CHRONOMETERS. 9-5 



middle of this plate is perforated with a hole, re- 

 ceiving that end of the arbor of the centre wheel 

 which carries the minute-hand. Near the plate is 

 fixed a pinion a b, of ten teeth ; this is called the 

 pinion of report ; it drives a wheel c d, of forty 

 teeth ; the wheel c d carries a pinion e f, of twelve 

 teeth ; and this drives a wheel g h, with thirty-six 

 teeth. 



As, in the body of the watch, the wheels every 

 where divide the pinions, here, on the contrary, 

 the pinions divide the wheels, and by that means 

 decrease the motion, which is here necessary ; for 

 the hour-hand, which is carried on a socket fixed 

 on the wheel g h, is required to move but once 

 round, while the pinion a b moves twelve times 

 round. To this end, the motion of the wheel c d 

 is one-fourth of the pinion a b : again, while the 

 wheeled, or the pinion e J' goes once round, it 

 turns the wheel g h but one-third part round ; con- 

 sequently the motion of g h is but one-third of 

 one-fourth of the motion of a b : but f of £ = T r . , 

 that is the hour-wheel g h moves round once in 

 the time that the pinion of report, on the arbor of 

 the centre or minute-wheel, makes twelve motions. 



Dr. Franklin contrived a clock to show the 

 hours, minutes, and seconds, with only three wheels 

 and two pinions in the whole movement. The 

 dial-plate (Plate 5. fig. 3.) has the hours engraved 

 upon it in spiral spaces, along two diameters of a 

 circle containing four times 60 minutes. The 

 index A goes round in four hours, and counts the 

 minutes from any hour which it has passed to the 

 next following hour. The time, therefore, in the 

 position of the index shown in the figure, is either 

 30 minutes past twelve, four, or eight ; and so, in 

 every other quarter of the circle, it points to the 



