John Perry (14 February 1850 – 4 August 1920) was a pioneering engineer and mathematician from Ireland. Life He was born on 14 February 1850 at Garvagh, County Londonderry, the second son of Samuel Perry and a Scottish-born wife. John's brother James was the County Surveyor in Galway West and co-founded the Galway Electric Light Company. One of his daughters, Alice, was the one of the first women in the world with an engineering degree. Perry worked as Lord Kelvin's assistant at the University of Glasgow, and later became professor of mechanical engineering at Finsbury Technical College. He was a colleague of William Edward Ayrton and John Milne at the Imperial College of Engineering in Tokyo, 1875–79, and was also a Fellow of the Royal Society. He was professor of mathematics at Imperial College in London from 1896 to 1913. In 1900 he was elected president of the Institution of Electrical Engineers, and from 1906–08 served as president of the Physical Society of London. Perry was a great admirer of his employer, Lord Kelvin. In the printing of his 1890 lecture on spinning tops, Perry inscribed the following acknowledgement: "This report of an experimental lecture is inscribed to Sir William Thomson, by his affectionate pupil, the lecturer, who hereby takes a convenient method of acknowledging the real author of whatever is worth publication in the following pages." The book was later reprinted by Dover Publications in 1957 as Spinning Tops and Gyroscopic Motions. Although others (Max Schuler in Germany, Sperry in the USA) had been working on developing practical gyrocompasses, Perry collaborated with Sidney Brown to further develop these and they were awarded : "Gyro-compass" by John Perry, Sidney George Brown, filed August 1917; granted 1919. Perry received an honorary doctorate (LL.D) from the University of Glasgow in June 1901. Challenging Lord Kelvin In 1895, Perry published a paper challenging Kelvin's assumption of low thermal conductivity inside the Earth, and thus disputing Kelvin's estimate that the Earth was only 20–400 million years old, but this had little impact. It was not until the discovery in 1903 that radioactive decay releases heat and the development a few years later of radiometric dating of rocks that it was accepted that the age of Earth was many times greater, as Perry had argued. Perry's reasoning held that if the interior of the Earth was fluid, or partly fluid, it would transfer heat much