224 Royal Society : — 



Meyer's principle, described in the Gottingen Transactions for 1814. 

 The size of the small spheres, or their distance from the needle, was 

 varied in the different experiments, so as to bring different parts of the 

 axle to rest on the agate planes. The mean of the ten experiments 

 was 70° 02'*9 N., corresponding to the epoch 1821 .65 : the extremes 

 being 70° 00 f, l and 70° ()5' # 9. The whole of the experiments were 

 made by myself, and are detailed in a paper in the Phil. Trans, for 

 1822, Art. *1. 



Epoch o/"1838. — The experiments on this occasion were made on 

 different days in 1837 and 1838, in the course of the magnetic survey 

 of Great Britain, by Messrs. Robert Were Fox and John Phillips, 

 Captain (since Admiral) Sir James Clark Ross, Captain Edward 

 Johnson of the Royal Navy, and myself. The instruments employed 

 were those of Robinson, Gambey, and Jordan : the particulars are re- 

 corded in the 8th volume of the Reports of the British Association for 

 the Advancement of Science (1839), Table 10, p. 64. The localities in 

 which the experiments were made were — I. The same spot in the 

 Regent's Park where those of 1821 had been made. 2. Kew Gardens. 

 3. Westbourne Green, a locality which has been since built over. 

 Separate determinations were made on 13 days between May 30, 1837, 

 and December 10, 1838, the mean epoch being 1838.3, and the mean 

 dip 69° 1 7''S N. The extremes of all the observers and of all the in- 

 struments were 69° 13'-3, and 69° 23''9. 



Epoch of 1854. — The experiments on which this determination 

 rests were made by the late Mr. John Welsh, of the Kew Observatory, 

 and myself in August and September 1854, with two inclinometers 

 made by Mr. Henry Barrow (successor to Mr. Robinson), fitted 

 according to the modern English construction with verniers and 

 microscopes, and each having two needles. The localities selected 

 were — 1. The station in the Regent's Park already named as that of 

 the experiments in 1821, and of a part of those in 1838 ; and 2. the 

 magnetic house of the Kew Observatory. The experiments had a 

 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 of ten 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 o/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 I860 inclusive, chiefly by four observers, viz. Mr. John 

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



