Snell, Willebrod
, son of the preceding, and an excellent mathematician, was born at Leyden in 1591, where he succeeded his father in the mathematical chair in 1613, and where he died in 1626, at only thirty-five years of age. He was author of several ingenious works and discoveries, and was the first who discovered the true law of the refraction of the rays of light; a discovery which he made before it was announced by Des Cartes, as Huygens assures us. Though the work which Snell prepared upon this subject, and upon optics in general, was never published, yet the discovery was very well known to belong to him, by several authors about his time, who had seen it in his manuscripts. He undertook also to measure the earth. This he effected by measuring a space between Alcmaer and Bergen-op-zoom, the difference of latitude between these places being 1° 1′ 30″. He also measured another distance between the parallels of Alcmaer and Leyden; and from the mean of both these measurements, he made a degree to consist of 55,021 French toises or fathoms. | These measures were afterwards repeated and corrected by Musschenbroek, who found the degree to contain 57,033 toises. He was author of a great many learned mathematical works, the principal of which are, 1. “Apollonius Batavus;” being the restoration of some lost pieces of Apollonius, concerning Determinate Section, with the Section of a Ratio and Space, in 1608, 4to, published in his seventeenth year; but on the best authority this work is attributed to his father. The present might perhaps be a second edition. 2. “Eratosthenes Batavus,” in 1617, 4to; being the work in which he gives an account of his operations in measuring the earth. 3. A translation out of the Dutch language, into Latin, of Ludolph van Collen’s book “De Circulo & Adscriptis,” &c. in 1619, 4to. 4. “Cyclometricus, De Circuli Dimensione,” &c. 1621, 4to. In this work, the author gives several ingenious approximations to the measure of the circle, both arithmetical and geometrical. 5. “Tiphis Batavus;” being a treatise on Navigation and naval affairs, in 1624, 4to. 6. A posthumous treatise, being four books “Doctrinæ Triangulorum Canonicæ,” in 1627, 8vo: in which are contained the canon of secants; and in which the construction of sines, tangents, and secants, with the dimension or calculation of triangles, both plane and spherical, are briefly and clearly treated. 7. Hessian and Bohemian Observations; with his own notes. 8. “Libra Astronomica & Philosophica;” in which he undertakes the examination of the principles of Galileo concerning comets, 9. “Concerning the Comet which appeared in 1618, &c.” ^{1}