I urge the reader to give careful attention to the remarkable interweaving of the first four voice parts. At the fifth bar of the melody, in the first voice, the second voice enters with the beginning of the air. When the second voice reaches the fifth bar of the air, the third voice comes in, and at the same time the first voice begins the second half of the melody. When the third voice reaches the fifth bar of the first half of the air, the fourth voice comes in, and the second voice begins the second half. And so it goes on, the entrances always being made according to the rule established at the start, and each voice singing the tune without the alteration of a single interval to make it fit into the scheme. Rigid imitation of this kind is called "canon." In this particular canon the two lowest voices have a bass in two parts, written in double counterpoint, which they sing over and over again all the way through. A constantly repeated bass is called a "basso ostinato," and this is the first example of it. This kind of writing possessed the merit of high organization, without which there can be no work of art. One might search in vain for evidences of artistic design in the early Gregorian chant, while in such works as those of the early Frenchmen and their English disciples they confront one in every measure. The result was that these writers developed several kinds of contrapuntal writing. But it must be admitted that their work was cold and mathematical, being wholly the result of ingenious calculation. Furthermore it must be borne in mind that they had no conception whatever of music as a means of expression. In a vague way they felt its suitability to the worship of their churches, and its Gothic complexity did indeed harmonize well with ecclesiastical architecture. But it never occurred to the composers of the French school to try to make music beautiful for its own sake. They were too busy exploring the resources of their art, and their _materia musica_ was as yet too scanty to allow them to treat their art with the command of mastership. But they served well the cause of music by discovering many of its essential rules, and by formulating in their treatises much of its fundamental theory. It is not at all surprising that in the last period of this school we meet with a large work. The last important master of the school was William of Machaut, who flourished between 1284 and 1369, and wrote the mass for the coronation of Charles V. of France.