NO. 7.] HORIZONTAL INTENSITY. 77 



n 

 log has not exactly the same value when calculated by C and /tt for 



distance e, as for distance E. The differences are, however, very small. 



Q 



Calling -, calculated for distance e, i) t , and for distance E, i] s , I have put: 



? 2 



With this mean value, and the values of p found in the above table, I have 

 then calculated log Cfi according to the formula 



log 0,* = log -^ + 2 log p. 



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 formulae 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 MI, u 2 , u 3 and t 4 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. 



