which means that he compares objects observed with objects shown on the map. Here the similarity ends and the problem becomes more complex.
If the flight is over water, uncharted land, or above cloud, the navigator must use what is known as dead reckoning to keep track of his position.
Dead reckoning is the method of determining position by keeping an account, or reckoning, of the course and distance flown from a known position.
The position of an aircraft is the resultant of two distinct motions-the motion of the aircraft in the direction in which it is headed due to the driving power of the engines, and the motion of the air itself-the wind.
An aircraft is always affected by the speed and direction of the air mass through which it is moving and if, after flying for some. time by dead reckoning, it is found that the wind has changed, and if for any reason it is impossible to determine accurately the effect of the wind, the position by dead reckoning will be in error by an unknown amount.
In order to ascertain the effect of the wind on the aircraft's position the aerial navigator has a number of aids of which the two basic items are the compass and the air speed indicator. However, these two items are by no means sufficient in themselves and must be augmented. All modern aircraft are equipped with radio, and the position of the aircraft can be determined from two or more bearings from directional radio stations.
This, of course, is dependent on the proper functioning of the radio and upon the proximity of the radio beacons.
Almost all long distance flying. over the water involves the use of celestial navigation and this method and the use of radio are the only methods that determine the actual position regardless of the wind, compass error, or other similar difficulties. The equipment required for celestial navigation consists of a chart, a sextant, an accurate timepiece, and tables for converting the observed data into the position of the observer. The introduction of radar has helped considerably in navigation, but as all radar is subject to unserviceability and to jamming or interference it is by no means the whole answer. As long as the sun, moon and stars can be observed they can be used to ascertain position. Much thought has been given in recent years to improving the equipment for celestial navigation and such items as the periscopic sextant and the stabilized sextant have emerged.
As much of the flying carried out by service aircraft over the sea is at low altitude it will readily be appreciated that if cloud cover is present celestial navigation is not possible. In this case the navigator must recourse to taking drifts on the surface of the sea and thus deduce the wind speed and direction. Such winds
must be found at frequent intervals in order that any wind changes may be known and used to determine position.
Radio and radar aids are constantly being improved and the day will certainly come when a world-wide fixing service will make all forms of navigation relatively simple.
However, all mechanical systems are subject to failure and the art of dead reckoning must not be lost. For as long as aeroplanes are flying with