18 Captain Sabine's experiments to determine the 
Whence 70° 03' may be considered as the mean dip of the 
needle towards the north in the Regent’s Park, in August and 
September 1821, within four hours of noon, being the limit 
within which all the experiments were made. 
In referring to the observations which are recorded to have 
been made for the purpose of determining the dip in London 
in former years, those of Mr. Nairne, in 1772, of Mr. Ca- 
vendish, in 1776, and of Mr. Gilpin, in 1805, appear to 
have received, and to be deserving of, principal consideration; 
the errors by which these several determinations may have 
been affected, in consequence of the imperfections of the in- 
struments, may be believed to have been confined within 
limits of no great extent, by the method of the observers, and 
by the precautions which they adopted ; as, however, the ob- 
servations were made in houses in close built parts of the 
metropolis, they were all subject to the influence of local 
attraction, from whence may have originated errors of greater 
consequence possibly than those of the instruments; nor 
can the application of a correction found by observing the 
difference of the dip, on the outside of the house, be consi- 
dered an effectual remedy, inasmuch as the needle may still 
have been attracted by iron in the adjoining houses, or in the 
neighbourhood. It needs only to try needles in different si- 
tuations in a city, to be convinced how little dependance should 
be placed in the accuracy of such results ; it may, no doubt, 
be principally owing to this cause, rather than to instrumental 
error, that the dip at the Apartments of the Royal Society is 
stated in the Philosophical Transactions for the present year 
( 1821 ), to be 71 0 & or 71 0 42'. 
As the observations of Mr. Nairne, in 1772, and of Mr. 
Cavendish, in 177 6 , do not differ very widely from each 
