Lovering and Bond on Magnetic Observations at Cambridge. 17 
Thus it appears that each position of the needle is determined 
from 13 separate observations; and as each reading is to? of a 
minute the mean of all may be considered as within a smaller error 
of observation, and only subject to the exception that the law of 
reduction is not rigorously exact when the change of declination 
during 1’ 48” is not uniform. If the arc of vibration be so large as 
to have a sensible decrease the effect is cancelled when the read- 
ings extend through twice the time of vibration. On Term-days 
this process is repeated every 5’ during the 24 hours, so that 
3,744 observations are made which give 288 mean _ positions. 
This was the rule of the Observatory till June 26th, 1840, when 
a slight modification was introduced which diminished the labor 
of observation and reduction without compromising the accuracy 
of the result. Thus the observation of June 26th, 3" 40’, P. M.* 
which was the first one made in this way, stands thus; it should 
be remembered that the column of figures to the right of the point 
are not tenths but eighths. 
Readings of the Times corre- 
H. | M.} S. Partial Results. sponding to Par- 
Scale. tial Resulis 
3 | 39] 10 196.3 
19 105.7 
28 105.3 
37 105.2 106. 39! 37! 
46 105.1 105.937 46 
55 105.2 105.812 55 
4 105.5 105.875 40 4 The Final Mean gives 105.854 as 
13 106.0 105.750 13 the number on the scale corre- 
22 106.2 105.750 22 sponding to the magnetic meri- 
5 106.4 dian at 35 39/ 59/15, 
40 106.3 
49 106.2 
Means. | 105.304 105.854 | 397 597.5 
By this method, which is the same in principle as the other, only 
12 observations are made, and the mean of them is the same as the 
* Gottingen Mean Time is to be understood wherever it is not otherwise stated. 
2* 
