Proposition 30 describes the construction of a curve of double curvature called by Pappus the helix on a sphere; it is described by a point moving uniformly along the arc of a great circle, which itself turns about its diameter uniformly, the point describing a quadrant and the great circle a complete revolution in the same time.
The historian's grandfather became within his own circle a highly esteemed interpreter of Scripture, and held fast his profession even in the time of Julian.
A conic may also be regarded as the polar reciprocal of a circle for a point; if the point be without the circle the conic is an ellipse, if on the circle a parabola, and if within the circle a hyperbola.
Within the Arctic circle a large number of Tertiary plants have been collected.
If we assign to a column of water at the equator the density S-7-= 1.02 2 at the surface and 1.028 at l000 fathoms, or an average of 1.025, and to a column of water at the polar circle a mean density of i 028, there would result a difference of level equal to (1.028 - 1.025) X moo = 3 fathoms in a distance from the equator to the polar circle of some 4600 m.
