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 named Ellicott, convinced him that he had reached the same result twenty years previously. They became fast friends. At the president's request Ferguson published in 1745 a large engraving of the curve generated by his ‘trajectorium lunare.’

His first literary attempt was in a pamphlet on ‘The Use of a new Orrery,’ printed in 1746, to which succeeded in the following year ‘A Dissertation upon the Phenomena of the Harvest Moon.’ In a paper ‘On the Phenomena of Venus, represented in an Orrery, agreeable to the Observations of Signor Bianchini’ (Phil. Trans. xliv. 127), he described before the Royal Society on 20 March 1746 the course of the seasons on Venus resulting from a supposed rotation in 241/3 days, on an axis inclined 75° from the perpendicular; and on 14 May 1747, ‘An Improvement of the Celestial Globe’ (ib. p. 535). In April 1748 he entered upon his career as a popular scientific teacher and lecturer, choosing for his theme the solar eclipse of 14 July (O. S.) 1748. His later courses, delivered in the provinces as well as in London, covered a wide range of experimental science. The chief part of the illustrative apparatus was invented and constructed by himself, and several of his machines kept a permanent place in the lecture-room. Among his inventions (besides eight orreries) were a tide-dial, a ‘whirling-table’ for displaying the mode of action of central forces, the ‘mechanical paradox,’ and various kinds of astronomical clocks, stellar and lunar rotulas. His ‘seasons illustrator,’ invented in 1744, became indispensable to lecturers on astronomy. His ‘eclipsareon’ for showing the time, duration, and quantity of solar eclipses in all parts of the earth, was described before the Royal Society on 21 Feb. 1754 (ib. xlviii. 520; Gent. Mag. 1769, p. 143), a new hygrometer on 8 Nov. 1764 (Phil. Trans. liv. 259), his ‘universal dialling cylinder’ on 2 July 1767 (ib. lvii. 389). He lectured in 1752–3 on the reform of the calendar and the lunar eclipse of 17 April 1753, and was collecting meanwhile materials for his best work.

Ferguson's ‘Astronomy explained on Sir Isaac Newton's Principles’ was published in July 1756, and met with immediate and complete success. The first issue was exhausted in a year; the thirteenth edition, revised by Brewster, appeared in 1811, and the demand for successive reprints did not cease until ten years later. It was translated into Swedish and German, and long excluded other treatises on the same subject. Although containing no theoretical novelty, the manner and method of its expositions were entirely original. Astronomical phenomena were for the first time described in familiar language. The book formed Herschel's introduction to celestial science.

Ferguson was now famous, but he was still poor. In the first edition of his ‘Astronomy’ he advertised himself as teaching the use of the globes for two guineas, and ‘drawing pictures in Indian ink on vellum at a guinea apiece, frame and glass included,’ but failing eyesight began to hinder artistic employment. On 17 Jan. 1758 he imparted to the Rev. Alexander Irvine of Elgin his thoughts of soon leaving London on account of the expense of living there. Some relief was afforded by the sale, for 300l., of the remaining copyright of his book, and an interview with the Prince of Wales (afterwards George III) at Leicester House, on 1 May 1758, finally decided him to maintain his position.

‘Franklin's clock’ was in 1758 turned into ‘Ferguson's clock’ (remembered as a horological curiosity), by an improvement to which the original inventor's assent had been obtained during his visit to London in 1757; and in 1760 Ferguson's ‘Lectures on Select Subjects in Mechanics, Hydrostatics, Pneumatics, and Optics,’ were published with a dedication to Prince Edward. A seventh edition of this popular book appeared in 1793; Brewster's revision in 1805 gave it fresh vitality; translations into several languages and repeated impressions in America further attested its value. The author received about 350l. for the copyright.

In February 1761 he published a pamphlet entitled ‘A Plain Method of Determining the Parallax of Venus by her Transit over the Sun,’ including a revised translation of Halley's memoir of 1716, and accompanied by a map of ingresses and egresses modelled on that of Delisle. It was appended to later editions of his ‘Astronomy.’ He himself observed the transit with a six-foot reflector from the top of the British Museum (Addit. MS. No. 4440, f. 604). He altogether left off portrait-painting in 1760, but a pension of 50l. a year was granted to him by George III in 1761, and he received gifts from persons of distinction. That his lectures were fairly profitable appears from the statement that he cleared 100l. during a tour of six weeks to Bath and Bristol in the spring of 1763. Unsuccessful as a candidate for a clerkship to the Royal Society in January 1763, he was, however, on 24 Nov. following, elected a fellow, and ‘on account of his singular merits and of his circumstances’ excused the customary payments.

On 17 Nov. 1763 he presented to the Royal Society a projection of the partial solar eclipse