320 Mr. Barlow on the temporary magnetic effect 
accomplish ; and having got the machine erected on my own 
premises, I was soon enabled to clear up the difficulties which 
had hitherto so much embarrassed my proceedings, although 
even here, in the first instance, I found some results very 
difficult to explain. 
The machine I now employed is shown in the annexed 
drawing. Plate XXIV. 
A B C D is a strong wooden frame, resembling that of a 
common electrical machine, the shell S being hung in the 
same manner as the cylinder ; the axis is made in two parts 
of gun metal, and very strong ; s s are two strong screw bolts 
and nuts, which were used for fixing the frame firmly to the 
top of the table, the bolt passing through from below. EGF 
is a substantial table with its feet sunk into the ground, and 
the floor of the room cut away where they passed through, 
in order to prevent any effect of shaking on the stand carry- 
ing the compass. 
The stand consisted of an upright pedestal filled with sand, 
to render it steady, and to this was fixed the table ML, with 
a semicircular hole cut in it, so that it might be placed near 
the shell. This table might be elevated or depressed at plea- 
sure, and it was divided into the points, quarter points, &c. 
of the compass. 
By means of different holes bored in the top of the table, 
the machine might be placed N and S, E and W, &c. at 
pleasure, and the motion of the shell be inverted by turning 
the handle to the right or left. The large wheel is six times 
the diameter of the small one ; and as it might easily be turned 
twice in a second, the number of revolutions of the shell 
were gradually about 720 per minute. The little apparatus 
