[Fig. 190] shows such a scale broken. An explanation of the 1″ and 1⁄2″ side will suffice for all. Where it is used as a scale of 1″ to one foot, each large space, as from 0 to 12 or 0 to 1, represents a foot, and is a foot at that scale. There being 12″ in one foot, the twelve long divisions at the left represent inches; each inch is divided into two equal parts, so from 0 to one division at the left of 9 is 91⁄2″ and so on. The 1″ and 1⁄2″ scales being at opposite ends of the same edge, it is obvious that one foot on the 1″ scale is equal to two feet on the 1⁄2″ scale, and conversely, one foot on the 1⁄2″ scale is equal to six inches on the 1″ scale; and 1″ being equal to one foot, the total feet in length of scale will be 12; at 1⁄2″ to 1 foot the total feet will be 24.
In working to regular scales, such as 1⁄2, 1⁄8, or 1⁄16 size, a good plan is to use a common rule, instead of a graduated scale. There is nothing more convenient for a mechanical draughtsman than to be able to readily resolve dimensions into various scales, and the use of a common rule for fractional scales trains the mind, so that computations come naturally, and after a time almost without effort.
The protractor shown in [fig. 193] is an instrument for laying down and measuring angles on paper; it is used in connecting with a scale to define the inclination of one line to another.
Protractors have the degrees of a half circle marked upon them; as the whole circle contains 360 degrees, half of it will contain 180, one-quarter 90, etc. Hence, protractors showing 180° exhibit all that is needed. To protract means to extend, so this instrument is also useful in “extending” the lines of inclination at the circle.
Fig. 192.
Fig. 193.
DRAWING-PENS.
A special pen called a drawing-pen, and also special ink, are required to ink a drawing; [figs. 194] and [195] represent two sizes of drawing-pens—one being best adapted for fine work, and the other for coarse or heavy line work. The points, as will be observed in the illustration, are made of two steel blades which open and close as required for thickness of lines by a regulating screw.