Several important points remain to be noticed in connection with the use of the instrument. As a rule, the areas to be measured in connection with ship designing are on a small scale, and the fixed or needle point about which the instrument moves can always be placed _outside_ the figure measured, in which case the process remains as above stated. It should be mentioned, however, that by placing the needle point _inside_ the figure, in such a position as to enable the operator to follow its contour a larger figure can be measured at one operation—the reading, however, being less than the true area by a constant number which varies slightly with the construction of each instrument, and which is found engraved on the small weight already referred to (on the top of the bar in the proportional planimeter). Adding this constant number to any reading taken by the instrument placed as described, gives the true area. The counting disc may go through more than one revolution forwards or backwards. If the needle point be _outside_ the figure traversed the counting disc can only move _forwards_ (as 9, 0, 1, 2, &c.): that is, provided the figure has been traced in the manner directed—in the direction of the hands of a watch. Then as many times as the zero mark passes the index line add 10·000 to the _second_ reading. If the needle point be _inside_ the figure, the disc can move either forwards or backwards. If moving backwards, as 2, 1, 0, 9, &c., then add 10·000 to the _first_ reading. Before passing from the subject of the planimeter it may be both interesting and useful to give an example of a calculation involving its use. Subjoined is a specimen displacement and longitudinal centre of buoyancy calculation, and any one familiar with the prodigious array of columns and figures pertaining to a “displacement sheet” of the ordinary kind cannot fail to appreciate the advantages of the specimen, both with respect to simplicity of arrangement and curtailment of the amount of calculation ordinarily involved:— EXAMPLE OF SHIP DISPLACEMENT, WORKED OUT BY PLANIMETER.