Possible Clues:

• Thus I forbid pressure along it, all the same
• Met line of equal pressure
• Line on map showing atmospheric pressure
• Map line showing atmosphere pressure
• I climb, covering black line on map
• Apparently thus I ban the pressure being the same all along the line
• Line showing atmospheric pressure on map
• Is nothing but a pressure line
• One's pub has nothing in such a pressure line
• Line joining places of equal barometric pressure