Lovering and Bond on Magnetic Observations at Cambridge. 19 
about this mean value.* Whenever observations have been made 
with the Gauss Magnetometer since June 26th, 1840, it has been 
the rule of the Observatory, recommended by its superior simplicity 
and freedom from all practical objections, to take 12 readings at 
intervals of 9”, commencing 50” before the real time, and to con- 
sider the mean of them as the final determination of position for 
that moment. Neither the Gauss method nor that of Cambridge 
which is based on it is practicable when the bar is agitated by unu- 
sual magnetic influences, as in seasons of violent disturbance, in 
consequence of the great extent of its motion. In such emergencies, 
the extreme of every excursion is recorded so long as this perturba- 
tion continues and an approximate time is obtained as exact as 
circumstances allow. The reduction is then made by this Formula 
4(a+2b+c) which has been already explained. After the mean 
results for every five minutes during the 24 hours of a day are 
obtained by any of these processes, they are used as the data for 
projecting a diurnal curve of magnetic declination. ‘Two lines are 
drawn upon a sheet of paper at right angles to each other and 
assumed as the axes of rectangular codrdinates. One of them is 
divided into 24 equal intervals each of which is subdivided into 
smaller parts according to the scale of the chart. ‘The other line 
is also divided in portions corresponding to degrees and minutes of 
arc. Any point that is most convenient may be selected as the 
origin of the codrdinates, and by considering the time as ordinate 
and the result of observation annexed to it as abscissa we obtain 
as many points of a daily curve as there are mean results of obser- 
vation. In ordinary Term-days the number is 288. When so 
many points are fixed upon the sheet they are connected by 
* The mean of 28 vibrations in April was 53.05; of 21 in July 53//.38 ; 
of 22 in September 53.45 ; of 12 in October 53.65. 
