It is difficult even to assist the imagination to form an idea of it.

I will essay a few words in that direction.

Suppose, then, a plain spire, 100 feet high, in the form of an attenuated cone, planted upon a heavy church tower.

Now, in imagination, plough this cone all around into deep ridges from top to bottom.

Then mount to the top, and, with a great iron wrench, give it an even twist clear down to the base, so that each ridge shall wind entirely around the spire between the bottom and the top.