[ 448 ] 
It is evident this incurvated furface may be infi- 
nitely extended on all Tides beyond the lines AM, 
M N, ND, AD, and as well below the given plane 
A D N as above it ; and therefore the various fedtions 
of it, if continued, will be infinite lines. 
The line AM, raifed above the given plane A DN, 
may be called the vertical Diredtrix. 
And if thro’ this diredtrix A M there be drawn a 
plane AMR in any given angle to the plane A D N, 
interfering the moving plane NMR in the line 
M R, and meeting the given plane A D N in R ; 
then the points A, R, and R, N, being joined, there 
will be a trapezium ARND, formed, that may be 
named the Bafe, and in which the line D N may be 
called the Diredtrix of the Bafe. 
And the plane AMR pafiing thro’ the vertical 
diredtrix and the moving plane M N R, together with 
the bafe ARND, and the curve furface, will make 
a folid A M R N D A, fomething in the form of a 
wedge. 
In this folid there may be made five fedtions in a 
given angle with the bafe, or parallel to it ; viz, one 
parallel to the moved plane ; one parallel to the di- 
redtrix of the bafe ; another parallel to the vertical 
diredtrix j a fourth parallel to the bafe ; and a fifth 
interfedting both the diredtrices. And in all thefe 
cafes, when the diredtrices are right lines, the fedtions 
will be either the conic hyperbola, or the parabola, 
or right lines. 
lit, If the folid A M R N D A be cut by a plane 
parallel to the moved or generating plane N M R, or 
the line A D, the fedtion will be a right line. This 
is evident from the defcription, Fig. i. 
2, 
