No. 7.] HORIZONTAL INTENSITY. 77 
log = has not exactly the same value when calculated by C and ye for 
distance e, as for distance EH. The differences are, however, very small. 
Calling a, calculated for distance e, n,, and for distance EH, 7,, I have put: 
C_ 1 
lu 2 
With this mean value, and the values of « found in the above table, I have 
then calculated log Cu according to the formula 
log Cu = log ents 4 Blog u. 
THE OBSERVATIONS AND THEIR REDUCTION. 
The following tables give, in chronological order, all the series of deflec- 
tion and vibration observations made during the expedition, and the calcu- 
lation, according to the formule on page 64, of the absolute value of the 
horizontal intensity from each separate series. 
OBSERVATIONS OF DEFLECTION. 
Under the date of each observation, the latitude and longitude of the 
place is given, the designation of the magnets used, and an indication of the 
distance e or E, at which the deflector was placed. These are followed by 
the readings on the horizontal circle, corresponding to the 4 positions of the 
deflector, each value of u,, #., #, and w, being the mean of the readings 
on the two verniers. In several cases, 2, 3, or even 4 and 5 settings have 
been made, and readings taken, with the position of the deflector unaltered. 
A statement of this is made every time in the notes below the table. The 
small letters, a, b, c, d, placed before each value of u indicate the order in 
which the settings were made. 
