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VII. On the Dip of the magnetic needle in London, in August \9i2S. 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. 

 Nairne and Cavendish in 1772 and 1773^ 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 coimtry, 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 



