CAO. If an arm OC, attached to AB and capable of rotation about 0, be provided, one rectilinear guide may be dispensed with. James Finney's ellipsograph (1855), shown in Plate I., fig. 5 with Stanley's improvements, is of the overhead type on this principle. It is provided with milled-headed screw adjustments for varying the radius and setting the pen. Edward Burstow in his ellipsograph of 1871 used a chain or wheel gearing to effect the combination of motions, only one rectilinear slide being em ployed.
If in addition to providing for the circular motion of C about 0 as centre, other gearing carried on OC and CA ensures that CA moves so that the angles CAO and COA are always equal, the single rectilinear slide becomes unnecessary. A few instru ments of this type have been constructed, since about 186o; one designed by Frank J. Gray in 19o1 is described in detail in the Journal of the Society of Arts, 1902.
When an ordinary drawing pen is used as the describing point in the instruments previously mentioned, the lines drawn are not of uniform thickness unless the pen is guided so that its nibs are kept tangential to the curve. It will be seen from the adjacent diagram that if AD and BD be drawn parallel to the arms of the trammel or lent trammel in any ellipsograph, D is the instantaneous centre of motion of AB and of any point P in AB produced. As OD is of constant length, D describes a circle about 0 as centre, and it is possible to arrange for a rod PD, passing through a sleeve fixed to the arm OD at D, and attached to the pen at P, to keep the pen always tangential to the curve. On this principle a steering device was applied to the ellipsograph by Prof. Alexander, and by F. J. Gray in his instrument of 1901.