SNELL, Willebrord (1591-1626), commonly known as Snellius, astronomer and mathematician, was born at Leyden in 1591. In 1613 he succeeded his father as professor of mathematics in the university of Leyden. In 1615 he planned and carried into practice a new method of finding the dimensions of the earth, by determining the distance of one point on its surface from the parallel of another, by means of a triangulation. His work Eratosthenes Batavus, published in 1617, describes the method and gives as the result of his operations between Alkmaar and Bergen-op-Zoom a degree of the meridian equal to 55,100 toises = 117,449 yards. (A later recalculation has given 57,033 toises = 121,569 yards, after applying some corrections to the measures indicated by himself.) Snell also distinguished himself as a mathematician, and discovered the law of refraction, which, however, is generally attributed to Descartes, who made it more widely known. Snell died at Leyden on 30th October 1626.

In addition to the Eratosthenes Batavus he published Cyclometria sive de circuli dimensione (Leyden, 1621, 4to), and edited Coeli et siderum in eo errantium observationes Hassiacae (ibid., 1618, 4to), containing the astronomical observations of Landgrave William IV. of Hesse. About his Tiphys Batavus s. Histiodromice, de navium cursibus et re navali (1624), see Navigation, vol. xvii. p. 255, note. A trigonometry {Doctrina triangulorum) by him was published a year after his death.