Colin McCallum's World War II RAAF navigation instrument, a Douglas Protractor.
The Douglas Combined Protractor and Parallel Rule was used by RAF and RAAF navigators for laying and reading a course or bearing, taking a bearing to or from a compass on a chart, position finding, finding magnetic course from true, position fixing by angles, and plotting lines of position [1].
In 1914 Captain HP Douglas Superintendent of Charts in the Office of the Hydrographer in the royal navy invented a new navigational protractor that was far simpler and lighter than previous designs. Although it would not see much use on naval vessels, except in very small craft, it was ideal for use by airmen and has continued to be a navigational tool right up to the present day[2].
This protractor was used by 0118019/425956 Flight Lieutenant Colin McCallum (1914-1996) RAAF during his 1942-45 World War II service, his name and number written on the cardboard case.
General information is printed on one side.
>W & G<
"DOUGLAS"
COMBINED PROTRACTOR AND PARALLEL RULE
AN ACCURATE INSTRUMENT
ENGRAVED BY >W & G<
MELBOURNE . . AUSTRALIA
USE SOFT PENCIL AND SOFT RUBBER
KEEP PROTRACTOR IN CASE SUPPLIED WHEN NOT IN USE
Specific use instructions appear on the reverse
INSTRUCTIONS AND EXAMPLES
To Lay Off a Course or Bearing. -Use the left side as a ruling edge and the graduation in italic figures on the meridian or parallel -rule the desired course or bearing. For example, try 10°, 170°, 230°, 350°. The left edge can always be distinguished by the writing: "Douglas Combined Protractor and Parallel Rule."
To Read Off a Course or Bearing. -Align the protractor to the course or bearing, and the angle can be read off at once from the nearest meridian or parallel.
To Use as a Parallel Rule, or to Take a Bearing to or from a Compass on the Chart. -Align the protractor by one of its parallel lines, as necessary, on the bearing or on the compass, and move the protractor to the point required by drawing pencil lines either side of the protractor as necessary.
To Find Dep. or Lat. Between Two Positions. -Align the protractor to the nearest meridian, and a perpendicular from the course line (at the distance given) will cut off on the 90°-270° line the number of divisions equalling the departure. Another perpendicular will cut off on the 0°-180° line the number of divisions equalling the difference of latitude The same principle applies to laying or reading off a position for lat. and long.
To Find Magnetic Course or Bearing from True. -Set the protractor to the course or bearing, e.g., 340°, and then, given the compass error as 5° W., treat the protractor as a movable compass card and move it 50° to the west. This will give 345° or N. 15° W. as the magnetic course. By using the protractor in the same way, the true course or bearing can be found from the magnetic, or, given both true and magnetic, the compass error may be determined.
To Fix Position by Angles. -Lay off the angles observed on the matt side in the opposite direction to that in which they are observed. Use italic figures. Then reverse the protractor so thật the pencil lines are in contact with the objects charted, and prick through position as usual
To Plot Lines of Position, and Similar Problems. -This can easily be done, as the upper surface of the protractor readily allows of lines being ruled and erased. This upper surface also allows of notes and calculations being made, positions trans-ferred from one chart to another by tracing, and so on. Constant use will present many advantages which cannot, through want of space, be entered into here.
