"I left my ancient black smock, which, for a hermetic like myself, was far from being hermetically closed."

I suspect Hugo has committed an anachronism here. Gringoire says he posses occult knowledged, but is not tightly sealed against wind chill. I'll go search for Helmholz, I think it was who first made sphere divided into two halves, in which he was able to reduce air pressure to a level so low, horse could not pull the half spheres apart. Gotta go look that up.