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VII. On the Dip of the magnetic needle in London, in August 1828. By Captain 
Edward Sabine of the Royal Artillery, Secretary of the Royal Society. 
Read January 8, 1829. 
The Philosophical Transactions contain the record of the dip of the mag- 
netic needle in London, observed at irregular intervals since the early part of 
the last century. In comparing these, and particularly the results obtained 
by Messrs. Whiston and Graham in 1720 and 1723, with those of Messrs. 
Mairne and Cavendish in 1772 and 1775, and both with the dip as it exists 
at present, we have satisfactory evidence of the progressive diminution of the 
dip in London during the whole of the period in question ; but the observa- 
tions are too few in number and infrequent, and the earliest ones particularly 
too doubtful in point of accuracy, to enable us to determine whether the 
annual diminution has been uniform or otherwise. 
In the Philosophical Transactions for 1822, Art. I. the Society did me the 
honour to publish an account of observations which I had made in the 
Regent’s Park, in August 1821, to obtain a correct determination of the dip in 
London at that time ; in which observations I employed, for the first time in 
this country, a needle constructed on a plan proposed by Professor Meyer of 
Gottingen, for avoiding the usual error of dipping needles arising from the 
non-coincidence of the centres of motion and gravity. Seven years having 
since elapsed, an interval perhaps not. too small to throw light upon the pre- 
sent rate of diminution of the dip, I repeated the observations in the August 
of last year, an account of which I now present to the Society; changing 
the place of observation, in consequence of the increase of buildings in the 
Regent’s Park, to the Garden of the Horticultural Society at Chiswick, distant 
about six miles, in a direction coinciding as nearly as possible with the line of 
equal dip passing through the Regent’s Park. 
The general apparatus employed is the same that I used in the observations 
