A Magnetic Survey of the Fortieth Parallel. 



141 



The times of vibration given in the table are, in all cases, obtained 

 from the means of 14 observations of the time reqnired by the magnet 

 to make 100 vibrations : no correction has been applied for semi-arc 

 of vibration which at the commencement of the observations was 

 always below 50' ; at two stations only, viz., at Salt Lake City and at 

 Council Bluffs — where the rate of the chronometer exceeded 3 seconds, 



was it necessary to apply the correction 1 The observed 



time of vibration has been also duly corrected for the torsion-force of 

 the suspending thread. The angular value of 1 division of the scale 

 of the vibration magnet is 2' 12" : that of the deflection apparatus 

 is 60"-5. 



The mean value of P in the formula V2 L= — (l— J—\ was —0*002357. 



X X' \ r 3 / 



Tables III and IIIa contain the details of the several observations. 



Table III. — Deflection Observations. 



Station. 



G. M. Time. 



Distances 

 of 



Centres of 

 Magnets. 



Tempe- 

 rature. 



Observed 

 Deflection. 



Log-. 





d. h. m. 



foot. 



o 



o 











New Haven .... 



July 17 11 14 A.M. 



10 



77-7 



L6 



42 



26 



9 



•16182 





11 58 „ 



1-3 



78-0 



7 



30 



55 



9 



16120 





„ 18 4 48 p.m. 



1-0 



99-8 



1(3 



36 



07-5 



9 



16136 





5 28 „ 



1 3 



100-0 



7 



28 



53 



9 



•16146 





„ 25 9 14 a.m. 



1 -o 



93 -0 



L2 



03 



34 



9 



•02471 





9 54 „ 



1 3 



93 -6 



5 



27 



51 



9 



•02492 



Manitou 



Aug. 6 7 05 a.m. 



1 -o 



86-4 



12 



33 



47 



9 



•04109 





7 46 „ 



1-3 



86 -0 



5 



40 



40 



9 



04113 





„ 8 11 44 a.m. 



10 



87-7 



12 



45 



52*5 



9 



•04847 





12 14 „ 



1 3 



87 '9 



5 



46 



52 



9 



•04875 



Salt Lake City .. 



„ 14 10 06 a.m. 



1-0 



94-9 



12 



39 



57-5 



9 



•04590 



10 42 „ 



1-3 



94-4 



5 



44 



13 



9 



•04609 





„ 24 11 16 a.m. 



1 -o 



90 -0 



12 



47 



36 



9 



•04967 





11 48 „ 



1 -3 



89 -4 



5 



47 



53 5 



9 



•05021 



Green River .... 



„ 26 5 39 p.m. 



1-0 



87-2 



13 



12 



05 *5 



9 



•06278 





6 03 „ 



1-3 



87'0 



5 



58 



16 



9 



•06263 



Grand Island . . 



„ 28 8 59 a.m. 



1 



83 -6 



13 



55 



31 



9 



•08518 





9 36 „ 



13 



83-8 



6 



17 



39'5 



9 



•08513 



Council Bluffs . . 



„ 30 5 30 a.m. 



1 -o 



84-9 



14 



33 



28 



9 



•10420 





5 57 „ 



1 3 



84-4 



6 



34 



36 



9 



•10416 





Sept. 2 8 50 a.m. 



1-0 



84-0 



15 



39 



30 



9 



•13500 





9 27 „ 



1 3 



83 5 



7 



03 



45 



9 



•13490 





5 8 09 „ 



1-0 



78-4 



15 



25 



41 



9 



•12820 





8 40 „ 



13 



79-0 



6 



57 



56-5 



9 



•12849 



Washington .... 



8 7 20 „ 



10 



77-9 



14 



36 



11 



9 



•10483 





V53 „ 



13 



78-1 



6 



35 



59 



9 



•10505 



