The lenses in the periscope are constructed to give a radius of 10°; that is to say, if the sun’s reflection appears in the disc’s outer edge, one can allow 10° before it disappears in the other outer edge. If one has set the solar-compass for a flight directly north, one will continue in the right direction so long as the flying machine has no deflection. In order to detect such deflections we had a combined speedometer and deviation measure which was also given to us free of charge by Goerz. Amundsen attended to these on the northward journey--Dietrichson on the southward. They both speak of them with high praise. Their uses are shortly as follows: Inside the instrument, on a move-able ring, is fastened a diametrical wire. One looks through the instrument down to the ground below or to the ice, and adjusts the wire the longitudinal way of the ship, then pays attention to the objects passing aftwards under the plane (icebergs, for example), noting whether they follow the direct line of the wire or deviate to the side. Should there be a deviation, one knows that they are not following the direct course in which the nose is pointing, so it has to be set at an angle allowing for the deviation. The wire must be drawn to the side quite slowly until one finds that the objects which one can notice now follow the line of the wire exactly. This points now in the direction one comes from, and the wire’s angle, compared with the boat’s nose, can be read directly in the instrument. That gives the angle of deviation. [Illustration: TAKING THE WINGS OUT OF THEIR BOXES] [Illustration: SETTING UP THE WINGS]