136 



Absteacts foe the Magnetic Declination. 



TABLE I. — The Monthly Means of the Declinometer Readings in Westerly Declination, for the Hours 20, 23, 



2, and 5. 



Month. 



201=. 



23''. 



21-. 



5". 



Mean of 

 20" and 5^. 



Range. 



1841. 



25° 



25° 



25° 



25°. 



25°. 





July 



25-95 



30-65 



35-70 



3303 



29-49 



9-75 



August 



33-68 



37-32 



42-55 



38-09 



35-88 



8-87 



September 



30-26 



34-27 



39-01 



33-87 



32-06 



8-75 



October 



30-42 



31-99 



34-75 



3002 



30-22 



4-33 



November 



31-63 



32-94 



35-15 



3201 



31-82 



3-52 



1842. 















January 



27-96 



28-47 



30-77 



29-25 



28-60 



2-81 



February 



29-46 



31-51 



35-74 



31-23 



30-34 



6-28 



March 



27-51 



31-13 



35-63 



31-92 



29-71 



8-12 



April 



30-66 



34-84 



39-96 



35-82 



33-24 



9-30 



May 



25-52 



30-53 



35-98 



33-23 



29-37 



10-46 



June 



24-94 



29-25 



34-92 



33-31 



29-12 



9-98 



July 



25-13 



28-65 



34-08 



31-98 



28-55 



8-95 



August 



24-18 



29-73 



'33-53 



28-80 



26-49 



9-35 



September 



26-53 



31-08 



34-25 



28-85 



27-69 



7-72 



October 



25-79 



28-42 



32-79 



28-31 



27-05 



7-00 



November 



26-80 



28-16 



30-38 



26-80 



26-80 



3-58 



December 



24-85 



25-86 



27-53 



25-36 



25-10 



2-68 



The mean of the Declinations at 20'' and 5'^ have been given as an approximate mean for the month. The 

 difference of the Declinations at 20'' and 2'' is given as the approximate diurnal range. The latter must be very 

 near the truth, as 20'' and 2^ are nearly the hours of least and greatest Westerly Declination throughout the year. 



It seems very probable, that the Declination in the months of July and August 1841, and of April 1842, are 

 considerably affected by the torsion of the suspension thread. While this would destroy the value of the mean for 

 the month, the diurnal range may be considered as unaffected. 



In order to shew the regular changes well, it is advisable to eliminate, as far as possible, those days of irre- 

 gularity or disturbance in which the positions of the magnet differ considerably from the mean. The following 

 rule was adopted for the determination of the days of irregularity which should be rejected. 



If a, b, and c, be the approximate diurnal ranges for three successive months, those days in the middle month 



are rejected in which the diurnal range exceeds ^ • 



For the first and last months, the sum b + c or a + b has been used. (See Table XXIII.) 

 This rule was formed chiefly for the two components of force, for which it is much better suited than for the 

 Declination. 



Table II. has been formed after rejecting the days of disturbance thus determined. 



