Sept. 22, 1887] 



NATURE 



485 



respect to the other, which has been permitted to go on 

 with the watch-train. 



The form of mechanism which is applied for the pur- 

 pose of maintaining and showing a calendar has under- 

 gone considerable development. Calendars are now 

 made to be perpetual, correcting themselves for every- 

 thing, including leap-year. The following (Fig. 10) is 

 the plan generally adopted in clocks, and is the invention 

 of the late M. Brocot. M m is a lever which is worked 

 by a pin, e, in a wheel in the clock-train going round every 

 twenty-four hours. As e advances in the direction indi- 

 cated by the arrow, M tn is moved to the left, and the 

 clicks G and H, which it carries, pass over the top of a 

 single tooth each of the wheels A and B, the wheels 

 being meanwhile held loosely in position by weak springs 

 called "jumpers." As soon as e has passed the end of 

 M w, the latter falls back by its own weight, dragging 

 back A and propelling B each one tooth respectively. B 



has seven teeth, and works the days of the week ; a has 

 thirty-one, and serves for the days of the month. All 

 months, however, have not thirty-one days, and provision 

 is made for the difference by a supplementary thruster, N. 

 In A there is a pin, /, which comes regularly below N 

 every twenty-eighth day. The tail of N rests against 

 a wheel, v F, which goes round once in four years, v F 

 is graduated with notches of different depths. These 

 notches correspond to the respective lengths of each 

 month, and those representing the Februaries are con- 

 spicuously noticeable ; that one which is the shallowest 

 of the four identifying the leap-year. During the months 

 of normal length, N maintains the position which is shown 

 dotted in the diagram, and does nothing. But whenever 

 there occurs a short month, the tail of N will enter one 

 of the notches ; in consequence, N will descend, and, 

 engaging /, propel A forward a day or more, depending 

 upon the depth of the notch. This happens whilst M m 



b 



Fig. 9. — Chronographlc watch-work. 



is travelling to the left. When M m falls back, the click 

 G will act in addition, and as usual. Fig. 1 1 shows the 

 dial ; the hands on dials right and left are in connexion 

 with A and B (Fig. 10). The hand upon the lowest 

 dial shows the month of year ; its progression is con- 

 tinuous. The hand at the top shows the equation of 

 time, and alternates on each side of noon, -f- or — , as 

 may be required. It is worked by a rack which reposes 

 against a cam of suitable form revolving once in twelve 

 -months. 



The phases of the moon are indicated (as may be seen 

 in the diagram) by the passage of three shaded disks 

 across a circular aperture. 



Magnetism exercises the most destructive influence 

 upon watches or chronometers, turning their balances into 

 compass-needles, and causing the coils of their balance- 

 springs to stick together. 



In these days of large magnetic engines it has there-- 

 fore been found necessary to revert to an idea of the 

 elder Arnold, and to construct watches for the use of 

 those having to do with such engines upon a plan which 

 shall render them indifferent to magnetization. This re- 

 sult is obtained by making the balances of silver and 

 platinum, or an alloy of iridium, or of some other non- 

 magnetizable material, and the balance-springs of gold or 

 palladium ; and the use of steel is avoided in certain 

 parts of the escapement. Watches carefully constructed 

 on such plans give results little inferior, as regards time- 

 keeping, to others. 



Amongst the multifarious purposes to which clocks 

 have recently been applied, we must not omit to mention 

 those which are designed for registering the proper per- 

 formance of a watchman's duty. The old-fashioned 

 principle was that there should be a separate clock at 



