VII. On the dip of the magnetic needle in London, in August 1828

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Abstract
The Philosophical Transactions contain the record of the dip of the magnetic 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 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 observations 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 Göttingen, 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 pre­sent 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

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