AT SEVERAL PLACES IN THE UNITED STATES. 



63 



The mean of the preceding observations furnish, 



^'■"^ °^ Temn 



Vibration. ^^™P- 



Hansteen, 869'. 64 88°. 8 



Sabine, 747 .03 85 .6 



Henry, 596 .61 87 .6 



Time of 

 Vibration. 



858^85 



732 .13 



592 .38 



Temp. 

 22°.9 

 23 .5 

 26 .0 



Then, by the usual formula a 

 For Hansteen's needle, a = 



T—T 



10.79 



858.85 X 65.9 



we obtain: — 



.000191. 



For Sabine's needle, a = 



For Henry's needle, a 



14.9 



732.13x62.1 



= .000328. 



4.23 



592.38 X 61 6 



= .000116. 



The standard temperature, to which the following results are reduced, is 60° 

 Fahrenheit. No correction is applied for the diurnal variation of intensity, but 

 the hours of observation are always stated. To test the permanency of the 

 magnetism of the needles, I have been furnished, by Prof. Renwick, with two 

 series of observations, made at New York, besides those made in September, 

 1839. The results are as follows : 



Needle. 



Date. 



iTime of 280 

 1 Vibrations. 



Temp. 



Corrected 

 Time. 



Hansteen 



1838, June 22, 0'' 6" 



P.M. 



873^60 



82°. 



869^94 



n 



1839, Sept. 9, 11 13 



A.M. 



869 .40 



86 .0 



865 .09 



(( 



1840, June 6, 4 



P.M. 



943 .60 



80 .2 



939 .97 



Sabine 



1838, June 21, 5 23 



(( 



744 .35 



77 .0 



740.21 



i( 



1839, Sept. 9, 10 48 



A.M. 



744 .34 



85 .2 



738 .24 



tc 



1840, .Tune 6, 10 23 



a 



740 .00 



78 .8 



735 .44 



Henry- 



1838, June 25, 10 21 



<( 



590 .80 



77 .0 



589 .64 



it 



1839, Sept. 9, 11 38 



(I 



596 .72 



86 .2 



594 .91 



t( 



1840, June 6, 11 26 



C( 



598 .20 



79 .0 



596 .88 



In Sabine's needle, the time of vibration continually diminished, and in 

 Henry's increased ; indicating, in the former case, a slight increase of magnetic 

 force, and in the latter a diminution. The inequality, however, does not much, 

 if at all, exceed the irregular fluctuations of intensity which may be observed 

 at a single station, within a moderate interval; and as the variation indicated 

 in the two needles are opposite in kind, and will consequently, in part, balance 

 each other in taking the mean, the magnetism of both is regarded as invariable. 

 In Hansteen's needle there is a striking increase in the time of vibration 



