58 Lovering and Bond on Magnetic Observations at Cambridge. 
is placed firmly on the table and then the line of collimation is adjust- 
ed in the meridian by means of Polaris and the large ‘Transit- 
instrument ; a section having been made in the roof for this purpose. 
The azimuthal circle is then read off. The circle is now turned 
until the line of collimation coincides with the direction of the mag- 
netic meridian, as indicated by the needle that accompanies the 
Variation-transit. At the same moment the circle is read off 
again and the scale is noted through the telescope. ‘The difference 
of readings from the azimuthal circle will indicate the angle which 
the magnet makes at that time with the astronomical meridian, and 
the reading from the scale shows to what number on it this absolute 
variation corresponds. Now since according to their arrangement 
on the scale at Cambridge an increase of numbers implies a de- 
crease of declination, we readily find the absolute declination of 100 
of the scale by adding or subtracting as the case requires the 
difference between 100 and the reading at the time. This will 
be easily understood from the following example : 
The azimuthal reading by Polaris, June 21, at 8 P. M. 
Gott. M. T. - = = - - = 310° 34! 40". 
The azimuthal reading at the Coincidence of the 
Needle - : = - = - = 3801s 15! 30". 
When the reading of the scale through the telescope 
was 100,835, the absolute declination - = SSSI iG 
The absolute declination at 100 of the scale - = 9° 20/00” 
As it may not always be possible to take an astronomical observa- 
tion on account of the state of the atmosphere, the azimuthal angle 
between some fixed mark and the true meridian is read off and the 
position of the magnetic meridian determined by reference to this. 
Thus it appears, June 25th, that a certain mark on Gore Hall, which 
