Sir John Leslie, physicist, mathematician and inventor, died on the 3rd of November, 1832.
Sir (by request) John Leslie was a marvellous man who admirably annoyed the Church of Scotland, which was always a good thing, taught Mathematics and Natural Philosophy (what we now call Physics) in Edinburgh, and invented lots of ‘scopes’ and ‘meters’. He invented the aethrioscope, the hygroscope and the pyroscope in addition to the atmometer, the differential and optical thermometers, the hygrometer, the photometer and the pyrometer. Ok, ok, the hygrometer and the hygroscope are the same thing; as are the pyroscope and the pyrometer, if you haven’t guessed. Leslie was a “self-taught mathematician and physicist” but he was also a prodigious inventor, whose main contributions to Physics were to do with the properties of air, heat and moisture. He invented all those ‘scopes’ and ‘meters’ the better to allow him to explore those properties, and his discoveries enabled the development of new equipment and processes.
His aethrioscope is used to measure very accurately the variations in temperature due to the condition of the atmosphere and his pyroscope or pyrometer is a kind of optical thermometer. The differential thermometer is also used for measuring difference in temperature and the atmometer is used to measure the rate of evaporation from a moist surface. The photometer is used in measuring luminous intensity and one of his inventions, the hygrometer, a device for measuring humidity, allowed him to discover, in 1810, a process of artificial congelation. That process in turn enabled him to become the first man to create ice artificially, with the use of water, mercury and an air pump. In 1802, Leslie was also the first man to give a true explanation of capillary action. Not content with all of that, in 1793, Leslie translated from the French all nine volumes of the ‘Natural History of Birds’ of the French naturalist, Georges-Louis Leclerc, Comte de Buffon.
Leslie also published a lot of books in his own right, with the most notable being ‘An Experimental Inquiry into the Nature and Properties of Heat’ in 1804, for which he won the ‘Rumford Medal’ of the Royal Society of London. He published numerous volumes, text books and papers on Mathematics, Geometry and Trigonometry, and contributed heavily to ‘Nicholson's Philosophical Journal’. He also contributed several articles to the ‘Encyclopaedia Britannica’ and the ‘Edinburgh Encyclopaedia’, and strangely enough, he somehow contributed to a popular work on polar travel. In 1813, he found time to publish a short account with a long title: ‘A Short Account of Experiments and Instruments depending on the relations of Air to Heat and Moisture’. Volume I of Leslie’s mathematical course went through four editions and was translated into French and German; a high honour indeed for those times.
John Leslie was born in Largo, in Fife, on the 16th of April, 1766. He learned mathematics at home from his father and elder brother and, for no more than a year, at a local school in Leven. At the age of twelve, he received a gift of mathematical books from the Minister at Largo and, in 1779, he entered the University of St Andrews where he completed an arts course. He benefitted from a scholarship, which was granted by the Principal on condition that Leslie would, not untypically, go on to join the Church. With that in mind, he went to the University of Edinburgh in 1785, to study Divinity. However, he became far more interested in mathematics and science. He attended lectures by a veritable ‘Who’s Who?’ of famous Scots, including John Playfair, Joseph Black, Alexander Monro, John Robison, and Dugald Stewart. When the Principle died, Leslie no longer felt obliged to follow the conditions of his scholarship. Instead, another famous Scot, Adam Smith, got Leslie a job as a tutor to one of his relations and, in 1788, Leslie sent his first paper to the Royal Society of Edinburgh. After that, Leslie spent some time as a private tutor in Virginia, before returning to Britain around 1790 and getting another tutoring job; to the Pottery Wedgwood’s in Staffordshire. It was while he was employed by Josiah Wedgwood that Leslie got involved with Buffon’s birdies. The income he got from the book of birds and the pension for life that he later got from Wedgwood, basically gave him the freedom to concentrate on becoming an academic and prolific inventor. The final hurdle was the antipathy of the Church.
After a celebrated dispute that went all the way to the General Assembly of the Church of Scotland, which had an unnatural influence on University appointments, Leslie was elected to succeed John Playfair as Professor of Mathematics at Edinburgh. Later, after Playfair’s death, he became Professor of Natural Philosophy, but the violent opposition of the narrow-minded clerics in 1805 was because Leslie, like many great thinkers and scientists, he had been accused of heresy; in his case by the Synod. Leslie, who had studied Divinity, was by that time practically an atheist and, quite apart from failing to have become a Minister, his politics were also of the wrong colour. The Church adduced that Leslie’s support of the doctrine of yet another celebrated Scot, the religiously sceptical David Hume, was tantamount to heresy. Leslie’s profanation? He had written that “causation was nothing more than an observed constant and invariable sequence of events,” thus negating established, but decidedly tenuous, theological principles.