All 4 Uses of

undulate
in
The Hunchback of Notre Dame

There were also fine edifices which pierced the petrified undulations of that sea of gables.†
Chpt 1.3.2

undulations = smooth wave-like motions

It is no longer anything but a mass of sonorous vibrations incessantly sent forth from the numerous belfries; floats, undulates, bounds, whirls over the city, and prolongs far beyond the horizon the deafening circle of its oscillations.†
Chpt 1.3.2 *

All at once, while he was scrutinizing the great city with that eye which nature, by a sort of compensation, had made so piercing that it could almost supply the other organs which Quasimodo lacked, it seemed to him that there was something singular about the Quay de la Vieille-Pelleterie, that there was a movement at that point, that the line of the parapet, standing out blackly against the whiteness of the water was not straight and tranquil, like that of the other quays, but that it undulated to the eye, like the waves of a river, or like the heads of a crowd in motion.†
Chpt 2.10.4

At the sight of this line of cuirassed backs, undulating as they rose through the gloom, one would have pronounced it a serpent with steel scales, which was raising itself erect in front of the church.†
Chpt 2.10.4

Definition:

a smooth wave-like motion (physical or auditory)

or:

having a wavy or rippled form or surface