16 Lovering and Bond on Magnetic Observations at Cambridge. 
whenever no remarkable disturbances are apprehended and the are 
of vibration is small; or within the same limitations as the other 
methods. If now the position of the magnetic meridian is desired 
for any definite moment, the first observation is made to precede 
this period by half the time of the bar’s vibration, and the second to 
follow the period at the same distance. ‘Thus, if ¢ be the time of 
vibration, and 7' the time of mean observation, the actual obserya- 
tion must be made at 7—23t and T+3¢. For greater accuracy, the 
final result is made to depend on several partial results as will be 
seen by an illustration, The time of vibration of the Gauss Mag- 
netometer used at Cambridge is about 54”. This is divided into as 
many parts as separate observations can be conveniently taken dur- 
ing that time. It has, therefore, been divided into 6 intervals 
of 9" each, and a separate observation is made at each interval. 
This is done during two vibrations of the needle, or 1’ 48’. By 
taking the mean of every two observations which have an interval 
of 54” we have a partial result for the middle time, and these partial 
results are combined so as to give a final result for any time when 
the declination is required. If this time is 2" 5’ the first observa- 
tion is made at 2" 4’ 6”, and repeated at intervals of 9” till 2° 5’ 54”. 
An example is given from the observations made June 26, at 0" 
of Gottingen mean time. 
Readings of the ‘Times corre- 
H. | M. s. Partial Results.|sponding to par- 
Scale. tial Results. 
23 | 59 6 108.750 
15 9.000 
24 9.500 B 
33 110.000 109.312 23 59! 33! 
42 10.125 9.062 42 |The Final Mean of the 7 partial re- 
51 10.000 9.250 51 sults gives 
0 9.875 9.375 00 0 109.294. 
9 9.125 9.312 9 
18 9.000 9.375 18 
27 | 8.750 9.375 27 
36 8.500 
45 8.750 
54 108.875 
Mean 109.294 0 0 
