drawing one line parallel witli another, to which one edge of the rule is applied. It is of rather limited use, and requires considerable care in manipulation. It is superseded for students' use by the pair of set squares as described on page 25.
Protractors—These are instruments for measuring and setting out angles. There are two forms, the circular or semicircular, and the rectangular. They are made of
various materials--boxwood, ivory, bone, brass, etc. Although differing greatly in appearance, the two forms are set out or divided in exactly the same manner, as will be seen by inspection of Fig. 2, whereon a part of the describing semicircle is shown, with the divisions radiating from the centre, a few of these are dotted in in each figure to indicate the principle of dividing the instrument, which may be briefly explained thus : A circle of any diameter is divided upon its circumference into 36« equal parts, lines are drawn from these points to the centre, and each division thus formed
is said to contain one degree, and, obviously, any other circle described upon the same centre will be divided into the same number of degrees by these radiating lines, therefore it does not matter what size circle or protractor is used, the number of degrees contained between any two lines forming an angle will be accurately shown by it, and on the instrument these are numbered for easy reference, commencing at the diameter of the circle: in the example only each tenth degree is numbered, and a few of the intermediate degrees shown (note, degrees are usually indicated by a .'-mall circle over the figure thus, 30°). The semicircular instrument contains only half of 360°—viz. 180°. For convenience of reference the degrees are numbered from r to 180, in both directions, that they may be read off readily from either hand. Upon the straight edge of the semicircular and the plain edge of the rectangular protractor in the middle of its length, is placed an index mark which indicates the centre of the circumscribing circle. When it is desired to measure an angle, the instrument is laid with its edge upon one side of the angle, and the aforesaid centre mark at the vertex or intersection of the sides, then the number of degrees contained between the two sides oi the angle, will be indicated on the edge of the protractor where the second side intersects it. In like manner an ang'e is laid off on any given line by placing the straight edge oi the protractor 011 the line, ticking off the required angle close to the edge, marking the central or index point, and drawing the second side between the two points marked. The scale shown in the middle of Fig. 2 is explained under Scales, page 22.
miscellaneous drawing accessories
Drawing Paper.—Of this there are several kinds ; for elementary work " cartridge " paper is the most suitable, but a smooth or nearly smooth surfaced kind should be choscn. A rough surfaced or coarse-graincd paper causes
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