448 



POLAR PROBLEMS 



aerial navigators. This instrument by an ingenious device takes a 

 sight simultaneously on the sun and the part of the horizon immedi- 

 ately beneath the sun and also on the part in the opposite direction, 

 thereby eliminating errors due to uncertainty in the amount of dip 

 correction and to the apparent change in the vertical caused by 

 accelerations in the speed of the aircraft and sudden turns. 



These errors in the apparent 



Approximate Refraction 



Giving Corrections to the Nearest 



5' OF Arc Only 



(Assumed temperature, 0° F.) 

 (Assumed barometer at sea level, 29.5 ins.) 



vertical are the chief objection to 

 the use of bubble sextants, and 

 attempts have been made in 

 France to overcome this dififi- 

 culty, and still not be dependent 

 on the natural horizon, by uti- 

 lizing the gyroscopic principle. 

 At first such sextants were not 

 satisfactory for use in aircraft, 

 but it is understood that the 

 method is being developed.® 



It is very important that the 

 navigator before embarking upon 

 a flight across unknown regions 

 should know the maximum pre- 

 cision possible of altitude obser- 

 vations in the air with a sextant 

 and also how far he personally 

 is capable of obtaining this pre- 

 cision. As for the former, opin- 

 ions on the subject vary;^ and it 

 appears that the best that can 

 be said at present is that, under 

 good flying conditions, namely 

 the best piloting and observation 

 and an absence of bumps, the 

 average error of one bubble-sextant observation is probably some- 

 where between 5' and 30'. Observations are likely to be more accu- 

 rate on sea than on land, more accurate if taken in the line of flight 



6 G. W. Littlehales: The Search for Instrumental Means To Enable Navigators To Observe the 

 Altitude of a Celestial Body When the Horizon Is Not Visible, U. S. Naval Inst. Proc, Vol. 44, 1918, 

 pp. 1808-1817. 



'Eaton, op. cii.; Beij, op. cit.; H. N. Russell: On the Navigation of Airplanes, Pubis. Astronom. 

 Soc. of the Pacific, Vol. 31, 1919, pp. 129-149, San Francisco; J. P. Ault: Navigation of Aircraft by 

 Astronomical Methods, Special Rept. in: Ocean Magnetic and Electric Observations, 1915-1921, 

 [constituting] Researches Dept. of Terrestr. Magnetism, Vol. 5, Carnegie Instn., Washington, 1926, 

 PP- 317-337; B. Melvill Jones: The Accuracy of Sextant Observations Taken from Aircraft, Technical 

 Rept. Aeronautical Research Committee for the Year 1Q22-23 {Repls. and Memoranda No. 810), Vol. 2, 

 pp. 589-606, H. M. Stationery Office, London, 1924; G. M. B. Dobson: Design of Instruments for 

 Navigation of Aircraft, Geogr. Journ., Vol. 56, 1920, pp. 370-389; Chart of Route Flown by Lieutenant 

 Commander R. E. Byrd ... Natl. Geogr. Mag., Vol. 50, 1926, p. 386. 



