


A Novel Instrument for Drawing Parabolas. 201 
a number of modifications and improvements—especially the 
replacement of slots by wheels running in grooves, which 
make the mechanism work with the ease and perfection of 
all that the Ztirich work shop turns out. The design remains, 
however, essentially Payne’s. Hence I propose to call it the 
Pa yne-Coradi Parabologr aph. 
In practice what we need is to construct a parabola with 
given vertex and axis passing through two given points 
symmetrically situated on either side of this axis, 7. e. we are 
given V the vertex, VX the axis, and the points is ab ot 
fig. Tec kiwi the actual instrument the lower “east and west ’ 
= gives the tangent at the vertex, the vertex itself is marked 
by the short vertical stroke seen on the bar*. The upper 
“east and west ” bar serves to car ry the vertex piv ot, whence 
we see the rectangular bars corresponding oto W/E an VP of 
fig. 1. The point P is the pencil or pen. It is ona carriage 
constrained to slip both along the VP bar and the “ north and 
south”? bar TFP; in each case by three horizontal wheels 
running in grooves, two on one and one on the other side of 
the respective bars. The “ north and south” bar is attached to 
a carriage much like that used in the Abakanowitz-Coradi 
= age which rolls along the lower “ east and west ” bar, 
. €. the tangent at the vertex. The same system of three 
Fonte wheels on a vertical swivel clamped to the “north 
and south” bar fixes the point T. 
The working of the instrument is very simple, the lower 
east and west bar being placed along the known tangent at 
the vertex, with the vertical stroke on the reguired axis, the 
clamp fixing the point T is released, and the pen or pencil 
brought to the point A. The point T is then clamped, and 
the required parabola struck at once, by simply pulling the 
pen or pencil round in the general direction of the curve by 
the handle provided for that purpose. The curve is traced 
right through the vertex at one sweep. 
Slight improvements in dimensions will be made in future 
machines, but the present one works excellently, as may be 
judged from the actual curves on fig. 2. The size of the 
machine is about 20 x 20 ins., but the “ tangent at the vertex,” 
i. ¢. the lower east and west bar, can be brought within a 
couple of inches of the top of the drawing-board. 
The parabola has been a known curve for at least 2000 
years, and therefore some excuse may be needed for speaking 
of it here. | think this is to be found in the fact that, strange 
as it may seem, this appears to be the first instrument con- 
structed for drawing it accurately and quickly in a manner 
satisfying the needs of actual drawing-office practice. 
* Really this stroke is someway behind the true vertex on the axis to 
allow of the pen or pencil passing right through the vertex. 
