72 Lovering and Bond on Magnetic Observations at Cambridge. 
ings of the scale to absolute values. It is only necessary to find 
the azimuth of the line of collimation of the telescope by a Varia- 
tion-transit, and then adding or subtracting the difference between 
the former and a fixed point of the scale, determined by observation, 
we readily convert every observation into absolute declination. In 
this respect the instrument of Lloyd is preferable to the Gauss Mag- 
netometer, where the passage from marks on the scale to their corre- 
sponding absolute values is difficult and uncertain. A more minute 
description of Lloyd’s apparatus may be found in the Report of the 
Royal Society to which we have made frequent reference. It was 
first proposed that the place of the bar should be marked at three 
successive excursions and the position determined by the familiar 
formula + (a + 2b +c); where a, 6 and ¢ express the three read- 
ings. But a circular was issued to the Magnetic Observatories, un- 
der date of January 15, 1841, and signed by Professor Lloyd, re- 
commending that the method of observing the Declination and Hor- 
izontal Intensity Magnetometers be so far modified as that, instead 
of the three successive readings just mentioned, they should be 
taken at the times T—?, T, and T'+4, of which T is the appointed 
epoch of observation and ¢ the mean time of vibration of the magnet. 
This is returning essentially to the principle of Gauss, which gives 
the position of the bar for the precise moment required, and which 
we should think to be generally superior, whenever the length of the 
arc of vibration admits of it, to the method of observing three suc- 
cessive excursions. To recur now to the Plate; b is the place 
of the Declination Magnetometer, g and g windows to allow the 
light to pass in so as to be reflected strongly on the scale ; e is the 
place of the fixed telescope ; @ is a pillar which supports the Varia- 
tion-transit. This instrument can be adjusted in the meridian by in- 
dependent observations of the stars, through a section in the roof at 
