64 



The plane of the instrument being made to coincide with 

 the magnetic meridian, and facing the East, the deflecting 

 magnet is to be fixed on its support at a given distance, 

 with its north pole toivarcls the needle ; and the angles of po- 

 sition of the deflected needle, a, and 02>— with its north pole 

 towards the north, and towards the south, respectively, — are to 

 be observed. The deflecting magnet is then to be reversed 

 on its supports, so as to have its north pole turned ^/'om the 

 needle, its distance being unchanged. Then 03 and a^ being 

 the corresponding angles of position, the magnetic inclination 

 is 



= i(ci + a2+«3+«4); 



and the angle of deflection is 



M = i(ai-«2 + a3-«4)- 



The observations are to be repeated, with the face of the in- 

 strument towards the West, and will give new values of Q 

 and u, which are be combined with the former. We have 

 only to observe that, in this latter case, the arithmetical mean 

 of the four observed angles is the supplement of the inclina- 

 tion, instead of the inclination itself. 



Dr. Allman exhibited and described a singular implement 

 discovered in an ancient copper mine in the parish of Skull, 

 Co. Cork. It consists of a tube formed of yew timber, gra- 

 dually increasing in diameter towards one end, and bent in 

 the manner of a siphon at an angle of about 80°, the point of 

 flexure being nearer to the narrower end. A slit nearly half 

 an inch in width extends for about the middle third of the con- 

 cave side through the thickness of the walls, and at the nar- 

 rower end are indications of wear, as if the implement had 

 been here fitted into a collar or tube of greater diameter. It 

 presents the following dimensions : 



