[ 2 43 ] 
Then let other rulers of like form, as Bb, A a, 
&c. be moveable upon the centers B, A, 8cc. and let 
thofe centers themfelves be moveable upwards and 
downwards in the groove or notch in the ruler SS, 
and in fuch manner, that the centers B and A may 
be placed upon one another, or upon C, if occafion 
requires, and let thofe centers be fcrewed down in 
their proper places, viz. the center A at A, the center 
B at B, &c. Then let kb and la reprefent other 
moveable rulers, fuch as the former, and fo confined 
that they fhall always continue parallel to themfelves 
and to the line D c and let MM reprefent another 
ruler of like form. And let the rulers kb and MM 
be connected with the fixed ruler C c by a Hiding pin 
palling through the interfe&ion of their notches at q, 
and let the rulers kb, B b, la and ha be conneded 
with one another and with MM and RR, by like 
pins palling through their notches at b, r , a , and s j 
and let the laft of thefe pins s, have the point of a 
pencil fixed therein ; then, I fay, that if the ruler 
MM be moved backwards or forwards, from or to- 
wards S S, and always held parallel thereto (or be fo 
confined as to be capable of moving in no other 
manner) the pencil s fhall defcribe the required 
curve. And the dillances from the point O, at 
which that pencil fhall crofs the bafe ZZ, on the 
right hand fide of SS, fhall denote the affirmative 
roots of the equation ; and the diftances from the 
fame point O, where it fhall crofs the faid bafe on 
the left hand fide of S S, fhall exprefs the negative 
ones ; and the places, where it fhall approach the 
faid bafe, but fhall return without reaching it, fhall 
indicate the impoffble ones ; the faid diftances being 
I i 2 always 
