the scale is given in fig. 39. Sup- 1 ' 1 1 1 pose the (¿rawing is to be re- ^
duced one-half, a scale half fig. 39 is to be made, a& in fig. 40; and as each measurement is taken in ■ . , ■ , , t ... . the compasses from fig. 38, it '» a Ts • r « i must be applied to the scale in fife; 4(t fig. 39. Suppose this distance is found to be 6 feet, then the distance of 6 feet must be taken from the scale of fig. 40; and the line thus obtained must be drawn in a situation corresponding to that in fig. 38. The result will be a reduced copy, one-half of the size, as shewn in
Example 38, fig. 41. To reduce by means of the proportional compasses : Having previously set them at the desired mark on the scale attached to each instrument, according to any proportion as desired, all that is necessary to be done is to take any measurement with one end; the distance corresponding to this, reduced or enlarged, is given in the other ends. This being transferred to paper, the desired distance is obtained at k once. To reduce by means of the ordinary compasses, without the use of a scale as just described in figs. 38-41, is a matter requiring greater time, and accuracy of adjustment of the compasses is indispensable. Suppose a b, fig. 41, to be the points representing the intersection of the centre-fines
of the parts a, b with the base-line ab, and that a line corresponding to the centre-line from a was drawn on paper, and that half the distance aft in the copy was to be transferred to the paper, half of a b would have to be found in the first place on the copy and transferred. By proceeding thus, a copy of £g. 41, but only half its size, would be obtained. The enlargement of figures is exactly the converse of what we have described in figs. 38-41.
Example 39, ^fig. 42, is a drawing which is reduced half in
Example 40, fig. 43.
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