From Strand, August 1906:

A circular canon is so named not because of its circular form, but because it completes the circle of fifths–i.e., it goes through all the keys, each a perfect fifth above the other, until it returns to the original key. The one under notice is written in triple counterpoint, any part sounding equally well in the top, middle, or lowest voice, and each bar is in three different keys at once, all harmonizing.

This rendering is a bit indistinct, I’m afraid — if I can find a clearer version I’ll post it.