This diminution in the number of schoolmasters, for some reason or other, seems to have continued into the following century. William Byngham, in the petition which he submitted to the king in 1439 for the purpose of obtaining permission to found a college at Cambridge for the training of teachers for grammar schools throughout the country, stated that, "on the East of the way between Hampton and Coventry and no further north than Ripon, no less than seventy grammar schools had fallen into desuetude because of the scarcity of teachers."[377] It is also extremely probable that the Black Death contributed considerably to the almost total disappearance of the French language from the schools. One effect of the Norman Conquest had been the gradual growth of French as the spoken language; after the pestilence period, the use of the native tongue of the English nation again became common. This is directly evidenced by statements contained in John de Trevisa's translation of Higden's _Polychronicon_. After showing that French was at one time very prevalent in this country because it was the language in common use at schools and that the children of "gentil men" were taught that language from the time they were "i-rokked in here cradel," Trevisa states that after the Black Death the knowledge of French had disappeared to so great an extent that "now children of gramer scole conneth na more Frensche than can hir lift heele."[378] The fifteenth century witnessed important changes in the economic condition of England. The most important of these changes was connected with the development of manufactures. At the close of the fourteenth century, we learn that wool was "la Sovereigne Marchandise and Jewel ... d'Angleterre"[379]; a century later, it is said that "the makeyng of cloth" was "the grettest occupacion and lyving of the poor people of the land."[380] Various enactments of the period testify to the growth of the woollen industry, and to the efforts which were made by the government to foster and develop it. But though the manufacture of cloth was the most important industry, yet it was not the only form of industrial occupation. Before the close of the fifteenth century, the manufacture of silk had been established in London, coal mining was carried on to a considerable extent, the manufacture of beer had been instituted, the making of bricks had been renewed, guns were being made, and ship-building was making progress.[381]