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 amount 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 1798, 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 -T- 14 =) 4'.93 a year; and for the second portion, containing sixteen 
years, of (68° 42' — 67° 5 8' = 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 -=-12=) 
5'.08 a year ; and for the last portion, containing eighteen years (68° 50' — 
67° 58' = 52' ~r 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 terrestrial magnetism. 
