As they could no longer see the sea beneath them, owing to the darkness and fog which lay between, John had to rely entirely upon intuition and his compass to strike Freetown.  Aerial navigation over immense bodies of water is similar to navigation on the seas themselves, except that the indispensable sextant of the mariner is of little use in the air, owing to the high speed of travel and the fact that allowances have to be made for the drift of the machine when side-winds are blowing--an extremely difficult factor to determine accurately.