14 Lovering and Bond on Magnetic Observations at Cambridge. 
The observations on the Term-days with the Gauss Magnetome~- 
ter were begun at the Cambridge Observatory in March, 1840, 
and no one has been omitted since that time.* The curves for only 
two of these days, the 29th of May and the 21st of October, are 
published; but the numbers which express the mean results for 
every five minutes during the 24 hours on all the Term-days are 
placed in Table I, at the end of the Paper. This mode is pref- 
erable in some respects to publishing the curves, as it enables 
any one who intends to make a comparison, to draw a curve for 
Cambridge on the scale used in observations made at other 
places. In our observations, the time was obtained accurately 
from the Transit-instrument and astronomical clock; and the scale 
which was reflected from the mirror at the southern extremity of 
the bar, and read off by the Variation-Transit, easily allowed of 
being marked to + of a minute. The experience of the observers 
has satisfied them that dependence may be placed upon each sep- 
arate reading within that limit of error. But other considerations 
make it necessary to determine the position of the Magnetometer for 
any assigned time by more than a single reading. As the bar in 
passing from one angle of declination to another is maintained al- 
ways in a vibratory state, it is necessary to eliminate what is due to 
the oscillation from what belongs to an absolute change of declina- 
tion. If the arc of vibration were constant, it would be eliminated 
by observing the limits of excursion of the magnetized bar and 
taking the mean between them. But the natural tendency of the 
* As the Report of the Royal Society containing the details of their plan had 
not been received when the observations commenced, two of the Term-days 
were incorrectly taken, and the observations in April, besides being undertak- 
en on the wrong day, were so imperfect that they have been left out of the 
account entirely. Instead of March 18, March 27, and instead of July 22, 
July 24 were observed at Cambridge. This must be considered in the com- 
parison of these days with observations made elsewhere. 
