James Ferguson:1 Account of Franklin’s Three-Wheel Clock
Printed in James Ferguson, Select Mechanical Exercises: Shewing how to construct different Clocks, Orreries, and Sun-Dials, on Plain and Easy Principles, … (Second edition, London, 1778), pp. 1–4.
Apparently Franklin never wrote a description of the three-wheel clock he devised, nor does he seem to have known that his friend James Ferguson published an account of it. He told Jan Ingenhousz in 1785, however, that he had seen several of his clocks in Paris, made by John Whitehurst of Derby, England.2 In the absence of any account of the construction and operation of this timepiece by its inventor, there is here reprinted Ferguson’s description, first published in 1773.3 It is placed with documents of 1758 (though no specific date can be assigned to this invention), on the basis of a statement by Ferguson’s biographer, who wrote: “The Clock usually called ‘Ferguson’s Clock,’ appears to have been invented and made by Ferguson in 1758, and is often alluded to in works on Horology, &c. It was contrived by him as an improvement on a singular Clock which had then been recently invented by the celebrated Dr. Franklin;—now known as ‘The Franklin Clock.’”4
A Clock shewing the Hours, Minutes, and Seconds, having only three Wheels and two Pinions in the whole Movement. Invented by Dr. Franklin of Philadelphia.
The dial-plate of this clock is represented by Fig. 1. of Plate I. The hours are engraven 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 it has passed by, to the next following hour. The time, as it appears in the figure, is either 32½ minutes past XII, or past IIII, or past VIII; and so on in each quarter of the circle, pointing to the number of minutes after the hours the index last left in its motion. Now, as one can hardly be four hours mistaken in estimating the time, he can always tell the true hour and minute, by looking at the clock, from the time he rises till the time he goes to bed. The small hand B, in the arch at top, goes round once in a minute, and shews the seconds as in a common clock.
Fig. 2. shews the wheel-work of this clock. A is the first or great wheel, it contains 160 teeth, goes round in four hours, and the Index A (Fig. 1.) is put upon its axis, and moved round in the same time. The hole in the index is round, it is put tight upon the round end of the axis, so as to be carried by the motion of the wheel, but may be set at any time to the proper hour and minute, without affecting either the wheel or its axis. This wheel of 160 teeth turns a pinion B of 10 leaves; and as 10 is but a 16th part of 160, the pinion goes round in a quarter of an hour. On the axis of this pinion is the wheel C of 120 teeth; it also goes round in a quarter of an hour, and turns a pinion D, of 8 leaves, round in a minute; for there are 15 minutes in a quarter of an hour, and 8 times 15 is 120. On the axis of this pinion is the second-hand B (Fig. 1.) and also the common wheel E (Fig. 2.) of 30 teeth, for moving a pendulum (by pallets) that vibrates seconds, as in a common clock.
This clock is not designed to be wound up by a winch, but to be drawn up like a clock that goes only 30 hours. For this purpose, the line must go over a pulley on the axis of the great wheel, as in a common 30 hour clock. Several clocks have been made according to this ingenious plan of the Doctor’s, and I can affirm, that I have seen one of them, which measures time exceedingly well. The simpler that any machine is, the better it will be allowed to be, by every man of science.5
1. James Ferguson (1710–1776), astronomer, mathematician, and horologist, also published some of BF’s magic squares and his magic circle; see above, IV, 392–402.
2. To Ingenhousz, April 29, 1785, Lib. Cong. Ingenhousz had asked permission to publish an account of the clock and BF replied that he did not know that his contrivance “has ever been publish’d. … You are welcome to do what you please with it.”
3. When the first edition of Ferguson’s Select Mechanical Exercises appeared.
4. E[benezer] Henderson, Life of James Ferguson, F.R.S., in a Brief Autobiographical Account and Further Extended Memoir (Edinburgh, London, and Glasgow, 1867), p. 231.
5. Ferguson went on to criticise BF’s clock on two counts: 1. it had to be rewound more than once a week; 2. “if a person wakes in the night, and looks at the clock, he may possibly be mistaken four hours in reckoning the time by it, as the hand cannot be upon any hour, or pass by any hour, without being upon or passing by four hours at the same time.” In his modification only the minutes of one hour were shown on the single dial, but small segments of revolving plates, set behind the dial and bearing numbers for the hours and seconds, appeared through small apertures in it. Ferguson readily admitted a major defect in his invention: for technical reasons it could not be as accurate as either a conventional clock or BF’s unless the device for showing the seconds were to be eliminated. Nevertheless, he recorded in his commonplace book that “I showed this Clock to Dr. Franklin, who approved very much of the alteration.” Ferguson, Select Mechanical Exercises, pp. 4–11; Henderson, Life of Ferguson, p. 235. Ferguson also devised a clock showing motions of the sun and moon and the times of high and low tide, and BF later had Whitehurst construct clocks for him on this design.