On the Secular Change in the Magnetic Dip in London. 225 



its present Director, Mr. Chambers, Assistant in the Observatory, 

 and Dr. Bergsma, Director of the Magnetical and Meteorological 

 Observatory of the Netherlands Government in Java. 



There were employed in these determinations, on different occasions, 

 twelve circles and twenty-four needles, all of the same form and 

 pattern ; the circles being 6 inches in diameter, fitted with verniers 

 and microscopes, and the needles 3| inches in length ; they were 

 all made by Mr. Henry Barrow. Every determination was 

 complete in the eight different positions of the circle and needle, as 

 described in Appendix 2 of the Article "Terrestrial Magnetism" in 

 the 3rd edition of the ' Admiralty Manual of Scientific Inquiry.' The 

 individual results are shown in the subjoined Tables, whereof Table I. 

 contains 115 determinations comprised between November 1857 and 

 December 1858; Table II. 96 determinations between January and 

 December 1859 ; and Table III. 71 determinations between January 

 and December 1860. The results in each year are reduced to the 

 1st of July in the same year, employing the proportional parts of an 

 annual secular change of — 2'* 6 : those which were obtained in the four 

 winter months, November, December, January, and February, have 

 also received a correction of — 0'*8, and those obtained in the summer 

 months, May, June, July, and August, a correction of + f, 8 in 

 compensation for annual variation, agreeably to an investigation con- 

 tained in the sequel. The Tables exhibit in every case the date, the 

 particular circle and needle employed, the azimuths in which the 

 observations were made, the name of the observer, the observed dip, 

 the reduction to a common epoch, the correction for annual variation, 

 and finally, the corrected result. 



The opportunity afforded at the Kew Observatory, of testing the 

 degree of accordance which may be expected in the results of different 

 instruments constructed on the plan which has been for several years 

 past approved and adopted at Kew, has thus been profited by, and the 

 conclusions appear such as to merit the consideration of those who 

 are desirous to possess reliable instruments. Several of the circles 

 are the property of foreign governments or of individuals, at whose 

 request they were provided subject to a verification at Kew. The 

 observations here recorded were for the most part made for the 

 purpose of such verifications, and were entered as they were made in 

 the books of the Kew Observatory, from which they are now taken. 

 No observation has been omitted. The circles were distinguished by 

 the numbers 20, 23, 27, 28, 30, 31, 33, 34, 35 and 36; and two 

 unnumbered, one known as the Kew Circle, the other an inclinometer 

 employed by Admiral Sir James Clark Ross in his recent magnetic 

 survey of a part of England. No. 20 was made for Professor Hansteeu 

 of Christiania, and is now in his possession ; 23 is the circle used by 

 the late Mr. Welsh in his magnetic survey of Scotland ; 27 was 

 supplied to the Austrian frigate * Novara ' for her voyage of circum- 

 navigation ; 28 was made for the Russian Government ; 30 was used 

 by myself in the recent magnetic survey of England, and has been 

 since supplied to the Observatory at the " Isle Jesus " near Montreal 

 in Canada; 31 was made for Padre Secchi of the Collegio Romano, 

 and is now at Rome ; 32 was made for the Rev. Alfred Weld of 



Phil Mag. S. 4. Vol. 23. No. 153. March 1862. Q 



