128 



TRANS. ST. LOUIS ACAD. SCIENCE. 



three exceptions — to retain the same arrangement of screws. 

 This is further evidenced by the satisfactory agreement of obser. 

 vations repeated at the same place. The precautions necessary 

 for the elimination of these errors are sufficiently evident. 



HORIZONTAL INTENSITY. 



All determinations were made under a tent, which was so 

 arranged that the wind could always be prevented from striking 

 the instrument. 



The determinations at Iowa City were made with Declinome- 

 ter No. 3 of the Coast Survey, and magnets Cg and C^^. All 

 subsequent work was done with a new magnetometer belonging 

 to Washington University. Magnets Cg and C^ 7 with the old 

 stirrup were, however, used for this work. 



At each station an oscillation series was made with Cg. The 

 experiments were conducted substantially as in 1878. The oscil- 

 lations were called by myself, and Mr. Shaub noted the time by 

 means of the watch (Jiirgensen 10890). In this way it was found 

 we could estimate tenths of a second with considerable precision. 

 It is scarcely necessary to say that all precautions were taken to 

 secure an unbiassed reading of the watch. 



The following will serve as a specimen determination : 



Jeff'crso7i City^ ■^^'-g- !-• 



