ON COxMPARING AND REDUCING MAGNETIC OBSERVATIONS. 



89 



the increase of horizontal force as 180 c.g.s., and the decrease of vertical 

 force as 80 c.g.s., the correction required to reduce the quiet day value to 

 the middle of the month, that is the correction to the ' Mean excess ' in 

 the respective columns of the table, would thus never exceed about the 

 -^^ part of the several annual variations above mentioned, and so may 

 be here disregai'ded. Neither would the numbers in the columns of 

 greatest and least monthly excess be sensibly affected. 



The following comparison, referring to Greenwich, may be of in 

 terest : — 



Declination 



Mean vahie of non-cyclic element ] 

 on quiet da3-s. Table X. f 



Mean excess of quiet day absolute 1 

 value above the all day absolute ^ 

 value. Table XII. J 



-0'-02 

 -1 c.g.s. 



+ 0'-08 

 -1- 4 c.g-..s. 



Horizontal 

 Force 



+ 39 c.g.s. 



+ 33 c.g.s. 



Vertical 

 Force 



-17c.g.s. 



9 c.g.s. 



In Sections 4 to G of the B.A. Report for 1896, Dr. Chree gave for the 

 various magnetic elements the excess of Wild's normal day absolute value 

 above the all day absolute value for St. Petersburg and Pawlowsk as 

 taken from Dr. MuUer's paper in Vol. 12 of the ' Repertorium fiir 

 Meteorologie ' and from the ' Annals of the Central Physical Observatory ' 

 respectively. Wild's normal days correspond generally in character 

 with the Greenwich selected quiet days. I have therefore compared the 

 results mentioned with the excess of the Greenwich quiet day absolute 

 value above the Greenwich all day absolute value, as follows : — ■ 



Excess of Absolute Values of Magnetic Elements on Normal or Quiet Days 

 above the Values for All Days. 



The horizontal and vertical force values are in c.g.s. measure x 10'^. 



The agreement in declination and horizontal force is on the whole 

 satisfactory, although the -|-0''6 for Pawlowsk in 1894 seems large. In 

 vertical force the agreement is not so good ; the — 30 at Pawlowsk in 

 1893 seems to diverge as much in the negative direction as does the +14 

 at Greenwich in 1894 in the positive direction. 



It appears that in the years 1895 and 1896 the introduction of iron 

 in the construction of new buildings at Greenwich affected to some extent 

 the determination of absolute values, but the differential results here 

 employed would not be sensibly influenced thereby. A new magnet 

 pavilion is in course of erection in Greenwich Park, in a position appa- 

 rently free from disturbing effect, to which the instruments will be removed. 



