Declinometer. 
XXI 
March 29. 1843. A small magnet, intended to be placed in the brass bar to 
facilitate the determination of the plane of detorsion, but which was never used, 
was found in the writing-desk ; the latter occupied a position to the east of the 
reading telescope of the declinometer, except on term-days, when, for convenience, 
it was moved to a position nearly midway between the declination and balance mag- 
netometers. In the usual position of the desk, the greatest effect of the small mag- 
net, on the reading of the declination magnet, might be from — 0'-2 to + 0'*2 ; and 
during term-days from + l'*l to — l'*l. It is probable that the magnet remained 
in the same position in the desk, and, therefore, the effect would be constant for 
each position of the desk. 
The large copper stove, occupying the position S in the plan, was removed 
early in 1844. Its effect was tried on the balance needle, by approaching and re- 
moving it, and was found to be nothing. 
19. From Nos. (11) and (13) we find the following corrections to be applied to 
the scale readings at the magnetic axis of the declination magnet, in order to obtain 
the readings which shall be used as the zeros of the scales : — 
Reading of the long scale at the magnetic axis, 257" 14 
Correction for the effect of the bifilar and balance magnet (11), + 0*16 
Correction for the effect of the copper ring or damper (13), — 100 
Adopted zero for the long scale, 256-30 
Reading of the short scale at the magnetic axis, 147'H 
Correction for the effect of the bifilar and balance magnet (11), + 0-14 
Correction for the effect of the copper ring or damper (13), — 0-90 
Adopted zero for the short scale, 146-35 
20. Time of vibration of the declination magnet with the long scale. 
The mean of 4 series of observations, given in the Introduction for 1841-2 (22), 
gives one vibration in 17 s, 84. The following series were made in 1843 : — 
d. h. s. 
Feb. 28 0. 32 vibrations, the observations made at the extremities of the arcs, give a mean of 17-84 
40 middle 17-84 
March 1 32 extremities 17-81 
32 middle 17-81 
April 17 3. 22 extremities 17-93 
The last series was not considered good ; the mean of the first ten vibrations of 
the series gives 17 3- 85. 
21. The observations of the declinometer are made in the following manner : — 
The points of the scale which coincide with the vertical wire of the reading tele- 
MAG. AND MET. OBS. 1843. / 
