i 9 8 SCIENCE PROGRESS 



of different magnitude, galactic latitude, and spectrum; and a 

 further publication is foreshadowed, dealing with the number 

 of stars of determined magnitude, galactic latitude, spectrum, 

 and proper-motion. The publication under review is divided 

 into two parts : in the first the mean secular parallaxes of 

 stars of different galactic latitude and magnitude are determined. 

 The various material is collected, reduced to a common basis 

 and discussed, and it is concluded that the logarithm of the 

 secular parallax can be expressed by the formula — 



log (secular parallax) = — 0*428 — 0^096 cos 26 — 0*13 73 m, 



where b is the galactic latitude and m is the apparent visual 

 magnitude. The secular parallaxes require to be multiplied 

 by 0*243 to reduce them to annual ones. In the second part 

 of the publication the result is extended, and formulae of a 

 similar nature are derived which are applicable to the several 

 spectral types separately. A comparison between observed 

 values and those computed from the formulae shows a very 

 satisfactory accordance. At the end of the publication are 

 given tables for the mean secular parallaxes of the separated 

 types, tabulated according to visual magnitude and galactic 

 latitude. 



The Navigation of Aeroplanes. — An interesting article by 

 Prof. H. N. Russell, in Pub. Ast. Soc. Pacific, 31, 130, 1919, 

 gives an account of some investigations made by him on the 

 determination of the geographical position of aeroplanes by 

 means of sextant observations made during flight. The in- 

 vestigations were made under the authority of the Division 

 of Science and Research of the U.S. Bureau of Aircraft Pro- 

 ductions, and Prof. Russell was assisted by Mr. J. P. Ault, 

 navigating officer of the non-magnetic vessel Carnegie, of the 

 Carnegie Institution. The development of long-distance 

 aeroplane flights renders this problem one of great importance, 

 which even the improvements in directional wireless teleg- 

 raphy do not lessen. As bearing upon this question, it is of 

 interest to recall that General Maitland, in his log of the voyage 

 of the R34, puts on record a statement by the navigating 

 officer that an error in position of fifty miles was easily possible 

 from sextant observations using a cloud horizon. 



Prof. Russell states that there is no difficulty in using a 

 sextant in an aeroplane provided that it is small and light, 



