induced in iron bodies in rotation. 
327 
ral laws which are known to exist. The formula, which we 
owe to Mr. Biot and to Mr. Kraft, expressing the law of 
the dips in different latitudes, viz. tan. dip. =: 2 tang. mag. 
lat. certainly agrees with observation in many cases, but the 
variation computed on the same principles, and which neces- 
sarily ought also to coincide with observation, is widely in 
error. It seems therefore very obvious that some disturb- 
ing cause exists, but whether any part of it can be attributed 
to the rotation of the earth is, notwithstanding the preceding 
results, very doubtful ; at the same time I may perhaps be 
allowed to observe, that one of the essential conditions for 
the production of such an effect has place in the earth, viz. 
that it does not revolve about its polarized axis ; and if the 
inductive principle through which it receives its magnetism 
be exterior to itself, then it would follow almost of necessity 
that some such an effect should take place. I beg however 
to be understood as advancing nothing in this letter, beyond 
the mere experimental fact above stated, which, even if it 
should find no useful application, may perhaps be thought 
sufficiently curious to be recorded in the Transactions of the 
Royal Society. 
PETER BARLOW. 
