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a square should be tapered as in Fig. 4, but shaped at its wide end as in Fig. 6, and have a stock wide enough to allow for the surface required in the washers of the fittings necessary to make the blade adjustable. These fittings, though requiring to be well made and neatly finished, are not expensive or difficult to make, as they consist merely of two washers, a square-necked bolt, and a fly-nut, articles that any one capable of making a pair of calipers could supply himself with. Fig. 5 shows a section of these fittings, which are generally made in brass. The top and bottom washers A, B, are slightly dished on their faces to ensure contact with blade and stock, and the spread of the wings of the fly-nut is such as to give sufficient leverage for a good grip. Reference is here made to an adjustable-bladed square, as one may possibly be required later on by the student ; but there is no present MECHANICAL AND ENGINEERING DRAWING 5 necessity for the provision of such a tool, as all lines that may be required other than those drawn with the tee- and set-squares in conjunction, are easily put in by a proper manipulation of the set- squares, which will be explained in due course. Set-Squares. Of the set-squares used conjointly with the tee- square, those of 45 and 60 are all that are required by the student in the earlier stages of study. A 6-in. 45 and an 8-in. 60 set-squares are the most useful sizes. Framed ones, well made, of foreign manu- facture, may now be obtained at a reasonable price, but the kind most generally in use are made of vulcanite. Those, however, of this material made with the middle part cut out to imitate framed wooden ones should be avoided, as they are very liable to fracture at the