Fig. 11 the square, a b c the outer square, d e f the inner square, a d e the stock or handle, b c f e the blade.
§56. The Square, Plate 13, Figure 11.
Consists of two rectangular prismatic pieces of wood, or one of wood, and the other which is the thinest [sic], of steel, fixed together, each at one of their extremities, so as to form a right angle both internally and externally; the interior right angle is therefore called the inner square, and the exterior one the outer square. The side of the square which contains the mortise, or through which the end of the other piece passes, is made very thick, not only that it may be strong enough for containing the tenon of the other piece, but that it should keep steady and flat when used; and the piece which contains the tenon is made thin, in order to observe more clearly whether the edge of the square and the wood coincide. The thick side of the square is called the stock or handle, and the narrow surface of the handle is always applied to the vertical surface of the wood. The thin side of the square is called the blade, and the inner edge of the blade is always applied to the horizontal surface of the wood. Squares are of different dimensions according to their use: some are employed in trying-up wood, and some for setting out work; the former is called a trying square, and the latter a setting-out square; the blade ought to be of steel, and always ought to project beyond the end of the stock, particularly if made of wood. The stock is always made thick, that it may be used as a kind of fence in keeping the blade at right angles to the arris.
§57. To prove a Square.
Take a straight edged board which has been faced up, and apply the inner edge of the stock of the square to the straight edge of the board, laying the side of the tongue upon the face of the board; with a sharp point draw a line upon the surface of the board by the edge of the square: turn the square so that the other side of the blade may lie upon the face of the board; bring the stock close to the straight edge of the board, then if the edge of the square does not lie over the line, or any part of the line, the square must be shifted until it does, then if the edge of the tongue of the square and the line coincide, the square is already true: but if there is an open space between the farther side of the board and the straight edge, that is, if the farther end of the edge of the tongue of the square meets the farther end of the line from the straight edge, draw another line by the edge of the tongue of the square, and these two lines will form an acute angle with each other, the vertex of which will be at the farther side of the board, and the opening towards the straight edge: take the middle of the distance between the two lines at the arris, and draw a line from the middle point to the point of concourse of the lines: then the blade of the square must be shot or made straight, so as to coincide with this last line. The same, or a similar operation, must be repeated, if the contrary way.
From Peter Nicholson’s “The Mechanic’s Companion.”