget to wind it at the proper hour of the day, we will give it a day extra, and make an “eight-day” clock of it. Hence then, while nine feet of cord is being pulled out by a weight which falls four and a half feet, the minute hand is to be turned round as many times as there are hours in eight days, viz., 192 times. This could be accomplished, of course, by winding the cord round the arbor of the minute hand. But this would require 192 turns. If our cord is to be ordinary whipcord, or catgut, say one-twelfth of an inch in diameter, in order that the cord could be wound upon it, the arbor would have to be 192/12 inches long = 14⅓ inches long. This would make the clock case unnecessarily deep. We must therefore again have recourse to an intermediate wheel.
Fig. 49.
If we put a pinion of eight leaves on the minute hand arbor c, and engage it with a wheel of sixty-four teeth on another arbor b, then b will obviously turn round once in eight hours, that is to say, twenty-four times in the period of eight days. And, if we fix on b a “drum” or cylinder two inches long, the twenty-four turns of our cord will just fit upon it, since, as has been said, our cord is to be one-twelfth of an inch in diameter. The diameter of the drum must be such that a cord nine feet long can be wound twenty-four times round it. That is to say, each lap must take (9 × 12)/24 = 4½ inches of cord. From this it is easy to calculate that the diameter of the drum must be rather less than one and a half inches. From this then it results that we want for a “Grandfather’s” clock a drum two inches long and one and a half inches diameter, on this a cogwheel of sixty-four teeth working into a minute hand arbor, with a pinion wheel with eight leaves, and a cogwheel of sixty-four teeth, an intermediate or idle wheel with an eight-leaved pinion, and a cogwheel of sixty teeth, engaging with a seconds hand arbor with a pinion of eight leaves. This is called the “train of wheels.” With it a weight such as can be arranged in an ordinary “Grandfather’s” clock case will cause by its fall during eight days the second hand arbor to turn round once in each minute during the whole time, and the minute hand arbor to turn round once in each hour.
Fig. 50.
We must next provide an arrangement for winding the clock up. It is obvious that we cannot do so by twisting the hands back. It is true that this could be done, but it would take about five minutes to do each time and be wearisome. In order to save this trouble, an arrangement called a ratchet wheel and pall must be provided. A ratchet wheel consists of a wheel with a series of notches cut in it, as shown in the figure A. A pall is a piece of metal, mounted on a pin, and kept pressed up against the ratchet wheel by a spring C. It is obvious that if I turn the wheel A round, and thus wind up a weight, fastened to a cord wound round the drum D, that the pall B will go click-click-click as the ratchet wheel goes round, but that the pall will hold it from slipping back again. When, however, I take my hands away, and let the ratchet wheel alone, then the weight E will pull on the drum D, and try and turn the ratchet wheel back the opposite way to that in which I twisted it at first. If the pall B is held fast, it is impossible to move it, but if the pall is fixed to a cogwheel F, which rides loose on the arbor of the drum D, then the pull of the weight E will tend to twist the cogwheel F round, and this, if engaged with a pinion wheel on the minute hand arbor, will therefore drive the clock. As the clock arbors move, of course the weight E gradually runs down, and, at last all the string is unwound from the drum D. The clock is said then to have “run down,” but if I take a clock key, and by means of it wind the string up upon the drum D, then the pall lets the drum and ratchet slip; the clock hands are not affected. When I have given twenty-four turns to the arbor, the nine feet of cord will then be wound upon the drum again, and the clock will be ready to go for eight more days, and will begin to move as soon as I cease to press upon the clock key.
