Lovering and Bond on Magnetic Observations at Cambridge. 59 
has been previously found to be 38° 11’ west of the north, reads on 
the circle 
272° 23' 30! 
Adding its azimuth or : ; 38 11 
The azimuthal reading of the true meridian = 310° 34’ 30 
The azimuthal reading at the Coincidence 
of Needle é 5 F = 301oe 14’ 50 
Absolute variation for 102.966 of the scale == 9° 19/ 40 
OG “ for 100. =) 9) 22 38 
The absolute variation corresponding to 100 of the scale being 
known, the real values of all the lower numbers are found from it 
by adding and of all the higher numbers by subtracting the differ- 
ence between them and 100. Here we suppose of course that all 
the readings on the scale are made in the same position of the tel- 
escope as the one by which the original absolute variation was de- 
termined. ‘To secure this condition, when the observations begin, 
the azimuthal circle must be firmly clamped at some place which is 
considered the fixed reading for this period ; and the vernier should 
be occasionally examined to see that the instrument has not been 
deranged. The absolute declination thus obtained cannot be relied 
on within so small a limit of error as that to which the changes of de- 
clination are subject. The chief uncertainty attaches to the coinci- 
dence of the needle with the line of collimation. Several readings 
repeated in succession are likely to vary three or four minutes so that 
their mean is only an approximation to the truth. Hence the diffi- 
culty of ascertaining the yearly change of declination which is so 
small as to be partially masked under accidental errors. If the feet 
of the Variation-transit were firmly secured to some durable founda- 
tion, the yearly variation might be found at once from the scale as 
we now find the daily ones. In this case the fixed reading should 
