58 On an Instrument for Drawing Parabolic Curves. 



It is based on that property of the parabola by which any 

 point in it is equally distant from its focus and from the 

 nearest part of its directrix. 



In the diagram, which represents the instrument as mounted 

 on a drawing-board, F is the focal pivot, which is adjustable 

 for larger or smaller parabolas by moving the piece support- 

 ing it in a slide to which it can be clamped by the screw K. 



AB is a slot formed by two steel straight edges with a 

 narrow opening between them. The centre line of this slit is 

 the directrix of the parabola to be drawn. 



Along the slit AB travel two pins L and G, which are parts 

 of a frame or system LMEG of such a nature that LM, ME, 



and MG are equal to each other, so that in any position E is 

 compelled to be vertically over G. 



A parallel frame HGFT is so constructed that one corner 

 rotates on F and the opposite corner rotates on G. The 

 diagonal bar CD can travel along between guides at H and I. 



A pencil or scriber is fixed at E, and the paper is placed 

 under this, and held by the clips N and 0. 



A handle is provided at G, on moving which along the line 

 AB it will be seen that the pencil E is compelled to describe 

 a curve such that it is always equally distant from F and from 

 G, and as G is on a straight line and F is a fixed point, that 

 curve mast be a parabola. 



The fact that any point in the diagonal of a rhombus must 

 be equally distant from one pair of its opposite angles is 

 obvious on a moment's consideration. 



The centre line of the diagonal bar CD is always a tangent 



