274 ME. a. B. AIET ON THE DIFFEEENCE IN THE MAONETIC PEOPEETEES 
upright in the same direction relatively to the direction of the maker’s inscription, and 
the distinctive number of the bar was painted on the upper end of each ; the end on 
which it was painted being that which is called “ the Lettered End.” The bars rested 
thus for several days, with the lettered end upwards. 
The following apparatus was prepared for the experiments : — A wooden frame was 
constructed about 11| feet in length; and this length, in the use of the frame, was 
placed very approximately in the direction of magnetic E. and W. Its ridge was 
21 feet N. of the old front, or 13 feet N. of the new front, of the anteroom of the 
Magnetic Observatory. The upper and essential part of the frame consisted of two 
planes, each about 2 feet broad ; of which one was very approximately in the position 
transverse to the direction of dip at Greenwich ; and the other, at right angles to the 
'former, included in its plane the direction of dip. These are called “ Equatorial Plane” 
and “ Dip Plane ” respectively. Ledges of wood were attached to the planes, for the 
support of flagstones resting on the planes and lying parallel to them. And whether 
the wooden surface or the stone surface was employed, a frame of laths was placed upon 
it, which retained the length of each bar in the position nearest to the vertical, and 
prevented one bar from touching another. In this position the bars were struck with a 
hammer. If they were upon the equatorial plane, any induced magnetism was instantly 
struck out of them; if upon the dip plane, they became powerfully magnetized, the 
lower end having the same properties as the marked end of a compass-needle, or being 
charged with austral magnetism. 
For testing the magnetism, a vertical wooden rod was provided, carryirig two horizontal 
planes or stages, also of wood. The upper stage supported a prismatic Rater’s compass, 
with which a well-deflned mark (the ball-mast) was viewed, and its apparent azimuth was 
read. The lower stage supported the bar under experiment, which was placed in a 
horizontal position below the compass, at the distance of 5 inches, as nearly as could ])e 
conveniently measured, between the centre of the compass-needle and the cenfre of the 
bar. The reading of the compass-card under view increases as the card turns in the 
direction opposite to that of the sun’s diurnal motion. From this it will appear that 
when the end of a bar, which has been downwards on the dip surface, is placed eastward 
on the lower plane, the austral magnetism of that end will attract the needle’s south 
end towards the east, and will cause the card-reading to increase. 
The bars were numerated as follows : — 
(Hot-Eolled Longitudinal) Nos. 1, 2, 3, 4, 5, 6 (painted in white) ; 
(Hot-Eolled Transversal) Nos. 7, 8, 9, 10, 11, 12 (painted in black); 
(Cold-Eolled Longitudinal) Nos. 13, 14, 15, 16, 17, 18 (painted in red); 
(Cold-Eolled Transversal) Nos. 19, 20, 21, 22, 23, 24 (painted in blue). 
In the conduct of the experiments, the following rules were uniformly followed : — 
The bars of the different classes were systematically intermixed, the same order being 
preserved in the whole series of experiments. Thus the order in which they were 
placed upon either plane (equatorial or dip), and the order in which then- effects in 
disturbing the compass were observed, was always the following ; — 
