90 Magnetic Dip in the United States, 



The average of all the preceding errors is ±5'. 5, which may be 

 taken as the probable error of a single reading entirely independ- 

 ent of instrumental errors, and the error frequently amounts to 

 about a quarter of a degree. What then are we to understand 

 by this result? Simply this, that if the instrument be properly 

 adjusted, and a number of different readings be made in the same 

 position of the instrument and needle, the needle each time being 

 raised from its supports and allowed to come to a state of rest, 

 the readings will not be identical. They will frequently differ 

 ±15' from the mean, and on an average ±5'. 5. This is the con- 

 clusion derivable from Prof Locke's observations, and the result 

 I presume coincides substantially with the experience of all who 

 have undertaken similar observations. My attention has been 

 particularly directed to this very annoying and almost disheart- 

 ening anomaly, and it has appeared to me that when the agate 

 supports and the axis of the needle are carefully wiped clean of 

 moisture and dust, the discordance of the readings arises mainly 

 from the needle's slipping upon the agates to the east or west ; 

 and that when the y''s which elevate the needle are so disposed 

 as to allow the least possible motion in that direction, the accord- 

 ance of the readings is the best. 



This uncertainty in the readings is of itself sufficient to entitle 

 the dipping needle to the character of ' one very ungrateful in- 

 strument.' Most of the other errors may be corrected by suita- 

 ble precautions and reversals ; but this cannot be thus annihilated, 

 and the only remedy with which I am acquainted is to multiply 

 observations. I am accustomed to make five observations in each 

 position of the needle and instrument, always reading at both 

 poles. I thus obtain eighty readings with each needle. 



Another error, and one which equally affects both needles, 

 arises from observations being made out of the meridian. At 

 Hudson, the dip increases less than one minute from being ob- 

 served two degrees out of the magnetic meridian. Where one 

 has leisure therefore to determine the magnetic meridian with ac- 

 curacy, this error may be pronounced insensible ; but on a tour 

 where observations are usually hurried, the error from this source 

 may become important. 



Another source of error is found in the imperfection of the 

 axles of the needles. It has long been known that different in- 

 struments would give different values of the dip at the same time 



