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Figs. I to 5. Architectural Scales. Fig. 6. Method of dividing a Scale. Fig. 7. Workshop Method of dividing J. Line into Equal Parts

Figs. I to 5. Architectural Scales. Fig. 6. Method of dividing a Scale. Fig. 7. Workshop Method of dividing J. Line into Equal Parts and each ij- in. a foot. Fig. 5 is a scale of 3 in. to 1 ft. or one-fourth lull size; the representative fraction is

The most common scales in architects' offices are J in., ^ in., 3 in. and full size. Engineers' draughtsmen use

22 DIAGONAL SCALES

these also : § in., £ in and i| in. to a foot and sometimes 6 in.—i.e. half-full size—perhaps the worst possible scale to work to, as it is fruitful of errors.

In laying down a srale, the feet, to the required fraction, should be set off with the dividers upon a line near the bottom of the drawing paper, and the divisions drawn in by the aid of the set square, and numbered towards the right hand, starting at the second division ; the first one marked o, is reserved for inch subdivisions. In the larger scales, these may be set off with the dividers direct; but in the smaller ones, it is better to adopt the method shown in Fig. i, and enlarged in Fig. 6. Set up a line at any convenient angle, from the left extremity of the line to be divided, and, opening the dividers to any convenient space, wider than the required division, step it up the inclined line umil twelve (or any other number required) spaces are marked. From the twelfth point, draw a line to the right-hand extremity of the line to be divided, as point b in Fig. 6. Next draw a series of parallel lines to this one from the other points, and, where they intersect the horizontal line, erect short perpendiculars, which will represent inches in this case. This is a rapid and accurate method of dividing any line. A favourite workshop method by aid of the 2-foot rule is shown in Fig. 7. Draw two parallel lines at right angles to the line to be divided, as at c and d, which shows a line 7 J in. long, that is to be divided into nine equal parts; lay the rule across the lines with the extremity 011 one line, and the required number upon the other, tick off the inches, with a pencil; then draw parallels to the first two through the points to cut the line c-d.

The Diagonal Scale shown on the protractor, Fig. 2, p. 15, is used for toking very minute measurements. The one shown will measure to two decimal places, or the hundredth part of one of the large or primary divisions, which may represent inches, feet, yards, chains, etc., as reauired.

It is constructed by drawing a rectangle whose height is

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