Roussel's piece for piano ascends by an octave with each repetition. This canon, though it rises with each repetition, is neither modulating nor a canon per tonos.
The instruction is given, "Reprendre au signe ⦻ en transportant les trois parties à l'octave supérieure, et continuer ainsi autant que le permettra l'étendue du clavier."
[Resume at the sign by carrying the three parts to the upper octave, and continue as far as the extent of the keyboard will allow.]
The piece is both tender and contemplative; the counterpoint both simple and complex. While the meter is triple, the treble voice is imitated two octaves below in the bass at a time interval of only two beats. All the while an accompanimental figure consisting beginning as an ostinato continues between the voices. Makes one wish that the piano's range went much higher.
I would reproduce the score here but I believe copyright laws in my country forbid it. It was written in 1913 but published (by Durand & Co.) in 1948, suggesting that by US copyright laws a score should not be shared here, but I believe it can be found at IMSLP.
I will link to a video, though: