John Leslie, Scottish mathematician and physicist, early 19th century.
© Science Museum / Science & Society Picture Library
Engraving of Sir John Leslie (1766-1832). After completing his studies at St Andrews and Edinburgh, Leslie travelled as a tutor in America and Europe. He invented a differential thermometer, a hygrometer (for measuring humidity), and a photometer (for measuring the intensity of light). In 1810 he created artificial ice by freezing water under an air pump.