MAGNETIC SURVEY OP GREAT BRITAIN. 65 



mean rate for the same year in M. Hansteeii's table is 2'* 93, 

 which must be regarded as a satisfactory accordance, the dif- 

 ference being less than exists between the rate for that year at 

 any one of the stations in M. Hansteen's table, and the mean 

 of the four stations. We may infer from the accordance, there- 

 fore, that both these numbers, 2'*93 and 2'* 73, are extremely 

 near the truth ; and I have employed that which results from 

 our own observations, namely, 2'"73 corresponding to 1830. 

 Following the progression in M. Hansteen's table, the rate of 

 decrease would become 2'*4 in 1836, which is the middle pe- 

 riod of the observations contained in this report. In the re- 

 ductions to a common epoch, 2'*4 has consequently been em- 

 ployed as the mean annual decrease of the dip in the British 

 Islands between 1834 and 1838. In the absence of any certain 

 knowledge in regard to the unequal distribution of the yearly 

 decrease in the different months of the year, I have regarded it 

 as taking place in the uniform proportion of 0'*2 per month. 



In a recent communication to the Royal Irish Academy, Mr. 

 Lloyd has stated the result of thirty-nine observations of the dip 

 in Dublin between October 1833 and August 1836, which, cona- 

 bined by the method of least squares, give 2''38 for the most 

 probable rate of the annual diminution of the dip in Dublin 

 during that period. This result, though drawn from so limited 

 a period, is in remarkable accordance with the deduction from 

 the observations in London, and furnishes a strong presumption 

 that the rate thus found is applicable both to England and Ire- 

 land. In regard to Scotland, no observations have as yet been 

 made, I believe, witli this particular object. The general 

 aspect of the observations in Scotland, at different dates, con- 

 tained in this report, would certainly indicate a less annual 

 change than has been deduced from the observations in England 

 and Ireland; and in every instance in Scotland where obser- 

 vations have been made at the same station and at different 

 periods, either by the same or different observers, the evidence 

 is of the same nature, — the results would be brought into better 

 accord if a smaller rate of decrease were adopted. In the case 

 of the Shetland Islands, the dip observed by Captain Ross at 

 Lerwick in August 1838, 73° 45', compared with that observed 

 by Sir Edward Parry and myself in June and November 1818, 

 74° 22', makes a decrease of 37' in twenty years, or a yearly 

 diminution of l'*85, corresponding to the mean epoch of 1828. 

 The observations of 1818 and of 1838 were made in the same 

 garden. The identity of the spot, — the length of the interval, — 

 and the repetition of the observations on different days on both 

 occasions, — all give weight to this comparison ; and strengthen 



VOL. VII. 1838. p 



