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To fix the position of a machine by cross bearings. 

Here we have two objects and we take the compass bearing of each in turn. Convert these into the true bearings and draw position lines through the two objects. It is odious that the machine is on both of these position lines and can be fixed by taking the intersection of the two.

Of course a period of the must elapse between taking two bearings; this will cause the machine to be slightly off the first bearing, so it is advisable to take an object well on the box whose bearing will change slowly. 

If the objects are a long distance away the bearings will change slowly, so the method will give a good result. This method of fixing position is being adopted by night bombing squadrons. 

To find the position by taking two bearings with an interval of time between the OBSERVATIONS.
Suppose the observation of an object A gives the position line AX. 

[[Graph]] A, B, O, P, Q, X, C, L [[/graph]]

Then if the machine is at a potion such as P (We know that it is somewhere on this line) and is moving in direction P.Q. Suppose that after an interval of time it goes a distance such PQ then if we draw a line BC thought Q parallel to AX the machine must be somewhere on this line. Now at this time we take another bearing either of the same object or another. If the same object, this lives a potion line such as AL. Where the position line cuts BC will give the position of the machine at the time of the second observation. 
This method is very often used in sea navigation and a particular case is  called doubling the angle on the bow. 
It is rather complicated for air navigation, but in a large machine flying over ground with few prominent objects, it might be some use. It is probable that the pilot will be able to make a good estimate of distance and fix his position by one observation. 

The use of drift wires. 

It is obvious that the relax of the machine over the ground is seldom the same as the course. Set the bearing plate to read the same as the compass and turn the drift wires so that objects appear to pass along the wires, the the direction of the drift wires clearly gives the track angle which can read off. 

If we merely require the drift angle the movable ring may be set so that the zero graduation is opposite the forward lubber mark and the reading will give the angle from the box or the drift angle. 
Note: Care must be taken if we make the reading on the port side of the machine. 

The use of the drift angle is well illustrated in using a bomb sight, where it is necessary to allow for drift and to have the accurate ground speed of the machine.
Changes in the wind can be quickly observed if we know of certain objects on the track. 

Transcription Notes:
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