Magnetic Dip in the United States. 91 



and place. This fact is strikingly exhibited in the observations 

 by Captain Ross, contained in the fifth report of the British As- 

 sociation. The dip at London, as given by eight different nee- 

 dles, was as follows : 



69° V.5 69° 18'.9 



6'. 3 19'. 6 



1K3 2K8 



16M 42'.6 



Here we have a difference of 41' in the results of two of the 

 needles. This discordance was satisfactorily traced to the im- 

 perfection of the axles, and its effect may be in a measure cor- 

 rected by making the axle turn in the needle, thus enabling the 

 points of the circumference of the axle in contact with the sup- 

 porting planes to be varied in successive trials ; or it may be cor- 

 rected by observations in different azimuths. The dip may be 

 deduced from the angles of inclination observed in any two azi- 

 muths 90° apart from each other, by the formula cot.2^ = cot.-«-{- 

 coi.^i' ] or it may be derived from the formula cot.(J= cot. i sec. 6. 

 Without some such trial or comparison with a standard instru- 

 ment, no needle can be certainly relied upon. I have made this 

 trial with my instrument, observing in every 10° of azimuth in 

 the usual manner. Thus one thousand three hundred and sixty 

 readings were made with each needle. I have made in all about 

 four thousand readings to determine the magnetic dip at Hudson, 

 and after all should not dare to use any stronger language than 

 Prof, Locke employs respecting his own results derived from six- 

 teen readings, that they "are accurate within at least two or 

 three minutes of a degree." 



Other errors arise from the presence of magnetism, as for exam- 

 ple, in the instrument itself, iron about the person of the observer, 

 which may sometimes inadvertently happen with the most cau- 

 tious, loose iron lying unperceived in the vicinity, etc. ; and 

 finally, local attraction sometimes causes the dip at a given place 

 to differ from that due to the geographical position by several 

 degrees. This will be especially noticeable in the vicinity of 

 iron mines, basaltic rocks, etc. 



From the preceding remarks, I think it will be seen, that in 

 magnetic observations we are not to look for the precision of as- 

 tronomy. We have not sufficient data for estimating ihe proba- 

 ble error of one of Prof. Locke's results ; yet I should not hesi- 



