The awkward physics of processional hymns

Singing hymns while marching is always a risky undertaking.  The physics of it produces some unwanted and embarrasing complications, and no congregation that I have yet observed, no matter how musical or devout, has surmounted them with any success.  But today a miracle happened.

The good Lutherans of St. Matthäus were invited one Sunday by their Catholic neighbours down the street to take part in a joint-service, with the first half taking place in the Lutheran chapel, and the second in the Catholic.  At half-time, as we processed down the sidewalk, someone in the middle of the train felt moved by the Spirit and began to sing a hymn.  One by one the rest of us joined in, until the whole procession was singing as one.  Well, for a while, anyway.  Strung out along the block as we were, hindered by imperfect human time-keeping and by the finite speed of sound on a windy day, different tempos began developing up and down the line.  The hymn began to wobble like a slinky.  The phase-shifts became so long that the vanguard of the parade would be wrapping up a verse right as those at the caboose-end were just gearing up to start it.  And we knew it.  That awful communal embarrassment that ensues when a group realizes that a spiritual moment has started to collapse, like a failed round of applause that one brave person tries to jump-start but then awkwardly withdraws when no one else joins in, leaving the unapplauded person feeling like a dope.

And now for that miracle I promised.  Too much of a bad thing became a good thing.  The temporal gap between front and back grew exactly long enough that in one magic instant the hymn became a canon!  We sensed it instantly and seized upon it.  We began singing in a round, first in two voices, then in three.  It was flash-mob harmony.  In a positive feedback loop, the resonance grew in strength.  Each mini-chorus could hold down its own part with confidence, and by the time the doors of the Catholic church came into view, the whole procession was bellowing praises to God, in perfect discipline, a choir of hundreds conducted by the unseen Spirit.