In addition Frisi was professor of philosophy at Casale Novara and Collegio Alessandro in Milan from 1753 to 1756. He left Milan in 1756 to take up a post as professor of philosophy at the University of Pisa. After holding the post in Pisa for eight years, Frisi returned to Milan becoming professor of mathematics at the Scuola Palatina in 1764. He was a leading authority on mathematics and science in his day.
Frisi made a number of contributions to mathematics, physics and astronomy. In physics he worked on light and electricity but, although his work was very up to date for its time, his explanations were based on vibrations in the ether and did little to advance the topic. He was, however, the first to introduce the lightning conductor into Italy.
His work on astronomy was based on Newton's theory of gravitation and is therefore of considerably more importance than his work in physics. He studied the motion of the earth and he was awarded a prize by the Berlin Academy for his outstanding memoir De moto diurno terrae. He also studied the physical causes for the shape and the size of the earth using the theory of gravity. Other astronomical phenomena which he studied included the difficult problem of the motion of the moon.
He studied kinematics and hydraulics and he was responsible for drawing up plans for a canal between Milan and Pavia. In fact the work on this canal was not undertaken in Frisi's lifetime, but in 1819, thirty-five years after Frisi's death, the canal was built to his plan. His major work on hydraulics is Del modo di regolare i fiumi, e i torrenti written in 1762.
In a paper of 1781 Frisi discussed isoperimetric problems. These were popular problems at this time with the Jacob and Johann Bernoulli having made important contributions and Euler, in 1744, having given a rule to determine a minimising arc between two points on a curve having continuous second derivatives. Frisi looked at problems involving both maximising and minimising.
Frisi also wrote on the contributions of Galileo, Cavalieri, Newton and d'Alembert and brought their ideas to a wide audience. Such writings were widely read for Frisi was considered, in Italy, to be :-
... a scientific authority and [he] was also well known abroad, so much so that his major works (which he wrote in Latin) were translated into French and English.These major works are Algebra e geometrica analitica Ⓣ (1782), Meccanica Ⓣ (1783), and Cosmografia Ⓣ (1785) which contain much of Frisi's earlier work, but written up in a polished form.
Frisi was also editor of the newspaper Il caffè. The paper was influenced by the ideas of the Illuminati (Enlightened Ones) which promoted free thought and democratic political theories. Through this paper Frisi had a major influence on the :-
... cultural, social, and political life of Milan ...Letters written by Frisi and to Frisi are discussed in  and . Letters between Teodorc Bonati and Frisi discussiong questions of mechanics and hydraulic mechanics are given in . A letter by Frisi written in 1753 on mechanics and geometry is given and discussed in .
Frisi received many honours and his talents were recognised on an international scale. He was elected to the Académie des Sciences in Paris in 1753, then elected a Fellow of the Royal Society in London in 1757. In addition he was elected to the Academy of St Petersburg, and the Academy of Berlin.
Article by: J J O'Connor and E F Robertson