AT SEVERAL PLACES IN THE UNITED STATES. 



67 



The results with needle No. 1 are quite satisfactory, the extreme range of 

 the values of the dip from observations in different azimuths being 4'. 2. With 

 needle No. 2 the extreme range is 12'.7. This discordance is ascribed to slight 

 rust which has formed upon one of the axles, but which is barely discernible 

 to the naked eye. The mean of the preceding 2720 readings with both nee- 

 dles is 72° 50'.2; the mean of the observations in the meridian is 72° 49'. 6. 

 Difference 0'.6. In these observations of azimuth is intended to indicate 

 the magnetic meridian. The dip may then be deduced by the formula 

 cot. h = cot. i. sec. 6. The following table gives the result of this comparison ; 



Azimuth. 



Needle No. 1. Needle No. 2. 



A A 



Inclination. 



Dip. 



Inclination. 



Dip. 







72° 45'.6 



72° 45'. 6 



72° 53'. 5 



72° 53'. 5 



10 



73 1 .7 



" 46 .9 



73 8.4 



" 53 .7 



20 



73 44 .9 



" 45 .9 



73 50 .9 



" 52 .2 



30 



74 57 .5 



" 45 .6 



75 0.4 



" 48.9 



40 



76 39 .8 



" 48 .2 



76 45.8 



" 55.8 



50 



78 46 .3 



" 50 .2 



78 52 .4 



" 59.2 



60 



81 17.2 



" 57.6 



81 7.6 



" 39 .7 



70 



84 0.0 



" 55.0 



84 3.6 



" 64 .8 



80 



86 49.2 



" 15.4 



86 55.1 



" 46.3 



Mean Dip, 



72° 45 '.6 



Mean Dip, 



72° 52', 7 



*That the dips obtained by this method should not perfectly accord with each 

 other will not appear strange when it is considered that an error of one minute 

 in the observed azimuth at eighty degrees causes an error of nearly two mi- 

 nutes in the computed dip; and an error of one minute in the observed incli- 

 nation causes an error of more than five minutes in the computed dip. The 

 mean result with the two needles by the last method is 72° 49'. 1; by the former 

 method of combination, 72° 50'. 2; mean of the two methods, 72° 49'. 6, which 

 accords perfectly with the result of observations in the meridian. 



The part of this paper which follows, was read November 20, 1840, 



