Fig. 27. THE POSITION OF THE PROTRACTOR ON PAPER
This done place the protractor on the opposite and left hand edge of the paper and the horizontal line and lay your rule with its edge crossing the middle of the straight part of the protractor as before and on the 30 degree line of the scale and so that when you draw the line it will cross the other 30 degree line as shown in Fig. 27.
If now you draw another line at 90 degrees, that is vertically, between the two crossed lines, also as shown in Fig. 27, each of the three lines will be exactly the same distance apart in degrees. You can go ahead now and draw lines ⅛ inch apart parallel with each of the three lines and you will have a sheet of isometric cross section paper of your own making.
How to Draw Isometric Ellipses.—An Easy, Rough Way.—There is just one more little thing you should know about making isometric perspectives and that is how to draw disks, wheels and anything else that is circular in form so that they will look right and be right.
Fig. 28a. THE PROPORTION OF AN ISOMETRIC ELLIPSE
In isometric perspective everything that is round in reality is drawn in the shape of an ellipse, that is a closed curve that is longer than it is wide as shown at A in Fig. 28; there are different shaped ellipses but there is only one used for isometric drawing and this is always in the ratio of 1¼ to 2; that is if an ellipse is 2 inches long it will be 1¼ inches wide; an ellipse 4 inches long will be 2½ inches wide and so on.
An easy, though rough way to draw an isometric ellipse is to make a line as long as the diameter of the disk or wheel you intend to represent; draw another line which is the width of the ellipse through the center and at right angles across it, see A again and then draw the curved line around the end of them free hand.
