There is, however, a more decided difference between West's system and those previously in use, for it is marked by the fact that the slabs composing the shell of the whole structure in many cases may be built up before the filling-in is deposited between the slabs, and in none of the other cases can this be done. In fact, only in the first two cases before mentioned can more than one course of slabs be laid before filling-in of some kind must be done. Compared with the ordinary method of building in concrete, this system avoids: 1. The charge for use and waste of wood casings; 2. finishing the face of the work (both inside and outside) after the structure is raised, and, therefore, the bursting-off of the finished face; and 3. the difficulties encountered in working mouldings and other ornaments on the face of the work by the ordinary plasterer's methods. It also provides a face of any of the usual colors that may be obtained in concrete, besides a facing of any other material, such as marble, etc., and produces better and more durable work, at the same time showing a saving in cost, especially in the better classes of work; all of which is effected with less plant than ordinarily required. For engineering work, such as sea walls, the hexagonal slabs, made of greater thickness than those employed for ordinary walling, will answer admirably, especially if the grooves be made proportionately larger. By the use of these slabs the work may be built up with great rapidity. For small domestic work, such as the dwellings of artisans, these slabs; which are of such a form as to render them easy of transport, may be supplied to the workmen themselves in order that they may erect their own dwellings, as, on account of the simplicity of this system and the absence of need of plant, any intelligent mechanic can do the work.