Having traced with the disc the line you wish to measure, carry the instrument to the zero of the scale and let it run inversely the length of that scale, until the zero of the disc returns opposite to the zero of the “reglet.” The point at which the disc is stopped on the scale indicates the length of the line measured upon the map. If the scale be smaller than the line measured, repeat the operation as many times as may be necessary.
If it is desirable to ascertain upon a map of a scale of 1/20000 the distance represented by 1,200 mètres, we have only to place the toothed disc so that its position marks four times the required distance—that is, 4,800 mètres on the map of 1/80000 (for 4 times 20 = 80). Then move the disc in the given direction until the zero returns opposite the zero of the “reglet”; this limit will mark the extremity of the length required.
Explanations are not easy upon paper, but the instrument is found very easy in actual use. It is employed by the military staff for calculating distances of any kind, curves or straight lines. On the march, or even on horseback, the campylometer can be employed.
Mysterious Clocks.
The clocks represented in the two following illustrations (figs. 878 and 879), are well worthy of being placed in the house of any amateur of science. They are made of transparent crystal, and though all mechanism is cleverly concealed they keep capital time. The former clock (fig. 878) is the invention of Robert Houdin, and consists of two crystal discs superposed and enclosed in the same frame. One carries the usual numerals, the other moves upon its centre with the minute hand attached, and its rotation induces by the ordinary method the movement of the hour hand. The requisite motion is transmitted to the dial by gear disposed along the circumference and hidden within the metallic frame, and is itself put in motion by clockwork, enclosed in the pedestal of the timepiece.
Fig. 878.—Houdin’s clock.
M. Cadot, in his clock (fig. 879), retains the plates, but adopts the rectangular form, so as to preclude all idea of rotation, and to puzzle those who are acquainted with the working of Houdin’s clock. The minute hand cannot, in this instance, be fixed to the second glass plate, it preserves its independence. This movable plate has only a very slight angular movement around its centre, which oscillation or “play” is permitted in the interior of the rectangular dial. A little spring movement, hidden in the central nut of the “hand,” provides in progressive rotation the oscillation of the transparent plate, which cannot be perceived to move.
To produce this “balance” motion the plate is supported upon a bar in the lower part of the metal frame. After the direct oscillation of which we have spoken, a little spring puts the machinery back. The direct displacement is produced by a vertical piston which raises the end of the bar. This piston rests upon a bent lever communicating with a wheel with thirty triangular teeth. Finally this wheel turns upon its axis once in an hour by a clockwork arrangement in the pedestal of the clock. Each tooth takes two minutes to pass, and the movement is communicated to the minute hand, which thus goes round the dial in the hour. The hour hand is controlled by a delicate arrangement hidden in the base. The illustration and notes will explain the working.