76 Lovering and Bond on Magnetic Observations at Cambridge. 
gle which the line 6 c makes with the magnetic meridian at b may 
be equal to 35° (theoretically it should be 35° 15° 52”.) Now, we 
are sure that the mean place of the declination instrument is not af- 
fected by the disturbance of ¢ upon 8, since it acts in the direction 
of the magnetic meridian. But the deviations from it will be af- 
fected by a constant error for which a correction must be applied. 
Again, the Horizontal Force instrument is practically affected only 
in regard to the divisions of the scale. The tangential part of the dis- 
turbance tends to move the needle from its transverse position but this 
is prevented by the suspension and enters unperceived into the cal- 
culation of the absolute value of the element. Again the angle which 
the line 6 d makes with the magnetic meridian is also 35°; so that 
b is affected by d in the same manner as by c. The action which 6 
would exert upon d, being in a horizontal direction, can have no in- 
fluence in deranging d, which admits only of motion in a vertical 
direction. 
These three instruments have been in adjustment two months, 
and regular observations were made with them on the Term-days 
of March and April, 1841; and will hereafter be continued. The 
same observer, by constant attention, is able to tend the three in- 
struments at once. Thus, if the time were 10 P. M. Gott. M. T., 
b would first be observed at 0"; then c at 2™ 30°, then 6 again at 5", 
then d at 7" 30°, then 6 a third time at 10", and c a second time at 
12™ 30°, ba fourth time at 15", and da second time at 17" 30°. In 
this way, which is continued during the whole period of observation, 
we have one observation of the declination every 5 minutes, and 
one of the Horizontal and Vertical forces every 10 minutes. On 
the March Term-day, which was the time when the transition was 
made from the Gauss to Lloyd’s Magnetometer, both declination 
instruments were observed in order to see how far they were com- 
