[ 3*7 ] 
always of a given or invariable Magnitude, when 
the Eilipfe is given, if the Ratio of thofe Diftances 
to the Diameter be given ; and when the Ratio of 
thofe Diftances to the Semidiameter is that of the 
Diagonal of a Square to the Side, (or of y/2 to 1) 
the Parallelogram has its Sides parallel to conjugate 
Diameters. It is like wife fhown here, how the Tri- 
angles, Trapezia , or Polygons of any kind are deter- 
mined, which, circumfcribed about a given Eilipfe, 
are always of a given Magnitude. 
There is alfo a general Theorem concerning the 
Fruftum of a Sphere, Cone, Spheroid, or Conoid, 
terminated by parallel Planes, when compared with 
a Cylinder of the fame Altitude on a Bale equal to 
the middle Se&ion of the Fruftum made by a parallel 
Plane. The Difference betwixt the Fruftum and the 
Cylinder is always the fame in different Parts of the 
fame, or of fimilar Solids, when the Inclination of 
the Planes to the Axis, and the Altitude of the 
Fruftum , are given. This Difference vanifhes in the 
parabolic Conoid. It is the fame in all Spheres 5 
being equal to half the Content of a Sphere of a 
Diameter equal to the Altitude of the Fruftum . In 
the Cone it is One-fourth of the Content of a fimilar 
Cone of the fame Height with the Fntftum ; and in 
other Figures it is reduced to the Difference in the 
Cone. 
In the Remarks on the Method of the Antients, 
the Author obferves, that they eftablifhed the higher 
Parts of their Geometry on the fame Principlesas the 
Elements of the Science, by Demonftrations of the 
fame kind 5 that they feem to have been careful net 
to fuppofe any thing to be done, till by a previous Pro- 
blem 
