1876.] Magnetic Dip fyc. at Kew Observatory. 233 



ings of the Boyal Society,' contributed by Dr. Balfour Stewart, the 

 Superintendent of the Observatory; it contained the results of the 

 monthly observations of Dip and Horizontal Force for the next series of 

 six years, viz. from April 1863 to May 1869. 



There is now presented a further series of six years' observations of 

 Dip and Horizontal Eorce, from April 1869 to March 1875 inclusive, 

 together with observations of absolute declination for the same period. 



The instruments employed in the determinations are the same as those 

 described by Sir E. Sabine in the Philosophical Transactions, 1863, 

 page 296, no change worth noticing having been made in either instru- 

 ments or magnets. 



As in the case of the six years discussed by Dr. Stewart, only those 

 dips have been considered which have been made with the two needles 

 Nos. 1 and 2 of circle Barrow No. 33 ; of these the general practice has 

 been to observe both needles on two successive days in the middle of the 

 month, giving the mean of the four observations as the mean for the 

 month. 



The values of the constants employed in the reduction of the observa- 

 tions of vibration and deflection have been redetermined, and found to 

 closely correspond with those found by Mr. "Welsh, which are given in 

 the paper quoted above. 



Two observations of vibration and two of deflection, at two distances, 

 have been made each month, the mean result being taken as the force for 

 the month. 



The Declination observations have been made in the Magnetic Obser- 

 vatory with the Kew 9-inch unifilar and a reversible collimator magnet 

 NE, the instrument being directed after each reading to the reference- 

 marks on the obelisk erected a quarter of a mile to the north of the 

 Observatory as a meridian mark for the transit instrument. 



The azimuth of this mark, as viewed from the declinometer, was care- 

 fully determined by Mr. Welsh in 1857, and has since been verified both 

 by Mr. Chambers and Mr. "Whipple. 



After each observation of the magnet it is removed and replaced by a 

 torsion-plummet of equal weight, the position of which is observed when 

 it has come to perfect rest a few hours after the observation. 



Should any torsion be found to have been introduced into the thread, 

 a suitable correction is applied to the observation. 



Observations have been made at noon on two consecutive days in each 

 month, the mean of which is inserted in the Table. 



Dip. 



In Table I. we have a record of the observed values of dip made with 

 circle 33 Barrow, each observation being the mean of two made with each 

 of the two needles belonging to that circle. 



