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Rh —a curve which does not go round and come back again, but goes on for ever. I can show you how to make a parabola, which is just as easy as to make an ellipse, I think almost easier. Take two straight lines, and set off on each a series of equal distances—let us say inches, or half inches, or whatever you like—and then join the corresponding points in the way shown in the diagram, outwards on one line and inwards on the other. I believe in some schools this is done as a needlework exercise, joining the points by threads, which shape out a parabola.

You need not stop at the point 8; you can go on past it, provided the lines are suitably extended, and there is no stopping-place: so you see a parabola is a curve that goes on without end. That is important to remember: for suppose we started on a journey from the Earth and moved under gravity we might get into one of these curves that goes on without end, and that would be inconvenient, for we should never get back again to Earth.

At one time it was thought that comets moved in parabolas and never came back; that they came to