Buy a quire of _isometric_ (pronounced i-so-met´ric) _cross-section paper_ 6 by 9 inches, at a cost of 15 cents, of any dealer in drawing materials. This paper is lined in faint colored ink in three directions, as shown in Fig. 22, and which represent length, breadth and thickness.
[Illustration: FIG. 22. A SHEET OF ISOMETRIC DRAWING PAPER. THE REAL SHEETS ARE PRINTED IN NEUTRAL TINTS, THAT IS, COLORS WHICH DO NOT INTERFERE WITH THE DRAWING]
Now _isometric_ comes from _iso_ which means _equal_ and _metric_ which means _measure_, so isometric means _equal measure_ and the three lines used in isometric perspective are at equal distances from each other. The lines which cross the vertical lines on isometric cross-section paper are 30 degrees from the base, or horizontal line and the vertical line is, of course, 90 degrees from the horizontal as shown in Fig. 23. Having everything at hand suppose you try to draw a square frame. Begin by making the first upright and you will see by looking at Fig. 23 that all you have to do is to draw three vertical lines and join the top and bottom by marking over the 30 degree lines. This done draw three more uprights in the same way and when you have these on paper it is easy to put beams on top or struts between them as shown at Fig. 24.
[Illustration: FIG. 23. FIRST STEP IN ISOMETRIC PERSPECTIVE DRAWING]
As all the lines are of equal measure you can mark on the exact dimensions as shown in many of the isometric perspective drawings in this book. For a drawing of some device, or of a whole machine, to give to some mechanic to make for you the better way is to hand him a perspective drawing together with the top, side and end views, rather than the latter views alone, and then he will not need to figure out how they are put together.
[Illustration: FIG. 24. THE NEXT STEP IN ISOMETRIC PERSPECTIVE DRAWING]
To show to a better advantage how isometric perspective works out look at Fig. 25 and you will see how the bearings of a crankshaft of a four cylinder gas engine stand out in a vertical line, up and down and in a horizontal line right and left as though they were real and made in three dimensions.
[Illustration: FIG. 25. A CRANK SHAFT DRAWN ON ISOMETRIC PAPER]