148 



Dr. T. E. Thorpe. 



Table VI A. 



Station. 



Date. 



Time. 



Circle 

 reading. 



Declination, 

 correct for 

 torsion. 









h. 



m. 



h. 



m. 

















July 



18 



8 



22- 



- 8 



47 A.M. 



291 



20 



38 



8 



40 



34 W. 







6 



11- 



- 6 



30 p.m. 



291 



23 



07 



8 



41 



55 „ 







29 



4 



55- 



- 5 



20 „ 



294 



06 



56 



14 



03 



55 E. 





Aug. 



8 



12 



17- 



-12 



34 „ 



299 



08 



57 



14 



43 



25 „ 





5) 



10 



10 



41- 



-10 



44 A.M 



230 



17 



27 



15 



33 



11 „ 



Salt Lake City . . . 



)> 



15 



8 



22- 



- 8 



40 „ 



305 



20 



59 



16 



48 



06 „ 



G-reen River .... 





26 



7 



41- 



- 8 



03 „ 



288 



57 



09 



16 



57 



49 „ 



Grand Island .... 



)J 



28 



10 



35- 



-10 



53 „ 



314 



19 



15 



12 



51 



39 „ 



Council Bluffs. . . . 



>5 



30 



7 



00- 



- 7 



19 „ 



316 



38 



58 



10 



39 



42 „ 





Sept. 



2 



10 



27- 



-10 



45 „ 



300 



40 



51 



4 



33 



05 „ 



Allegheny 



5 



10 



26- 



-10 



48 „ 



117 



59 



51 



2 



22 



46 W. 



Washington 



5> 



5 



4 



52- 



- 5 



08 „ 



117 



49 



44 



2 



20 



24 „ 



J) 



8 



10 



08- 



-10 



33 „ 



111 



36 



20 



3 



42 



51 „ 



I have not attempted to correct these results for diurnal range. 

 Accurate observations of daily change have been made at Philadelphia, 

 Washington, and Key West in Florida, and on the assumption that 

 the range of the daily variation is inversely proportional to the hori- 

 zontal force, the data derived from these observations may perhaps be 

 considered as fairly applicable to any place in the United States. But 

 such corrections are at best very unsatisfactory, as the epochs of the 

 extreme positions are subject to very great fluctuations on account of 

 disturbances : these are usually most prevalent at about the time at 

 which my observations were made, viz., August and September. The 

 magnet, however, was always quiet during the time of observation ; 

 hence the correction due to diurnal range would seldom exceed 3' or 4'. 



Observations of declination have been made in the district of 

 Columbia, in which Washington is situated, from as far back as 1792. 

 These have been collected and discussed by Mr. Schott in the memoir 

 above referred to, and represented by him by the formula — 



(I.) D = l°-79-r-l°-90 sin (l-5»-24°-l), 



in which n is the number of years since 1830, and D the declination, 

 + when west, expressed in degrees. 

 The annual variation v was given by the formula — 



(II.) v=2'-985 sin (1-5^ + 66°). 



The preceding formula gives the value of the declination for 1870 as 

 2° 54', and for 1880 as 3° 16'. The observed declination, however, 

 already considerably exceeds the latter value. Mr. Schott informs me 

 that an observation made on June 15, 1878, gave 3° 47' ; my own 



