Mag7ietic Dip in the United States. 



89 



parison of the observations eighty eight vahies of the same ele- 

 ment, the accordance of which with each other will enable us to 

 judge of the confidence which may be placed in a single obser- 

 vation. It will be seen that the dip is nsaally the greatest when 

 the face of the instrument is east. Subtract then the dip ob- 

 served with the face west from that found with the face east 

 with the same position of the needle. For example, in the first 

 observation 72° 47' from 72° 5' gives - 42', and so of the rest. 

 We thus obtain the following table, which exhibits the observed 

 values of twice the zero error. 



The mean of all these observations is +4'. 5, which may be 

 taken as equal to twice the zero error. The difi"erence between 

 this and the preceding observations will show the errors of the 

 observations, which when classified are as follows : 



By far the greatest error here is — 46'.5, which was obtained 

 from the first observation. The difference between the readings 

 with the face of the instrument and needle both east, and that 

 with the former west and the latter east, instead of being — 42', 

 should be +4'.5. This does not inform us which reading is most 

 in error ; if, however, we apply the correction to 72° 5', making 

 it read 72° 5r.5, the resulting dip would be 71° 54'. 1, corre- 

 sponding nearly with the other observations at the same place. 



Vol. XL, No. 1.— Oct.-Dec. 1840. 12 



