made in the Coquille by M. Duperrey. 283 
west as far as the Atlantic Ocean. The observation of which 
we have been speaking was made at the Portuguese island of 
St. Thomas. Captain Sabine found indeed, in 1822, for the 
value of the dip, 0° 4’ S. The magnetic equator then 
actually passes by this island, the latitude of which is 24! N., 
or some minutes only more to the west. Its point of inter- 
section with the terrestrial equator is about 5° of east longi- 
tude, whilst, according to the observations of 1780, MM. 
Morlet and Hansteen have placed it at least 13° more to the 
east. 
According to these several approximations, the existence of a 
translatory movement in the magnetic equator is very probable. 
M. Morlet had already pointed it out, but with the proper doubt 
which measures of the dip obtained without change of the 
poles of the needle justly excited in his mind. In this re- 
spect we can now obtain complete certainty in investigating 
under the same point of view the whole of the observations on 
the dip made in the open sea in the equinoctial regions. The 
journals kept on board the Uranie and the Coquille include 
all the elements of these researches, in our opinion one of the 
most important that can now be undertaken on the phezeno- 
mena of terrestrial magnetism, It would appear, in short, 
that it is the form and position of the line of no dip, which 
determines from one pole to the other, in what direction, in 
every place, the annual variations of the magnetic needle shall 
manifest themselves. This conjecture, inasmuch as there is 
question of change of inclination, is to be found in the inter- 
esting memoir of M. Morlet, which the Academy some years 
ago honoured with its approbation. Ifthe appellation of mag- 
netic latitude of a point be given to the angular distance from 
this point to the line without dip, measured on the magnetic 
meridian considered as a great circle, we shall find in general, 
according to M. Morlet, that the inclination of the needle dz- 
minishes, where the translatory movement of the equator 
tends to diminish the magnetic latitude; and that it zncreases, 
on the contrary, every where where the magnetic latitude e- 
comes greater. Some places, such as New Holland, Teneriffe, 
&c. seem notwithstanding to form an_exception to it. The 
observations collected in the voyages of the Uranie and Co- 
quille have enabled us to submit this rule to a greater number 
cof verifications, and to learn that it agrees with experience in 
a very remarkable manner, even in the stations which M. Mor- 
let had excepted. We see in this manner that if the south 
inclination increases rapidly at St. Helena, whilst the north 
inclination diminishes rapidly at Ascension, it is because in its 
translatory movement, the magnetic equator, which is consi- 
2N2 derably 
