bis 
DECENNIAL PERIOD OF MAGNETIC VARIATIONS, ETC. 569 
made with magnets suspended by silk fibres ; BEAuroy observed with a needle 
and agate cup suspended on a steel point; the imperfections of such instru- 
ments are so well known, that observations made with them are always regarded 
with suspicion (2). Brauroy employed two needles, weighing 48 and 651 
grains respectively, making always seven observations direct, and seven observa- 
tions inverted, with each needle.* While the consistent results obtained by 
this observer for the monthly and annual mean positions of the needle are 
much in favour of its sensibility, we have fortunately other evidence in the 
simultaneous observations made by ARAGo during the eleven months, February 
to December 1820; the mean ranges, according to both observers, during this 
period are as follow :—February to December 1820, Breauroy, 745; Arago, 
1007. 
BEAvFoy’s observations were made near 8° 40™ a.m., and 1" 25™ p.m., and the 
ranges are derived from the means at these hours. ARraco made generally an 
average number of eleven observations daily, beginning at 7 A.M. and ending at 
11 p..t, and the ranges are the means of the largest observed oscillations in 
each day; they are therefore the mean daily ranges, and not the ranges of 
the hourly means for each month, as in other cases. An approximation to 
the ratio of the two kinds of ranges may be obtained from the Makerstoun _ 
observations for the years 1843 and 1846, during which observations were made 
two-hourly, from 5 am. to9 p.m. From these we obtain the mean movement 
from 85 40™ a.m. to 12 10™ p.m. (nearly, as in BEAUFoy’s series)=6"98. The 
mean of the daily ranges for the same two years=11"51, and the ratio is— 
Ga =1054 
If we multiply BeauFoy’s range by this ratio we obtain 7-45 x 1°65=1273, 
which is the quantity to be compared with ARrAGo’s range of 10°07. Allowing 
for the irregularity in the number of observations made daily by the French 
astronomer, by which the daily ranges may have been somewhat diminished, 
and for any difference of the ranges for Paris and London, the monded needle 
seems to have been sufficiently sensitive. 
BEAvFoY’s observations were made from April 1813 to September 1815, and 
from April 1817 to December 1820. The yearly mean ranges corresponding to 
* TxHomson’s Annals of Philosophy, vol. iii. p. 372. 
+ Ciuvres de F. Arago, t. iv. p. 427. 
t A similar comparison of the Greenwich two-hourly observations for the four years, 1843 to 
1845, Bee the range from 8h 40™ a.m. to 1" 20™ p.m.=7'4, while the mean of the daily ranges 
=12':2, whence the ratio— 
12/2 
cer a =1°65 
is exactly that found from the Makerstoun observations. The ratio varies somewhat with the year and 
the amount of disturbance, the values for the four years at Greenwich being 1°53, 1°76, 1°64, and 1°67. 
As 1820°5 was upwards cf two years from the epoch of maximum disturbance, the ratio found cannot 
be far from the truth. 
