It may be interesting for some of the readers to know bow to draw an ellipse of any size, as well as an egg shaped figure or oval, Fig. 1, similar to the cross section of a sewer, writes W- McD., in "Carpentry and Building. " The instrument for doing this con-gists of two parts, one of which is cross shaped, the pieces being at right angles to each other and having a groove in their top surfaces. The other part of the instrument consists of a beam with two sliding buttons having round pins on the under side to fit into the grooves of the cross piece, also a pencil fixed into a button at the end.
then the end of the batten is the point of the ellipse In order to generate an ellipse make C' B' of the diagram equal to C B, which is half the minor axis of the ellipse, and make A' B' equal to A B, which is half the major axis of the ellipse. Now place the buttons in the slots or grooves and slide them along so that when B' moves along the horizontal slot A' will simultaneously move along the vertical slot, then C will generate an ellipse.
To generate an ellipse by means of a cord we first draw the major and minor axes and then take half the-major axis A B and with C as a center draw an are bi
I might mention that I saw this apparatus about 25 years ago in a large horticultural concern where I was designer. The trammel is exactly the same as the beam of a beam compass, with this difference, that it has a circular button to fit into the groove instead of a needle. Again, the beam or trammel of the beam compass is usually I-shaped, as shown at A in Fig. 2, but this is flat, as the sleeve S, which holds the knob or button, keeps it sufficiently rigid, as indicated in the larger section. The salient feature of the apparatus is not so much the trammel as the grooved cross shaped frame in which it works. The principle could be applied to the generation of a large ellipse by simply having two lines laid off at right angles representing the major and minor axes of the ellipse, and then taking a batten and putting in nails at the length of the half major and half minor axes. Then when the nail representing the minor axis is placed anywhere on the major axis, and the nail representing the major axis is swung around until it rests on the minor axis, secting the major axis in D and E. By this means D and E are established as the focii of the ellipse. Place pins at D and E with a cord around them and then stretch it until it comes to C; now with the cord kept taut, generate the ellipse. While this is an easy method of doing the work, the one first described is more exact. For generating a still larger ellipse a wire can be used instead of a cord, as the latter stretches and the whole value of the operation consists in its exactness.