16 
ME. J. E. H. GOEDON ON THE DETEEMINATION OF 
Tan l. 
The scale, which consisted of a wooden board on which was pasted a printed paper 
scale of millims., was fixed on a massive wooden tripod-stand fixed to the wooden floor. 
The telescope was, as before mentioned, bedded with plaster of Paris in a block of stone 
cemented on to a brick pier. [N.B. All the brick piers and tables were built on the 
solid ground and came up through holes in the wooden floor.] The scale was so 
adjusted that, when no current was passing, the line from O to the figure seen in the 
telescope was perpendicular to the scale. This adjustment was made by hanging a 
plumb-line near the scale, between it and the mirror O, so that on looking into the tele- 
scope it was seen in the centre of the field. A second was then hung in the line 
between the first and the mirror. The edge of a T square being placed so as just to 
touch both lines, its base gave the direction of the scale. 
As it was not certain whether or not the paper scale had stretched in the pasting, the 
deflections were determined by taking, with a pair of compasses, the distance between 
the zero reading and that corresponding to any current, and measuring it on the 
standard brass scale. The numbers thus obtained are hereafter spoken of as “ corrected 
deflections.” 
The readings were taken in each direction. Their mean was called x. 
The distance from O to the mean zero-point was called a, and by measurement 
we have 
«=102’50 centims. 
The values of y = tan c), where cS is the deflection of the suspended magnet, were 
obtained by the following formula, due to Professor Maxwell. We have 
y = tan ci=tan tan -1 ^j 
=aA+|-S 
(26) 
Determination of the Meridian. 
The best way to have determined the meridian of the helix would have been to have 
deduced it from that of the suspended mirror. Owing, however, to the arrangement of 
the supports, this meridian could not he transferred to other parts of the table. The 
meridian was therefore determined by means of a needle furnished with a straw pointer 
20 centims. long (by Elliott), belonging to a large tangent galvanometer. Part of a 
* This is a very simple formula to use, as we have 
y — i log -1 (log x — log «) — § log -1 3 (log x — log a). 
