1877, forcing Marshall to leave his position as a Fellow of St John's College, Cambridge, to comply with celibacy rules at the university. He became the first principal at University College, Bristol, which was the institution that later became the University of Bristol, again lecturing on political economy and economics. In 1885 he became professor of political economy at Cambridge, where he remained until his retirement in 1908. Over the years he interacted with many British thinkers including Henry Sidgwick, W.K. Clifford, Benjamin Jowett, William Stanley Jevons, Francis Ysidro Edgeworth, John Neville Keynes and John Maynard Keynes. Marshall founded the Cambridge School which paid special attention to increasing returns, the theory of the firm, and welfare economics; after his retirement leaderships passed to Arthur Cecil Pigou and John Maynard Keynes. Contributions to economics Marshall desired to improve the mathematical rigour of economics and transform it into a more scientific profession. In the 1870s he wrote a small number of tracts on international trade and the problems of protectionism. In 1879, many of these works were compiled into a work entitled The Theory of Foreign Trade: The Pure Theory of Domestic Values. In the same year (1879) he published The Economics of Industry with his wife Mary Paley. Although Marshall took economics to a more mathematically rigorous level, he did not want mathematics to overshadow economics and thus make economics irrelevant to the layman. Accordingly, Marshall tailored the text of his books to laymen and put the mathematical content in the footnotes and appendices for the professionals. In a letter to A. L. Bowley, he laid out the following system: He perfected his Economics of Industry while at Bristol, and published it more widely in England as an economic curriculum; its simple form stood upon sophisticated theoretical foundations. Marshall achieved a measure of fame from this work, and upon the death of William Jevons in 1882, Marshall became the leading British economist of the scientific school of his time. Marshall returned to Cambridge, via a brief period at Balliol College, Oxford, during 1883–84, to take the seat as Professor of Political Economy in 1884 on the death of Henry Fawcett. At Cambridge he endeavoured to create a new tripos for economics, a goal which he would achieve only in 1903. Until that time, economics was taught under the Historical and Moral Sciences Triposes which failed to provide Marshall the kind of