Desaguliers, John Theophilus

(b. La Rochelle, France, 12 March 1683; d. London, England, 10 March 1744)

experimental natural philosophy.

Desaguliers was taken to Guernsey when he was less than three years old by his Huguenot parents, who in 1694 settled in Islington, where the father taught school and educated his son. After his father’s death Desaguliers entered Christ Church, Oxford (28 October 1705), whence he proceeded B.A. in 1709. About this time James Keill abandoned the lectureship in experimental philosophy at Hart Hall that he had held for some ten years; he was succeeded by Desaguliers, who took his M.A. from this college on 3 May 1712. In that year he moved to Channel Row, Westminster, no doubt in the hope of gaining a more remunerative audience. His first book, a translation of A Treatise on Fortification from the French of Ozanam, had already appeared (Oxford, 1711).

Continuing in London the style of scientific lecturing he had inherited from Keill, and having taken orders, he was given the living of Whitchurch and Little Stanmore, near Edgeware, to which royal favor later added other benefices. Before long Desaguliers was initiated into No. 4 Lodge of the Freemasons, meeting at the Rummer and Grapes Inn, Channel Row; and by 1719 he had become the third grand master of the recently constituted Grand Lodge of the order. It is said that Desaguliers induced Frederick, prince of Wales, to become a Freemason and also that through him “Freemasonry emerged from its original lowly station and became a fashionable cult” (Stokes). It was at the behest of one such fashionable past grand master, the duke of Wharton (Pope’s “scorn and wonder of our days”), that in 1723 Desaguliers (then deputy master) dedicated to the grand master (the duke of Montagu) James Anderson’s Constitutions of the Free-Masons (in a preface). Others of the Royal Society, to which Desaguliers belonged, joined this distinguished fraternity.

Desaguliers’ practical abilities aroused the Royal Society’s interest soon after his arrival in London. Late in the winter of 1713/1714, at Newton’s suggestion, he was invited to repeat some of Newton’s experiments on heat; before long he had become a de facto curator of experiments. He was elected a fellow on 29 July 1714, being excused his admission money because of his previous services. Desaguliers continued to furnish the society with experiments until his death. For some time Sir Godfrey Copley’s benefaction (1709) of £100 per annum was paid to him; and after the Copley Medal was instituted in 1731, it was awarded to Desaguliers three times as a mark of his experimental ingenuity. Between 1716 and 1742 he contributed no fewer than fifty-two papers to the Philosophical Transactions, the earlier ones chiefly on optics and mechanics, the later ones on electricity. In the age of the Bernoullis, Clairaut, Euler, and Maupertuis, Desaguliers’ contributions to theoretical mechanics cannot be called outstanding. “Dissertation Concerning the Figure of the Earth” (Philosophical Transactions, 33 [1726]) is the most important, yet it follows Keill (correcting his chief error), who followed Huygens (I. Todhunter, A History of the Mathematical Theories of Attraction... [1873], pp. 103–108). This dissertation was criticized anonymously by Maupertuis in 1741.

In attempting a reconciliation of the measurement of “motion” by the Newtonians (momentum = mv) and by the Leibnizians (vis viva = mv²), Desaguliers correctly argued that the quantities so expressed were different, and hence the dispute was merely verbal; yet he seems also to have held that the Newtonian concept was better supported by experiment. Desaguliers never employed analytical methods in these papers. In practical mechanics he was highly skilled, being the first English writer to give theoretical analyses of machines on the basis of statics, the ancient treatment of the five simple machines, and elementary dynamics. He was himself a practical improver of various devices, among them Musschenbroek’s pyrometer, Stephen Gray’s barometric level, Hales’s sounding gauge, Joshua Haskins’ force pump, and Savery’s steam engine. Desaguliers claimed that his improved form of Savery’s engine was twice as efficient as the Newcomen pump. He also devised a centrifugal air pump for ventilating rooms, which was employed at the House of Commons. He had the advantage of relying upon Bélidor and his friend Henry Beighton (a Warwickshire engineer) in compiling a very up-to-date account of mechanical practice, including the railroad and steam engine. He clearly understood that a man or a horse could do only a finite amount of work in a given time, no matter what machinery might be used, and understood the fallacy of perpetual motion.

Desaguliers’ optical experiments (Philosophical Transactions, 29 [1716]) were for the most part repetitions of those described by Newton, made in order to vindicate Newton’s accuracy—which had been challenged—and the theoretical conclusions Newton had drawn. Some of them were improved in detail—for example, by the use of a camera obscura. Desaguliers taught that light is a “body” and that reflection, refraction, and diffraction are caused by the varying attractions between light and the media through which it moves. He also hinted at a similarity between the force of electricity and the force of cohesion, which he investigated experimentally. He made little use of the concept of ether (although not wholly avoiding the term), speaking, for instance, of “vacuities” between the particles of matter (the existence of which he thought he could demonstrate experimentally); rather, Desaguliers clung to that part of the Newtonian tradition which emphasized the duality of forces: “There seem to be but two Powers, or general Agents in Nature, which, according to different Circumstances, are concern’d in all the Phaenomena and Changes in Nature; viz. Attraction (meaning the Attraction of Gravity, as well as that of Cohaesion, etc.) and Repulsion” (Course of Experimental Philosophy, II, 407). Accordingly, Desaguliers, following Newton’s hint, firmly attributed the elasticity of air to a repulsion between its particles. On all such points it might be said that he was “plus Newtonien que Newton,” writing of the Opticks that it contained a “vast Fund of Philosophy; which (tho’ he [Newton] has modestly delivered under the name of Queries, as if they were only Conjectures) daily Experiments and Observations confirm,” and citing Hales’s Vegetable Staticks as a book that put several of the “Queries” beyond doubt and showed how well they were founded (Course of Experimental Philosophy, preface, pp. vi–vii).

Desaguliers described and demonstrated a great many electrical experiments to the Royal Society (Philosophical Transactions, 41, 42 ; Course of Experimental Philosophy, II, 316–335), although he refrained from so doing until after the death in 1736 of Stephen Gray—who, it is said, lived with Desaguliers and assisted him. This work certainly contributed greatly to the popularization of electrical science. Desaguliers studied charging, conduction, discharge in air, attraction and repulsion, the effects of dryness and moisture, and so forth, using a fragment of thread as detector. He distinguished “electrics per se,” which could be charged by friction and so on, from “non-electric bodies,” which were incapable of receiving charge directly although they were capable of being electrified indirectly when suitably suspended. Desaguliers did not make a parallel distinction between insulators and conductors, nor did he realize that a “non-electric body” could become an “electric per se” if properly insulated. Nor did he understand the role of leakage to earth in conduction experiments. At a very late stage he commented on the distinction between vitreous and resinous electricity established by Du Fay.

Until the end of his life Desaguliers retained his preeminence as a demonstrative lecturer in the Royal Society, at court, and in his own home (where he took in student boarders). By 1734 he had repeated his course on astronomy, mechanics, hydrostatics, optics, electricity, and machinery more than 120 times. Although he acknowledged that Keill had first “publickly taught Natural Philosophy by Experiments in a mathematical Manner” (the instrument maker Hawksbee had also begun to demonstrate experiments to the public at about the same time), it was Desaguliers who popularized the demonstrative lecture in Britain. (A sample of the scene is provided in two well-known pictures by Joseph Wright of Derby, ca. 1760.) “Without Observations and Experiments,” he wrote in the preface to the first volume of his Course of Experimental Philosophy (1734), “our natural Philosophy would only be a Science of Terms and an unintelligible jargon.” By deliberate choice he demonstrated to the eye not only things discovered by experiment but also those “deduc’d by a long Train of mathematical consequences; having contrived Experiments, which Step by Step bring us to the same Conclusions,” for he recognized that the Newtonian philosophy was not accessible to all through mathematics. Thus Desaguliers occupies a leading position (along with Keill, Pemberton, and Maclaurin) among those who gave Newtonian science its ascendancy in eighteenth-century England. Not that Desaguliers wholly avoided mathematical reasoning; on the contrary, he employed it continually, but only in simple terms and as an adjunct to empirical evidence. Desaguliers did nothing for serious mathematical physics.

Naturally, Desaguliers was eager to publicize rather than to publish his material. In 1717 he issued as Physico-Mechanical Lectures an eighty-page abstract of the twenty-two lectures in the course for the benefit of auditors who did not wish to make their own notes. Two years later one Paul Dawson edited A System of Experimental Philosophy, Prov’d by Mechanicks... As Performed by J. T. Desaguliers. The lecturer did not produce his own version until 1734 (A Course of Experimental Philosophy, Volume I) when he took occasion to denounce this unauthorized version. Meanwhile he was content to print only short syllabi of his lectures: Mechanical and Experimental Philosophy (1724) and Experimental Course of Astronomy (1725). The former exists in both French and English versions, Desaguliers advertising his willingness to teach in these languages and Latin. At last the long-promised first volume of the Course appeared in 1734, containing five long lectures and many additional notes. It is devoted wholly to theoretical and practical mechanics, including both a simple treatment of Newton’s system of the world and a description of Mr. Allen’s railroad at Bath. Desaguliers attributed the ten-year delay before the appearance of his second tome to his desire to improve the treatment of machines, especially waterwheels; he excused himself for omitting optics altogether, referring the reader to Robert Smith’s Complete System of Opticks. Continuing with mechanics, in seven lectures he discussed impact and elasticity, vis viva and momentum, heat, hydrostatics and hydraulics, pneumatics, meteorology, and more machines. This second volume is even more concerned with applied science and engineering than the first and entitles Desaguliers to be considered a forerunner of the more advanced knowledge of machinery that characterized the Industrial Revolution. Certainly its influence was greater upon practical men and inventors than upon physicists.

Desaguliers was married on 14 October 1712 to Joanna Pudsey, by whom he had several children; the youngest, Thomas, distinguished himself as an artilleryman. About 1739 the construction of the approaches to Westminster Bridge (upon whose design Desaguliers was consulted) necessitated the destruction of Channel Row; he moved his home and classes to the Bedford Coffee House in Covent Garden, where he died. He was buried in the Savoy Chapel, and there is no good reason for supposing him indigent. There are (or were) at least two portraits of Desaguliers: by H. Hysing (1725), engraved by Peter Pelham, and by Thomas Frye (1743), engraved by R. Scaddon.

BIBLIOGRAPHY

I. Original Works. Desaguliers published translations besides that already mentioned: Fires Improv’d: Being a New Method of Building Chimneys, so as to Prevent Their Smoaking, from the French by Nicolas Gauger (London, 1715), mostly about an elaborate form of fire grate; The Motion of Water and Other Fluids, from the French by Edmé Mariotte (London, 1718); The Mathematical Elements of Natural Philosophy, from the Latin by W. J. ’sGravesande (London, 1720); The Whole Works of Dr. Archibald Pitcairne, from the Latin (London, 1727), with G. Sewell; and An Account of the Mechanism of an Automaton, from the French by J. de Vaucanson (London, 1742). Most were reprinted.

Desaguliers’ own writings, in addition to those already discussed, were The Newtonian System, an Allegorical Poem (London, 1728), written on the accession of George II; an Appendix on the reflecting telescope, pp. 211–288 in William Brown’s translation Dr. Gregory’s Elements of Catoptrics and Dioptrics (London, 1735), which contains most of the correspondence between Newton and others relating to the development of Newton’s form of that instrument in 1668 and subsequently; and A Dissertation Concerning Electricity (London, 1742), the French version of which (Bordeaux, 1742) received a prize awarded by the Académie de Bordeaux (Course of Experimental Philosophy, II, 335).

There are MSS by Desaguliers in the Sloane and Birch collections of the British Museum and in the archives of the Royal Society.

II. Secondary Literature. Modern studies of Desaguliers include Jean Barlais, in Les archives de Trans en Provence, 61 , 281–288, for more on his freemasonry; I. Bernard Cohen, Franklin and Newton (Philadelphia, 1956), esp. pp. 243–261,376–384, mainly on electricity; D. C. Lee, Desaguliers of No. 4 and His Services to Freemasonry (London, 1932); Paul R. Major, The Physical Researches of J. T. Desaguliers, M. Sc. thesis, London University, 1962, with bibliography; and Jean Torlais, Un Rochelais grand-maître de la Franc-Maçonnerie et physicien au XVIIIe siècle: Le Reverend J.-T. Desaguliers (La Rochelle, 1937).

A. Rupert Hall