OVAL. 
to fee the curve, - it is traced beneath by the drawing-pen. 
The circles are united by fcrews at c, c, — “ the 
lower circle, and ‘the two bars of the being 
keep the two together, es at the 
in the mann 
tapped into the upper circle, se retaining ‘the lower one 
in the The centre-pin for the pinion K is 
fixed on one of the arms of the lower circle, and acts upon 
a rack, a fips to the upper circles, fo that it fepa- 
rates t when turned a nd by the finger and 
a a to the milled head upon it. 
he fr 
of a ftraight rulers 3 
own upon the former, and are therefore in plane above 
them, as fhewn by fg. 4, plates of brafs cae interpofed 
between them at the angles, to feparate them to a proper 
diftance. The lower circle, A, 1s fitted in between the 
two lower rulers D, E, and flides el in the direGtion of 
their length, but has no fhake fideways: in like manner, 
the upper circle, B, is included between the edges of the 
rulers F, G, and therefore moves in a right line, in a direc- 
tion perpendicular to the former. Thus the frame forms 
two grooves at right angles to each other, in which the 
circles revolve with an oar i Lapa the upper circle 
d the lower 
on any part of the circles, when they are rendered excentric, 
will defcribe an ellipfis on the furface beneath; but the 
only tracer which is ufed in drawing is feiated between 
the bars a,a: thus, into this {pace a {mall carriage, or frame, 
g is fitted, to flide freely from one end of the opening to 
the other, by ae her a rack, 4, {crewed on one fide of it, 
on acentre-pin, fixed into the 
awing compafle 
as in pen, M, traces the curve upon 
the paper, but si admit of cae turned up on the centre- 
pin of the focket H, and then, for the convenience of fetting 
the inftrument, the circles can be turned about without 
The tran{verfe Aarts ( ews 
it 18 htted into the 
een on its centre-pin, 
* and -he two pair of racks d, A, and pinions K, L ; the former 
for the purpofe of eParaiing the pease and the latter for 
moving the pen-frame, g, along — — the frame, g, is 
fo fitted, that it ponte at the int with refpe& 
to the upper circle, when the carties 5 of the two are fepa- 
rated from each other by the pinion K, and may, therefore, 
be confidered as immoveable when the lower circle only is 
smoved, 
To keep the frame of the inftrument ftationary upon the 
paper whilft it is ufed, two fharp pins are fixed in the ruler 
P, which ae er ee the paper, and make it quite faft, 
when they are held down by the finger ard thumb of the 
left hand applied upon the heads of nuts N, O; thefe are in- 
troduced to unite the ruler P with the ends of the two upper 
rulers F, G, but the fcrews of thefe nuts pafling through 
grooves in the ruler P, admit feet whole inftrument to be 
moved on the aper, a fmall q 
fet with any aed degree of excentricity, and the tracing~ 
point or pen can be removed to any required diftance from 
the centre of the upper circle. The compafies being opened 
to the extent of two marks made upon one of the bars, G, of 
the frame, and then being fixed in the es aray by pufhing 
them into their focket H, it is ready for In this 
ftate, fuppofe the two circles fet by the ciclo, K, exa&l 
concentric with ~ other, ane the pions, L, turned till the 
end of the frame, g, comes in conta& with the rim of the 
inftrument, then ae point of i: tracer will come exatly in 
the centre of both circles, and the circles being turned round 
in the frame, the pen w 
y be mad By turning the pinion, L, the 
breadth of the ellipfe will be determined, and by the other 
tion of in may be defcribed. 
The conjugate diameter will, in all cafes, be equal to 
double the aliens from ‘oa point of the pen to the centre of 
e upper circle, and the difference of the conjugate and 
ate diameters wil ‘- always equal to twice the excen- 
tricity of = two circles 
To prove i fuppofe PP and » (fg. 6.) 
two rove of the eda nd O o, ; 4g. Wf the t 
points, d, e, and f, are fixed upon an inflexible bar, and the 
points d oa € once applied to the two saree while 
a tracing-point, fituated at f, is carried round by the bar ; 
the latter will defcribe an ellipfis. This is the aera of the 
trammel ; but its defects, as at prefent | conftructed , are well 
own. 
frame containing the crofs grooves reprefenting the two dia- 
meters P P and QQ; nor can it draw much larger, uniefs 
the diameters are nearly aioe 2. Itis difficult to fix the 
crofs firmly on the paper, or to bring it to the exact point 
required for the centre of the ellipfe. And 3. When pods is 
5 
