Henri swept with the pole, and collided with an invisible force, that slightly moved the pole further away. He felt the wall, but found that it did not extend through it-- only the stone of the wall.

This was curious. Either the barrier was spherical, or some other shape that was confined to its radius. A cylinder with a hemispherical top, or some other form. The simplest would be a sphere, and would be the most likely. Curious. 

He moved further down the ledge of the canal, then deployed the hook to climb up just enough to peek above the water.