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Art. XXI .— On the Magnetic Influence of the Island s oj 
St Mayo and the Great Salvage . By Lieutenant W 
Mudge, R. N. 
In the course of the year 1819, while engaged with Lieutenant 
Vidal in surveying the Island of St Mayo, we found the hills 
on which we were carrying on our operations so strongly mag- 
netic, that the needle belonging to the theodolite became wholly 
useless ; the dip increasing so much, that the needle could not 
traverse, in consequence of one end of it being drawn down to 
the face of the instrument. In order, therefore, to obtain a mag- 
netic bearing, the latter was obliged to be inclined at a very 
considerable angle ; and even then the direction became so un- 
certain and variable at different stations, as to be entirely useless 
for the purposes of the survey. 
I did not at this time make any farther observations ; but in 
our recent voyage, I have been induced to examine a similar 
circumstance which we met with in the Island of the Great Sal- 
vage near Teneriffe, a little more minutely : the result of which 
will be better seen by the annexed delineation of the island, (See 
Plate XI.), in which I have marked the several stations used in 
the survey, and the bearing of the compass at each of them, 
from which it will be seen, that the extreme difference in the 
variation of the compass amounted to about 72°, although the 
two stations were less than a mile asunder. 
The island is obviously of volcanic origin, and consists prin- 
cipally of a dark coloured black rock, the detached parts of 
which, as well as the whole, exhibit strong marks of fixed mag- 
netic polarity ; but it is remarkable, that a large fragment, 
Weighing about 20 lb., which I brought away with me, has lost 
much of its power, although its effect on the compass is still 
strong* and its polarity w T ell defined. Even the dust of the 
roads, and of the floors of the cottages, has the same character 
as the rock itself, and may be gathered up like steel filings by 
means of a bar magnet. I have brought with me a small sam- 
ple of this dust, and have placed both specimens in the hands of 
Mr Barlow of the Royal Military Academy, Woolwich, at 
whose suggestion I have been induced to lay this and the pre- 
vox.. v. NO, 10 OCTOBER 182L C C 
