MAGNETIC OBSERVATIONS. 



203 



of the table give the place where, and the date when, each observation was made. 

 The third and fourth columns give respectively the observed deflections when the 

 north ends of the deflecting magnets were directed towards the west and towards 

 the cast; the distance of their centres from the centre of the compass needle being 

 in both cases eleven inches. The fifth column gives the mean of the four observed 

 deflections recorded in the third and fourth columns. The sixth, seventh, and 

 eighth columns contain, in precisely the same manner, the observed deflections, and 

 their mean, when the centres of the deflecting magnets were at a distance of fifteen 

 inches from the centre of the compass needle. Now, let r be the distance, 

 expressed in feet, between the centres of the deflecting magnets and the centre 

 of the compass needle ; v, the observed angle of deflection given for each value of 

 r in the column headed "mean"; in, the combined magnetic moment of the two 

 deflecting magnets; and II, the earth's horizontal force at the place where the 

 observation was made, taken from the table on page 01. Then we shall have 



tan u 



// 



and the ninth column contains the mean of the two values of lo< 



11 



computed 



respectively from the angles of deflection observed with ;•= 11 inches =0.917 

 foot, and » , = 15 inches = 1.250 feet. The tenth column contains the value of 



log. in, found by adding to log. the known value of log. H. 





Date. 











Detlec 



tions 













1 '" 

 L01;. — • 





Station. 





r- 



= 1 1 



inches. 







- 



=."5 



inches. 





Log. m. 





West. 



East. 



Mean. 



West. 



East. 



Mean. 







Oct. 30, 1S65 



1 9 



30' 



22° 



40' 







14° 



30' 



17 



30' 















19 







22 



20 



20 



52' 



>4 



20 



17 



40 



*i6° 



0' 



9.1617 



98344 



St. Thomas .... 



Nov. 13, 1865 



15 



20 



'4 



5° 







4 



20 



6 



40 















15 



3° 



14 



40 



15 



5 



4 



3° 



6 



40 



5 



V- 



8.9961 



9.8251 



Salute Islands . 



Nov. 2S, 1S65 



H 



35 



'5 











5 



20 



5 



20 















14 



35 



15 



5 



14 



49 



4 



55 



5 



20 



5 



'4 



S.9799 



9.8079 



Bahia 



Dec. 27, 1865 



15 



40 



16 



10 











10 



5 



30 















16 



40 



16 



10 



16 



10 



5 



40 



5 



3° 



5 



4? 



9.01S4 



9.S10S 



Rio Janeiro .• . 



Jan. 6, 1S66 



17 







17 











6 



40 



6 

















17 







17 



10 



17 



2 



6 







6 







6 



10 



9.0476 



9.S216 





Jnn IS, 1866 



16 



40 



16 



40 







6 



20 



5 



30 















17 







16 



40 



16 



45 



6 



10 



5 



3° 



5 



52 



9.0328 



9.8130 



Sandy Point 



Feb. 7, 1S66 



16 



3° 



16 



20 







5 



40 



6 



40 















16 



40 



16 



20 



16 



27 



6 







6 



3° 



6 



12 



9.040S 



9.8270 



Valparaiso .... 



March 2, 1S66 



17 







15 











7 



20 



5 



















16 



40 



14 



40 



>5 



5° 



7 



3° 



5 







6 



12 



9.0320 



9.8326 



Valparaiso .... 



April 7, 1S66 



14 



14 



40 

 3° 



17 

 17 



40 

 3° 



16 



5 



4 

 4 



3° 

 20 



7 

 7 



3° 

 40 



6 



O 



9.0284 



9.S290 





April 26, 1866 



14 



3° 



14 



3° 







5 



20 



5 



10 















H 



3° 



14 



3° 



H 



3° 



5 



10 



5 



3° 



5 



l8 



8.9777 



9.S222 



Panama 



May 14, 1866 



12 

 >3 



5° 

 10 



>3 



'3 



3° 

 3° 



13 



>5 



4 



4 



30 

 40 



5 



5 



20 

 



4 



52 



8.9387 



9.8195 



Acapulco .... 



May 30, 1S66 



12 



3° 



40 



12 

 12 



20 

 10 



12 



25 



4 



5 



40 

 3° 



4 

 4 



3° 

 40 



4 



5° 



8.9227 



9.8107 



San Francisco . 



June 26, 1866 



17 

 iS 



40 

 



17 

 16 







40 



>7 



20 



7 



7 





 10 



6 

 6 



10 

 3° 



6 



42 



9.069S 



9.8208 



* In this observation r= 12 inches. 



