The ribbon, which is extremely slight, breaks under the weight of the grains of sand that stick to it and are lifted with it.

If it does not break, the caterpillars at the back, however delicately we may go to work, feel a disturbance which makes them curl up or even let go.
There is a yet greater difficulty: the leader refuses the ribbon laid before him; the cut end makes him distrustful.

Failing to see the regular, uninterrupted road, he slants off to the right or left, he escapes at a tangent.

If I try to interfere and to bring him back to the path of my choosing, he persists in his refusal, shrivels up, does not budge, and soon the whole procession is in confusion.

We will not insist: the method is a poor one, very wasteful of effort for at best a problematical success.
We ought to interfere as little as possible and obtain a natural closed circuit.