145 



distant at that time from buildings or iron implements, railing, or 

 pipes. The experiments, ten in number, were made on six different 

 days, between the 3rd and 10th of August 1821 ; and all between 

 8 A.M. and 4 P.M. The circle employed was 11| inches in diameter, 

 made by Nairne, a celebrated artist in his day for instruments of this 

 description : the needle was made by Dollond on Professor Tobias 

 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 OO'-l and 70 05''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 of 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 1. 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'*3 N. The extremes of all the observers and of all the in- 

 struments were 69 13 f 3, and 69 23 r '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 oa part of those in 1838 ; and 2. the 

 magnetic house of the Kew Observatory, The experiments had a 



