Dury next judiciously adds that room must be left in the library for the increase of books, an indispensable condition not always easy of fulfilment; and that "in the printed catalogue a reference is to be made to the place where the books are to be found in their shelves or repositories." That is, the catalogue must have press-marks; in which suggestion Dury was two centuries ahead of many of the most important foreign libraries. It will be observed that he takes it for granted that the catalogue shall be printed, and in this he was ahead of almost all the libraries of his time, and until lately of the British Museum. In fact he could not be otherwise, for a printed catalogue is an essential condition of his dominant idea that the librarian should be a "factor" to "trade" with learned men and corporations for mutual profit. Hence he prescribes "a catalogue of additionals, which every year within the universities is to be published in writing within the library itself, and every three years to be put in print and made common to those that are abroad." The full plan of communication is unfolded in the following passage:— "When the stock is thus known and fitted to be exposed to the view of the learned world, then the way of trading with it, both at home and abroad, is to be laid to heart both for the increase of the stock and for the improvement of its use. For the increase of the stock both at home and abroad, correspondence should be held with those that are eminent in every science to trade with them for their profit, that what they want and we have, they may receive upon condition; that what they have and we want, they should impart in that faculty wherein their eminence doth lie. As for such as are at home eminent in any kind, because they may come by native right to have use of the library treasure, they are to be traded with all in another way, viz., that the things which are gained from abroad, which as yet are not made common and put to public use, should be promised and imparted to them for the increase of their private stock of knowledge, to the end that what they have peculiar, may also be given in for a requital, so that the particularities of gifts at home and abroad are to meet as in a centre in the hand of the Library Keeper, and he is to trade with the one by the other, to cause them to multiply the public stock, whereof he is a treasurer and factor. "Thus he should trade with those that are at home and abroad out of the university, and with those that are within the university, he should have acquaintance to know all that are of any parts, and how their view of learning doth lie, to supply helps unto them in their faculties from without and from within the nation, to put them upon the keeping of correspondence with men of their own strain, for the beating out of matters not yet elaborated in sciences; so that they may be as his assistants and subordinate factors in his trade and in their own for gaining of knowledge."