ON THE MAGNETIC INTENSITY OF THE EARTH. 3 
stances required to be known in order to estimate rightly the 
value of the results. In the case of observations which are 
either wholly or partly new, these particulars are not to be 
found elsewhere; and in the case of those series, the published 
accounts of which are contained in foreign works rarely met 
with in this country, it has appeared desirable,—whilst giving 
every direction which may facilitate a reference to the original 
publication,—to make the account here given complete in all 
particulars essential to a just estimation of the value of the 
results, independently of such reference. The details neces- 
_ sary for this purpose may render this portion of the report 
i ap 
occasionally tedious to the general reader, who will be princi- 
pally interested by that section which contains the general con- 
clusions. 
Section I.—Historicat Notices. 
It is to France we owe the first rightly directed experimental 
inquiry on this subject. The instructions, drawn up by the 
members of the French Academy of Sciences for the expedition 
of La Perouse, contain a recommendation that the time of vibra- 
tion of a dipping needle should be observed at stations widely 
remote, as a test of the equality or difference of the magnetic 
intensity ; suggesting also with a sagacity anticipating the result, 
that such observations should particularly be made at those 
parts of the earth where the dip was greatest and where it 
was least. 
The experiments, whatever their results may have been, which 
in compliance with this recommendation were made in the ex- 
pedition of La Perouse, perished in its general catastrophe ; 
but the instructions survived, and bore fruit in the earliest re- 
corded observations of the variations of the magnetic intensity, 
which are those published by M. de Rossel in the second volume 
of the Voyage de Dentrecasteaux in search of La Perouse. 
Rossel, 1791—1794.—These observations, though made in the 
years above-mentioned, were not published until 1808. They 
were made with a needle vibrated in a dip circle of Le Noir, 
coming to rest disadvantageously soon for the purpose of experi- 
_ ments on the intensity. The needle continued in vibration little 
_ more than three minutes; consequently incidental errors would 
_ bear a very large proportion to the total time of vibration; a 
_ disadvantage which appears to have been in a great degree coun- 
_ teracted by the very great care bestowed on the observation. 
The needle was vibrated at Brest in 1791, before the voyage 
commenced ; and, successively, at Teneriffe; Van Diemen’s Land, 
B2 
