[9] Suppose the dimensions of a proposed vessel to be 320 × 36 × 26½ feet, then, according to a method of approximation largely in use, the sum of these dimensions divided by 100 gives what is known as the “cubic number”—(320 × 36 × 26½ ÷ 100) = 3052 cubic number. Suppose that for a vessel already built, similar in type and dimensions, or of similar proportions, to the one proposed, the cubic number, when divided into the ship’s actual weight (_i.e._, the displacement _minus_ the weight of machinery and the dead-weight carried), gives say ·53, then this figure represents the “co-efficient” of ship’s weight, and applying it in the case supposed gives:—3052 × ·53 = 1620, the weight of hull for proposed vessel. This example illustrates the manner in which the weight of machinery is estimated, and indicates the nature and use of the general term “co-efficient:” frequently employed in this chapter. [10] One such method, devised and followed by Mr C. Zimmermann in his daily practice as chief draughtsman to the Barrow Shipbuilding Company, and described by him before the Institution of Naval Architects in 1883, gives with very little preliminary calculation, and at once, a close approximation to the correct displacement. Another system, originated and used in practice by Mr Chas. H. Johnson, chief designer to Messrs Wm. Denny & Brothers, consists of an analysis of the lines of vessels of various degrees of fineness and fulness previously built, formulated for daily use in a series of curves of areas, giving, for sections at certain fixed distances from midships—in terms of percentage to the midship area—the particular area specially suited to afford the required displacement; and at the same time to maintain the general form of hull which in actual practice has proved satisfactory with respect to speed. In his later practice, Mr Johnson has found it preferable to use the block form of analysis of Mr A. C. Kirk (considered further on in matters relating to speed), using the three sides of that form as a basis upon which to group the water-lines. [11] For illustrated descriptions of this and other improved calculating instruments referred to in this chapter, see Appendix.