18(5 Lieut. Foster's comparison of the diurnal changes, &c. 
V-— 
“ certain number of years describe a circle of about i$gr. 
“ radius round the magnetick poles northerly and southerly. 
“ This I have for several years suspected, and have had some 
“ reason for it too ; and three or four years ago, mentioning 
“ it at a meeting of our Royal Society, they were pleased to 
“ cause it to be entered in the Journals ; but I have not yet 
“ been so happy to procure a tolerable good dipping needle, 
" or other proper one to my mind, to bring the thing to 
“ sufficient test of experience ; as in a short time I hope 
“ to do, having lately hit upon a contrivance that may do 
“ the thing." 
Mr. Lovett next proceeds to illustrate Mr. Derham’s 
theory by appropriate diagrams, and then to compute the 
latitude of the magnetic pole from the best recorded obser- 
vations at the time on the variation of the compass at two 
well known places. Having thus obtained 13 0 51' for the 
north polar distance of the magnetic pole, or radius of the 
orbit which it describes round the pole of the earth, he then 
fixes the year of no variation of the magnetic needle in 
London to be 1660, from the observations of Dr. Halley in 
1672 ; and from a similar observation by Dr. Bradley in 
1750, he deduces the longitude of the pole for that time, and 
by this interval of 90 years, he infers the progressive rate of 
the pole westerly to be in longitude 7 0 7' 1 2" every ten years. 
With these data he has computed a table of variations of the 
compass for every ten years between 1660 and 1910, in 
which he has predicted, with near approximation to what 
has since been observed, considering the distance of time 
and want of correct knowledge of its quantity, not only the 
