62 



OBSERVATIONS TO DETERMINE THE MAGNETIC INTENSITY 



dingly distinguished by the names of Hansteen, Sabine, and Henry. They were 

 enclosed in a small cylindrical box of wood, supported by levelling screws, and 

 having a glass tube fitted to the top, from which the needles were suspended by 

 a few filaments of the silkworm's thread. At the bottom of the box was a di- 

 vided circle for the purpose of noting the arc of vibration, and the temperature 

 was shown by an enclosed thermometer. The bottom of the box being ren- 

 dered horizontal, and the needle properly placed in the stirrup, it was drawn 

 aside from the magnetic meridian by bringing near it another needle. The 

 registry of the oscillations was commenced when the half arc of vibration was 

 reduced to 30°, and continued to 320 oscillations, the instant of the comple- 

 tion of every tenth vibration being noted. The amplitude of the final arc was 

 generally recorded, being about five degrees. Five intervals of time were thus 

 obtained, each corresponding to 280 vibrations, namely, the interval between the 

 0th, and 280th vibration, between the 10th, and 190th, etc., and between the 

 40th and 320th; and the mean of these is taken as the result. 



At Dorchester, Princeton, and Philadelphia, the times were noted by a chro- 

 nometer. At the other stations, a lever watch was used. At Hudson, the watch 

 was compared with the Observatory clock, immediately before and after the 

 observations. At the remaining stations there is a little uncertainty with 

 regard to the time, yet, it is thought, its influence upon the results will not be 

 great. 



No correction has been applied for the arc of vibration. In order to deter- 

 mine the correction for temperature, the apparatus was placed upon a large 

 earthen plate, covered by a bell glass, alternately heated from below by a lamp, 

 and surrounded by a freezing mixture. The usual mode of observation was 

 employed, and the results are shown in the following table, the first column of 

 which indicates the time of commencement of each series of observations. 



Date.; 



Needle. 



Time of 280 

 Vibrations. 



Temp. 



Dale. 



Needle. 



Time of 280 

 Vibrations. 



Temp. 



1839, 



Dec. 



11, I'' 32"" 



P.M. 



Sabine 



732^56 



30°.8 



1840, 



Jan. 



4, l^ 28"" 



M. P. 



Sabine 



732^06 



17°.9 



ii 



(( 



1 51 





a 



732.10 



28.1 







1 45 



(( 



a 



731 .79 



17 .1 



" 



(( 



2 41 





Hansteen 



859.36 



29.6 







2 32 



<( 



Hansteen 



858 .47 



19 .9 



" 



(( 



2 59 





u 



858 .42 



26.5 







2 53 



(( 



(( 



859.14 



15 .7 



a 



(( 



3 41 





Henry 



592 .36 



34.0 







3 47 



ti 



Henrv 



591.31 



21 .7 



tt 



n 



3 55 





(( 



592 .20 



30.4 







4 3 



a 



n 



593 .66 



17 .9 



11 



«( 



4 44 



(( 



i( 



595.18 



89.4 







4 45 



i( 



a 



597 .26 



85 .0 



(( 



11 



4 59 





i( 



595 .64 



90.9 







5 1 



(( 



a 



598 .38 



85 .0 



(( 



<( 



7 17 





Hansteen 



869 .93 



91 .0 







7 



(( 



Hansteen 



870.10 



84 .3 



" 



t( 



7 38 





(( 



868 .29 



91 .1 







7 20 



(( 



<( 



870.31 



88 .8 



(( 



(( 



8 9 





Sabine 



745 .87 



84.2 







7 53 



(( 



Sabine 



748.11 



89 .4 



(( 



(( 



8 26 





(( 



747 .31 



85.0 







8 11 



i( 



u 



746.83 



84.0 



