Castelli, BENEDETTO, mathematician and physicist; b. at Perugia, Italy, 1577; d. at Rome, 1644. He was destined by his parents for the service of the Church and entered the Order of St. Benedict, at Monte Cassino. There he became abbot, and in 1640 he was transferred to the Abbey of San Benedetto Aloysio. He was specially interested in the mathematical sciences and their application to hydraulics. Galileo, his teacher, and Toricelli, one of his pupils, speak very highly of his scientific attainments, and both of them frequently asked his advice. In 1623 Urban VIII invited him to Rome and later appointed him chief mathematician to the pope and public professor of mathematics in the University of Rome. In 1625 he was sent with Monsignore Corsini to study the disorders occasioned by the waters of the Romagna, and to propose a remedy. Here he completed his important work on the "Mensuration of Running Water", in which he developed the important relations, that the speed of a current varies inversely as the area of its cross section, and that the discharge from a vessel depends on the depth of the tap below the free surface of the water. He was often consulted in other provinces of Italy in connection with drainage, water-supply, prevention of floods, and the like.
His chief work is "Della misura dell'acqua corrente" (Rome, 1628; 3d ed., 1660), translated into English by Salusbury (London, 1661), and into French by Saporta (1664), reprinted (Bologna, 1823) in Cardinali's collection "d'autori italiani che trattano del moto dell'acqua". Another work is "Risposta alle oppositioni del Sig. Lodvico, etc., center al trattato del Sig. Galileo, Delle cose che stanno sopra acqua" (Bologna, 1655). According to Poggendorf, the invention of the helioscope is ascribed to him.