Given the revolution of the 24-hour wheel, which supports the hand for the bottom subdial (Fig. 27), and that all the ratios are either 8 or 7.5, we can conclude that the 1st wheel completes one revolution every hour and thus supports the 60 minutes hand (top left, Fig. 27). Consequently, the 3rd wheel completes one revolution every 60 seconds and supports the seconds hand (top right, Fig. 27).

 

As we noted, the escape wheel is located at the centre of the movement and completes one revolution per second. It therefore supports the large hand that points to the outer circle of 60 'railway'-style markers, which are divided into units of 6. This outer circle marks what were known in French as 'tierces' ('thirds' in English). This term was previously used to denote one sixtieth of a second but subsequently disappeared with the introduction of decimalisation, which instead divided seconds into tenths and hundredths.

 

As we can see, this escape wheel has 30 teeth and, in view of the type of escapement that I will discuss in greater detail below, each of the teeth provides 2 vibrations (i.e. 60 vibrations per turn). The hourly frequency is therefore 216,000 vibrations/hour (V/h) (3,600 x 60), or 108,000 oscillations.

 

The period is 0.01/3 of a second (3,600:108,000), which is clearly exceptional for a mechanical device of that time. 

 

Fig. 27: Display details