Mechanical paradox with orrery

Made by Smith, Richard Bartholomew in Sydney, New South Wales, Australia, 1927.

The chief significance of the mechanical paradox is that it was associated with Richard Bartholomew Smith, the builder of the Museum’s Strasburg Clock model.

The object is good evidence for the interest that Smith had in the astronomical instrument making work of James Ferguson, especially devices that were used to demonstrate the movement of planets around the Sun.

The mechanical paradox is one of the few available objects (other than the Strasburg Clock) which indicates Smith’s range of int...


Mechanical Paradox with orrery, metal / wood, made by Richard Bartholomew Smith, Sydney, New South Wales, Australia, 1927

The mechanical paradox consists of two wooden plates, which are separated and joined by four small timber dowels. Between the plates rest two sets of timber gears. The gear on the left is a single unit with large spaces between the gear teeth. The gear on the right is a composite one, and comprises three small gears stacked together to form a single unit. The gears can be rotated in either direction by turning the short brass rod that protrudes from the lower plate.

A four-planet orrery has been placed on top of the upper plate.


90 mm
110 mm


Smith made his mechanical paradox device 38 years after he had completed his model of the Strasbourg Cathedral Clock. Nevertheless, he would have been interested in the work of James Ferguson while building the clock and orreries generally, as the model has an orrery.

Smith's mechanical paradox is crudely made by comparison, and he does not appear to have developed it into an accurate orrery of the solar system. No literary evidence is currently available that details how Smith made his mechanical paradox. On the other hand, Smith's interest in the astronomical instrument making work of James Ferguson is evidenced by his dedicatory note of 1887, which Smith placed opposite the title page in his copy of Ebenezer Henderson's 1870 biography on Ferguson.

James Ferguson (1710-1776) was a leading astronomical and horological instrument maker, Fellow of the Royal Society, and the author of ten books and several technical papers on astronomy, mechanics, electricity, optics and Newtonian natural philosophy, in Georgian England. His formal education consisted of three months attendance at Keith Grammar School (England) in 1717, thus making his life's achievements all the more remarkable, although his work was, according to one contemporary historian of science (Laurens Laudan), careful and extensive but neither original nor distinguished.

The most detailed and complete account of the life and work of Ferguson remains the 1870 publication by Ebenezer Henderson (Life Of James Ferguson F.R.S. In A Brief Autobiographical Account, And Further Extended Memoir (with numerous notes and illustrative engravings). Ebenezer Henderson (1809-1879), whose father was a Dunfermline watchmaker, was a Fellow of the Royal Astronomical Society and held the higher degree of LL.D (Doctor of Laws).

Sometime between 1750 and 1755, Ferguson developed his mechanical paradox device. He wrote three tracts on his invention, and published his work in 1764, 1767, and 1770, respectively entitled; 'The Description and Use of a New Machine, called The Mechanical Paradox, invented by James Ferguson, F.R.S.'; 'Tables and Tracts'; 'Selected Mechanical Exercises'.

Ferguson's one clear definition of the mechanical paradox appeared in his 1767 publication, where he states that "the mechanical paradox is a small kind of orrery" (Henderson, 1870, 140). Henderson (1870, 139-148) provides a detailed account of Ferguson's published work on the mechanical paradox. Ferguson did not elaborate on his concept of a 'paradox', but he does allude to a paradox, as exemplified by the direction of movement of the device's stacked wheels, as being contrary to 'common reason' and observation. For many in the eighteenth century, the peculiarity in the wheels' motions was akin to being against the uniformity of the workings of Nature.

In developing the mechanical paradox, Ferguson set himself a challenge to silence "a London watchmaker who did not believe in the doctrine of the Trinity" (Henderson, 145). Briefly, the doctrine of trinitarianism is the belief that there are three persons in one God. It holds that in God are three distinctions: God the Father; God the Son; and God the Holy Ghost. Ferguson did not expound his views on his belief in the Trinity in his natural philosophical work, nor did he explain clearly how the mechanical paradox device gave proof to the existence of the Trinity. There is, it seems, only a vague notion of the anomaly of the motions in the wheels of the mechanical paradox (see below) given a certain cause, and a belief in the idea of the Trinity. Ferguson was drawing a long bow between a demonstration of mechanical principles and the existence of the Christian Trinity. Nevertheless, Ferguson's demonstration is itself fascinating as it provides a good insight as to how mechanical devices could be used in the eighteenth century to illicit a belief in the particulars of Christian theology. On the other hand, the mechanical paradox led Ferguson to develop fine orreries that demonstrated mechanically the varying orbital movements of the inner and outer planets around the Sun.

Ferguson converted his mechanical paradox device into several orreries between 1750 and 1760. The purpose of his orreries was, as he was to write in his 'Select Mechanical Exercises' (1770); "The machine is (as altered) so much of an orrery, as is sufficient to show the different lengths of days and nights, the vicissitudes of the seasons, the retrograde motion of the nodes of the Moon's orbit, the direct motion of the apogeal point of her orbit, and the months in which the Sun and Moon must be eclipsed" (Henderson, 1870, 193-194). The full details of the astronomy and the workings of the converted mechanical paradox, as retold by Ferguson can be found in Henderson (1870, 194-201), a copy of which is held in the Museum's research library.

Smith's mechanical paradox, while purporting to copy the work of Ferguson does not demonstrate the paradox of wheeled motion as envisaged by Ferguson, nor did Smith successfully convert his model into an orrery. Smith's mechanical paradox reveals no paradox (as envisaged by Ferguson) in the motions of the wheels. In Smith's model, all of the stacked wheels turn in the opposite direction to the main drive wheel and at the same rate. At best, Smith developed an incomplete prototype of a mechanical paradox, while claiming knowledge of Ferguson's work.
Smith, Richard Bartholomew 1927


Smith made his mechanical paradox in 1927, forty years after he commenced work on his Strasburg clock. There is no reference to it in any of Smith's extant writings, although orrery making was clearly of crucial importance to Smith. Furthermore, the religious controversy on the nature of the Trinity (see below) and the laws of nature as exemplified by Newtonian natural philosophy, which engaged natural philosophers in Georgian England, did not, it seems, have any interest for Smith, as they did for Ferguson when he was developing his mechanical paradox.

As indicated by the rough inscriptions on the top and bottom plates of Smith's device, he was interested in understanding the practical consequences of making wheels with a specified number of teeth and the alignment of the inner planets in a certain configuration around the Sun.
Smith, Richard Bartholomew


Ex Museum, 2005
28 June, 2005

Cite this Object

Mechanical paradox with orrery 2017, Museum of Applied Arts & Sciences, accessed 18 November 2017, <>
{{cite web |url= |title=Mechanical paradox with orrery |author=Museum of Applied Arts & Sciences |access-date=18 November 2017 |publisher=Museum of Applied Arts & Sciences, Australia}}
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