146 



double purpose, viz. 1, to ascertain the difference, if any, in the 

 dip in the Regent's Park and in the magnetic house at Kew ; and 

 2, to obtain a determination of the dip in August 1854 which might 

 be strictly comparable with the result obtained in August 1821 . The 

 experiments were made on five different days, and comprised eighteen 

 determinations, ranging between 68 29''25 and 68 33'- 73 ; the mean 

 being 68 31'*13 N., corresponding to the epoch 1854.65. The 

 mean of eight determinations in the Regent's Park was 68 30'*55, 

 and often determinations at Kew, 68 31'* 6 ; the difference of either 

 from the mean being 0'*52, which is within the limits of probable 

 error. A detailed notice of these experiments was published in 

 1855 in an Editor's note in p. 364 of the translation, edited by 

 myself, of Arago's Meteorological Essays. 



Epoch of 1859. 5. The dip corresponding to July 1, 1859 (now 

 first discussed), is derived from 282 determinations made in the 

 magnetic house at Kew on 121 different days between November 1857 

 and December 1860 inclusive, chiefly by four observers, viz. Mr. John 

 Welsh, late Director of the Kew Observatory, Mr. Balfour Stewart, 

 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 

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

 annual secular change of 2 ff 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 



