A T-square of mahogany, at once' simple in its construction and affording all necessary service, may be thus made: let the stock or handle be seven inches long, two and a quarter inches wide, and three eighths of an inch thick; the blade, twenty inches long (exclusive of the stock), two inches wide, and one eighth of an inch thick. In joining the blade to the stock, a very firm and simple joint may be made by dovetailing it - as shown at Fig. 324.
The set-square is in the form of a right-angled triangle; and is commonly made of mahogany, one eighth of an inch in thickness. The size that is most convenient for general use is six inches and three inches respectively for the sides which contain the right angle, although a particular length for the sides is by no means necessary. Care should be taken to have the square corner exactly true. This, as also the T-square and rulers, should have a hole bored through them, by which to hang them upon a nail when not in use.
One of the rulers may be about twenty inches long, and the other six inches. The pencil ought to be hard enough to retain a fine point, and yet not so hard as to leave ineffaceable marks. It should be used lightly, so that the extra marks that are not needed when the drawing is inked, may be easily rubbed off with the rubber. The best kind of india-ink is that which will easily rub off upon the plate; and, when the cake is rubbed against the teeth, will be free from grit.
The drawing-instruments may be purchased of mathematical instrument makers at various prices; from one to one hundred dollars a set. In choosing a set, remember that the lowest price articles are not always the cheapest. A set, comprising a. sufficient number of instruments for ordinary use, well made and fitted in a mahogany box, may be purchased of the mathematical instrument makers in New York for four or five dollars. But for permanent use those which come at ten or twelve dollars will be found to be better.
The best scale of equal parts for carpenters' use, is one that has one eighth, three sixteenths, one fourth, three eighths, one half, five eighths, three fourths, and seven eighths of an inch, and one inch, severally divided into twelfths, instead of being divided, as they usually are, into tenths. By this, if it be required to proportion a drawing so that every foot of the object represented will upon the paper measure one fourth of an inch, use that part of the scale which is divided into one fourths of an inch, taking for every foot one of those divisions, and for every inch one of the subdivisions into twelfths; and proceed in like manner in proportioning a drawing to any of the other divisions of the scale. An instrument in the form of a semi-circle, called a protractor, and used for laying down and measuring angles, is of much service to surveyors, and occasionally to carpenters.
In drawing parallel lines, when they are to be parallel to either side of the board, use the T-square; but when it is required to draw lines parallel to a line which is drawn in a direction oblique to either side of the board, the set-square must be used. Let ab (Fig. 325) be a line, parallel to which it is desired to draw one or more lines. Place any edge, as cd, of the set-square even with said line; then place the ruler gh against one of the other sides, as ce, and hold it firmly; slide the set-square along the edge of the ruler as far as it is desired, as at f: and a line drawn by the edge if will be parallel to a b.
To draw a line, as kl (Fig. 326), perpendicular to another, as a b, set the shortest edge of the set-square at the line a b; place the ruler against the longest side (the hypothenuse of the right-angled triangle); hold the ruler firmly, and slide the set-square along until the side ed touches the point k; then the line lk, drawn by it, will be perpendicular to ab.
In like manner, the drawing of other problems may be facilitated, as will be discovered in using the instruments.