572, MR J. A. BROUN ON THE 
employed, which needle was so insensible that, according to GILPIN’s exp 
statement, the accidental deviations could amount to 8’ or 10’ or even more.”* 
An attentive consideration of GILPIN’s observations induces me to conclude 
that Dr LAamont’s view cannot be accepted without a considerable qualification ; 
for although the needle sometimes did not return to its previous position within 
10’ or more, yet we have the evidence of G1LPin’s monthly and yearly means, show- 
ing with considerable exactness the small westerly movement of the magnet then 
taking place, from 23° 50’ in 1793, to 24° 9’ west in 1805, as proof of the general 
exactness of his observations.t At the same time, there is no reason to believe 
that the occasional inconsistencies, which GILPIN sought to correct by frequent 
observations, would diminish the range more than increase it. We have, however, 
as in the case of BEAUFoy’s instrument, the means of verifying the sensibility of the 
London needle, by the simultaneous observations made at Paris at the com- 
mencement of GILPIN’s series ; the comparative mean ranges are as follow:— 
CASSINI. GILPIN. 
LBL, roe COO) ct ee re miter eek 
IASC, Dope GS eg a IR C7 
LiS835 be lO esi (a awe Cte lise 
19. These quantities give no reason to doubt the sensibility of Gripin’s needle 
at these periods. The principal objections to the results of GiLprn are to be 
found later, in the small ranges for the maximum of 1797-7, to which I shall 
refer immediately. It should be observed that, as far as I am aware, no one has 
suggested that GILPIN’s observations show a maximum in the year just ‘men- 
tioned; but a consideration of the facts allow, it seems to me, little doubt that 
a maximum occurred at that time. 
A second maximum appeared in 1803°5; but Grprn’s ranges for the last 
four of the five months of observations in 1805, show an average increase of 
0’52 on the ranges for the corresponding four months of 1804; it is by no means 
certain then that the second maximum did not occur after 1805°5. We may 
now compare the results obtained from the whole series of observations. 
* “Hinige Bemerkungen iiber die zehnjihrige Periode,”’ &c., Sitz. b. der k. Akad. d. W. 1864. 
Ginpin’s statement is—“Sometimes the needle would be extremely consistent with itself, so as to return 
exactly to the same point, however often it might have been drawn aside; at other times it varied 2’ 
or 3’, sometimes 8’ or 10’, or even more.”—Phil. Trans. 1806, p. 416. 
+ Araco has also indicated the large diurnal oscillation obtained by Giuprn and its varying amount 
with the season (as elsewhere) as evidence of the free movement of Ginrtn’s needle. His chief difficulty 
has reference to the small annwal variation of the mean position compared with that found by Casstn1, 
a result which I believe to be wholly in favour of Ginpin’s observations, since no such large annual 
movement as that found by Cassini has been shown by any careful series of observations since his 
time. See “Cfiuvres de F. Arago,” t. iv. p. 482. I regret that I have not been able to find the 
‘original observations which were made in the Royal Society’s Apartments, Somerset House, from 1786 
to 1808 by Gitpry, and continued thereafter by Mr Lzn, the librarian (See Beauroy, Annals of Philos. 
p. 339). It is not improbable, however, that they may yet be discovered. 


