AIR NAVIGATION 449 



than athwart, and more accurate in airships than in airplanes. It 

 will be obvious that further investigation is needed on the subject 

 of the precision of sextant observations, especially in respect to flying 

 conditions in the polar regions. 



In order to reduce errors, about six observations should be taken 

 in rapid succession and the results meaned. The only correction re- 

 quired if a gravity or Baker sextant is used is that for refraction.* 

 Very little is known about atmospheric refraction in the polar regions, 

 and the question is one that needs investigation. However, for aerial 

 navigation it need only be known approximately to perhaps the 

 nearest 2' or 3', and extrapolations from the ordinary refraction 

 tables are in all probability sufficiently precise. A convenient type 

 of refraction table for use while flying is shown in the adjoining table. 



Magnetic Checks in Position Finding 



Another check on position suitable for aerial navigation can be 

 obtained by observing the horizontal angle between a heavenly 

 body and the direction indicated by a magnetic needle;^ but this 

 assumes a knowledge of magnetic declination and also an approximate 

 knowledge of position. Consequently it is not likely ever to be of 

 much value in the polar regions, for, apart from the large errors of 

 observation due to the weakness of the horizontal component of the 

 magnetic force, local magnetic disturbances, and instrumental errors 

 due to acceleration, etc., the magnetic declination itself changes very 

 rapidly and thus the probable error in the assumed position will 

 create too large an error in the assumed magnetic declination. 



Nevertheless there is a possible independent method of checking 

 position by utilizing the earth's magnetism and obtaining the direction 

 of total intensity with a dip compass. By measuring the angle of 

 dip while flying,^" the navigator could determine his magnetic lati- 

 tude. Of course this method cannot be used until the polar regions 

 have been properly charted magnetically, and it is also a matter for 

 investigation as to how accurately dip compasses could be read in 

 aircraft, but the method is suggested here for the reason that if it 

 should prove practical it would provide an immediate check on position 

 while flying through or above fog and, together with the astronomical 

 position line, would provide a complete method of determining position 

 independent of dead reckoning. 



8 The correction for parallax can be considered negligible. 



^ Baker and Filon, op. oil.; Ault, op. cii. 



1" This method was referred to by G. M. Dobson, op. cit., p. 383. A similar suggestion was made 

 by D. Boykow and others: Die Navigation in der Arktis, Appendix 2 to "Das Luftschiff als Forschungs- 

 mittel in der Arktis," Internatl. Studiengesell. zur Erforschung der Arktis mit dem Luftschiff, Berlin, 

 1924, pp. 43-51. In this latter article it was proposed to measure both the horizontal and vertical 

 intensity of the magnetic force and thereby locate the aircraft on position lines of equal magnetic 

 intensity. 



