chronometers by the proximity of masses of iron. 367 
ridian, agreeably to the division of the magnetic sphere, as 
described in my “ Essay on Magnetic Attraction.” 
I now first placed the compass at O, eighteen inches from 
the centre of the shell, and observed the number of seconds 
which the needle employed to make forty vibrations ; then, 
still keeping the needle in the circle QO', I placed it 30° from 
Q towards E, or in longitude 6o°. I then brought it 30° 
nearer to E, or into longitude 30° ; then to E, or longitude 
o° ; and so on at every 30° through the whole circle. The 
same was then repeated in the circles ab , cd, ef , See. ; and by 
taking the mean of the results for each corresponding situ- 
ation on each side of the meridian, I obtained the numbers 
given in the following table : 
TABLE, 
Shozuing the time of making ten vibrations with a fine magnetic 
needle in different situations, eighteen inches from the centre of 
an iron shell, eighteen inches in diameter, weighing 496 lbs. 
Latitude. 
Mean time of making io vibrations. 
Long. 
90° N 
Long. 
60° N 
Long. 
3o Q N 
Long. 
0» 
Long. 
30° S 
Long. 
Co°S 
Long. 
90° S 
O 
go N 
60 N 
45 N 
30 N 
0 
30 S 
45 s 
60 S 
go S 
II 
30-25 
26-00 
26- 25 
27- 25 
34-50 
46-50 
43-50 
38-25 
II 
27-25 
27-50 
27-50 
34 - 75 
41-25 
39 2 5 
35 - 75 
n 
28-00 
28-25 
2g-00 
34-75 
34- 00 
35 - 2 5 
34 ’ 5 ° 
II 
29 - 75 
30- 00 
31- 25 
35 - 5 ° 
31-25 
30-00 
29-75 
II 
34 - 50 
35 - 25 
34- 00 
35- 00 
2g-oo 
28-25 
28.00 
II 
35-75 
39-25 
41-25 
35-00 
27-50 
27-50 
27-25 
II 
38-25 
435 ° 
46-5° 
35-00 
27-25 
26 25 1 
26-00 
30-25 
Mean time of making ic vibrations detached from the iron 
ball 32"- 50. 
