Another letter, written a century and a half later, makes reference to the practice of exchanging books or of using them as pledges. A prior writes to an abbot in 1150: “To his Lord, the Venerable Abbot of---- wishes health and happiness. Although you desire to have the books of Tully, I know that you are a Christian and not a Ciceronian. But you go over to the camp of the enemy not as a deserter, but as a spy. I should, therefore, have sent you the books of Tully which we have, _De Re Agraria_, the Philippics, and the Epistles, but that it is not our custom that any books should be lent to any person without good pledges. Send us, therefore, the _Noctes Atticæ_ of Aulus Gellius and Origen on the Canticles. The books which we have just brought from France, if you wish for any of them, I will send you.” The Abbot replies at the end of a long letter: “I have sent you as pledges for your books, Origen on the Canticles, and instead of Aulus Gellius (which I could not have at this time), a book which is called _Strategematon_, which is military.”[207] The custom of securing books by chains, which prevailed with the libraries of all the earlier religious institutions, did not originate with these. Eusebius mentions that the Roman Senate in the time of Claudius ordered the treatise of Philo Judæus on the Impiety of Caligula to be chained in the library as a famous monument. There appears to have been an early appreciation on the part of certain of the monastery scholars of the importance of indexes. Fosbroke quotes among others the example of John Brome, Prior of Gorlestone, who, in the fifteenth century, put indexes to almost all the books in his library. From an examination of the catalogues of various of the ecclesiastical libraries, Fosbroke arrives at the calculation that the proportion of the works contained under the several main sub-headings was approximately as follows: Divinity, 175; scholastic literature, 89; epistles and controversial literature, 65; history, 54; biography, 32; arts, mathematics, and astrology, 31; philosophy, 13; law, 6.[208] This classification does not give any separate heading for allegory, although this was a subject in which not a few of the earlier monkish writers largely interested themselves. As an example of monkish allegorical literature, Fosbroke mentions a work written in 1435, under the instructions of a cloth shearer in France, whose name he does not give. The cloth cutter, being a great lover of tennis, had written a ballad upon that game. When he was old, he wished to atone for his early sins and frivolities, and he secured the services of a Dominican monk, who wrote, at his instance and expense, an allegory on the game of tennis. The wall of the tennis court stood for faith, which should always rest on a solid foundation, while in the other conditions of the game the Dominican finds the cardinal virtues, the evangelists, active and contemplative life, the old and the new law, etc.