412 THIRD REPORT — 1833. 



other data towards the completion of the theory of this curious 

 subjectj the patient investigation of which cannot fail to give 

 us new and penetrating views into the constitution of the at- 

 mosphere. 



II. PHILOSOPHICAL INSTRUMENTS AND 

 MECHANICAL ARTS. 



On a peculiar Source of Error in Experiments with the_ DijJ- 

 ping Needle. By the Rev. William Scoresby, F.R.S. 



Certain discrepancies, at the time apparently unaccountable, 

 in observations made with a beautiful dipping-needle, by Dol- 

 lond, entrusted to Mr. Scoresby's care by the Board of Lon- 

 gitude, in an arctic voyage, led him, after a considerable interval 

 of time, to reflect upon the cause. Whatever might be the 

 apparent consistency of any particular series of observations 

 in the ordinary use of the instrument, the diflferences perceived 

 when the poles of the needle were changed indicated that 

 the preceding results were not accurate. But as the different 

 results thus obtained were capable of being combined for ob- 

 taining the true position of the needle, the formula of Professor 

 J. Tobias Meyer, given in his treatise De Usu accuratiori 

 Acus Inclinatorice Magneticce, was adopted for this purpose. 

 To verify the position thus obtained, another series of obser- 

 vations on the same spot was then made, with one of the arms 

 of the needle weighted, so as to render its position more de- 

 cisive from being the resultant of two forces, gravity and 

 magnetic attraction. The results, however, of the different 

 series were again anomalous ; but the cause of the discrepancies 

 thus observed not being fully apprehended at the time, the 

 consequence was that the observations were set aside for want 

 of consistency. 



Subsequently, however, it occurred to the author that the 

 cause of the discrepancies was to be found in the alteration of 

 the magnetic intensity of the needle when the polarity was 

 changed ; a circumstance furnishing a new element in these 

 calculations, not hitherto, he believes, taken into account. For 

 when the poles of the dipping needle are changed, (unless 

 magnetized with extraordinary care and some perseverance in 

 repeating the process,) the magnetic intensity of both positions 

 will not be the same. It is indeed a fact which the author fre- 

 quently observed, that the capacity of a propei'ly tempered 

 steel bar for the magnetic power (provided it have been kept a 

 considerable time in the same magnetic condition,) is the great- 



