Instrument for Drawing Conics. 275 
to the rhombus at G. It is slotted for a portion of its length 
and slides between the rhombus and BC. A pencil-holder P 
moves in both slots together. 
Ta) Ty Ys rel 

= Two pins are driven into the drawing-board at the foci of 
the required conic and the pencil P brought to a point on 
the curve. The corner EF’ of the rhombus is pivoted on one 
focus and FH turned round to the other focus, which can be 
fitted into one of the holes in FH. For an ellipse the second 
focus must be near the end F; for an hyperbola near the end 
H. Suppose that we wish to draw an ellipse, F and FE’ 
being the foci. By elementary geometry PF’=PG, there- 
fore FP+PH’=FG, which is a fixed length on the rod. 
Thus P describes an ellipse. If the second tocus were at H, 
we should have HP—PF’=HG=const., so that P would 
describe an hyperbola with EF” and H as foci. 
In either case the line AB bisects the angle FPF’ exter- 
nally (or F’PH internally) so that AB moves as a tangent 
to the curve. Thus if a drawing-pen were made to slide 
along AB without turning, but turning freely in the slot of 
FH, it would always keep tangential to the curve. This 
property is not possessed by the ingenious instrument de- 
scribed by Prof. Pearson. On the other hand, my instrument 
is open to the objection that it will only draw a little more 
than half an ellipse in one position. To describe the other 
half it must be reversed on the focal pins. Similarly, after 
drawing one branch of an hyperbola, it must be reversed on 
the pins to draw the other branch. 
