262 Analysis of Mr Barlow’s Essay on Magnetic Attractions. 
having a hole in its centre, through which the ball might be 
made to pass when required. The plane of this table was divid- 
ed i*ito equal portions of 2\° each, and properly numbered, pro- 
ceeding from the line forming the magnetic meridian ; and the 
whole rendered perfectly secure and steady, by placing the ap- 
paratus on piles driven into the earth, through the floor of the 
model room of the Royal Military Academy at Woolwich. 
With this machine, it is obvious, the compass might be placed 
at any azimuth from the magnetic meridian ; at any distance 
from the centre of the table, and the ball itself placed at any 
height above, or depth below, the plane of the table. 
We have already stated in our second number, in the article 
above alluded to, that Mr Barlow had, prior to these experi- 
ments, discovered that there exists in every ball of plain unmag- 
netized iron, a plane of no attraction, which passes from north to 
south, and forms, in these latitudes, with the horizon an angle 
of about 195 °, as the complement of the dip ; and his first course 
of experiments is accordingly directed to the confirmation of 
this fundamental fact. The result of these experiments gives 
for the mean inclination of the plane 19° 24' ; and the actual dip, 
as determined by an excellent dipping needle, is in the same 
place 70° 80J', of which the complement is 19° 29|', leaving an 
error of only 5J', which is probably to be attributed to the daily 
variation, or to defects in the instruments, and errors of observa- 
tion. 
Before we proceed farther in our analysis, let us examine how 
far this result, which the author has certainly put in a strong 
point of view, may be considered as a newly discovered fact. 
For more than two centuries it has been known, that if a bar 
of soft iron be placed vertically, or nearly so, in these latitudes, 
its lower end will become a north pole, and its upper end a south 
pole ; that is, the lower extremity of the bar will attract the 
south end of the needle and repel the north ; while, on the 
contrary, the upper extremity will repel the south, and attract 
the north end of the needle. It may, therefore, easily be imagi- 
ned, that if a needle were made to descend vertically near such a 
bar, it must pass through a point where the effect of the bar will 
vanish, and this will be the case in every vertical, whatever may 
be its azimuth and distance. But the question is, Had any one 
