“ In 1785 when Dugald Stewart succeeded Ferguson in the Edinburgh chair of moral philosophy, Playfair succeeded the former in that of mathematics. In 1802 he published his celebrated volume entitled Illustrations of the Huttonian Theory of the Earth. To its publication the influence exerted by James Hutton on the progress of geological knowledge is largely due. In 1805 he exchanged the chair of mathematics for that of natural philosophy in succession to Dr John Robison, whom also he succeeded as general secretary to the Royal Society of Edinburgh. ”