MAGNETIC NEEDLE IN LONDON, IN AUGUST 1828. 53 



seven years, or 2'.5 in each year. This rate of diminution is sensibly less than 

 the general average resulting from the comparison of the most authentic obser- 

 vations, at considerable intervals apart, in the century preceding 1821. These 

 results fall variously between the limits of 2'.9 and 3'.2. Did the observations 

 of 1821 and 1828 stand alone, in indicating a decrease at the present time of 

 the amoimt of the annual change in the dip in this part of the world, it would 

 appear the more probable supposition that either of those observations might 

 be in error the few minutes which would be sufficient to make their difference 

 correspond with former observations ; and still more probable that they might 

 contain between them an error of that small amount. But if we examine the 

 very correct and consistent series of observations on the dip at Paris, com- 

 menced by M. Humboldt in 1798j and continued in subsequent years by 

 MM. Gay Lussac, Humboldt, and Arago, we find in them a similar indica- 

 tion of diminution latterly in the annual decrease of the dip. If, for example, we 

 divide the interval of thirty years between 1798 and 1828, into two nearly equal 

 portions by means of the observations made by M. Arago in 1812, we have for 

 the first portion, containing fourteen years, a diminution of (69° 51' — 68° 42' 

 = 69 -r 14 =) 4'.93 a year; and for the second portion, containing sixteen 

 years, of (68° 42' — 67° 58' = 44 -4- 16 =) 2'.75 a year. And if instead of di- 

 viding the interval by M. Arago's observations in 1812, we take for that pur- 

 pose the conjoint observations of MM. Humboldt and Arago in 1810, we have 

 for the first portion, containing twelve years, (69° 51' — 68° 50' = 61 -f- 12 =) 

 5'.08 a year ; and for the last portion, containing eighteen years (68° 50' — 

 67° 58' = 52' -7- 18 =) 2'.89 a year: all which indications are of the same 

 character and accord well with the observations of 1821 and 1828 in London. 

 A repetition of the observations in London at the expiration of another seven 

 years, and a continuation of those at Paris, will probably show decisively 

 whether the annual change in the amount of the dip in this part of the world 

 is diminishing, as there now appears reason to suspect. Should it prove the 

 case, careful and frequent observations of the dip will possess a more than ordi- 

 nary interest, since the correct determination of the precise period when the 

 dip may become stationary, and its amount at that time, which would be its 

 minimum limit, will form most important additions to our knowledge of the 

 phsenomena of teiTestrial magnetisni«k>wiiOil m qxa-uUi aoii;>iiv. 



