amount of the dip of the magnetic needle , in London , &c. 19 
other, either in the date, or in the amount of the dip, their 
mean, 72 0 25', in 1774, may be considered as the best approxi- 
mation which can now be made to a knowledge of the 
amount of the dip in London at an early period. 
By comparing this amount with the dip in the present year 
as above determined, we obtain 3 , ,02 as a mean annual rate 
of diminution between 1774 and 1821; which is less by two- 
fifths than the mean annual diminution at Paris between the 
years 1798 and 1814, as deduced from the observations of 
Messrs. Humboldt, Gay Lussac, and Arago ; whence it 
might be inferred, if sufficient dependance could be placed on 
the accuracy of the observations, that the annual variation of 
the dip in this part of the world, is greater now than it was 
30 or 40 years since. 
It is, however, worthy of notice, as being at least a curious 
coincidence, that if we take Mr. Whiston’s determination of 
the dip in 1720, 75° io' (of which Mr. Cavendish remarked 
in the Philosophical Transactions for 177 6, Art. 21,) that 
“ he believed it to have been pretty accurate, as Mr. Whis- 
ton observed in many parts of the kingdom, and his obser- 
vations agreed well together,” we obtain between the years 
1720 and 1774, an annual diminution of 3' ,05, which differs 
only three hundredths of a minute from the rate which has 
been now found for the succeeding 47 years. 
It may not be useless, briefly to examine how far the know- 
ledge of the amount of change, by direct observation, is 
capable of receiving confirmation by the effect which a dimi- 
nution of dip must produce on the vibrations of a needle sus- 
pended horizontally. 
If the intensity of the magnetic force be considered to varv 
