Mathematics and General PJil^sIcs. 367 



These have been found not generally parallel with lines of equal 

 dip ; but in Ireland they are not greatly different from parallel. 

 At the south-east point of the county of VVaterford, the least 

 dip was observed ; the greatest was on the north-west point of 

 Mayo, or Donegal. In Limerick, the dip was 71°; in Dublin 

 it was less. It increased towards the north-west, in the direc- 

 tion of the west magnetic pole, 71 i°. In Armagh it was about 

 71*15i°. The author remarked the utihty of having magnetic 

 observations made at different stations. An interchange of 

 needles was made between Captain James Ross and Captain 

 Sabine, by which their respective observations would be made 

 to correspond. 



39. ]\Ir Hamilton on the Theorij of Varying Orbits. He 

 explained the general nature of orbits, under the influence of 

 perturbations, expressed by Lagrange as a varying ellipse. The 

 author's method is a modification of this, by different ellipses, 

 founded on a former general method, in a somewhat simplified 

 form. 



Mr M'CuUagh mentioned some investigations of his own up- 

 on the same subject ; and stated, v^erbally, a rule which he had 

 deduced. 



Professor Powel stated certain difficulties of calculation which 

 had occurred in his optical researches, with a view to elicit any 

 assistance which the members of the Association might be able 

 to give in the solution of them. 



33. Mr Kane on the Interference of Soiaid. He referred to 

 Herschel's suggestion of the distinction of sound by interference, 

 as analogous to the absorption of light. In trying the experi- 

 ment he found anomalies in certain cases. 



]Mr Addams remarked that the anomalies might easily be ex- 

 plained, by considering the actual conditions of the case. 



Mr Wheatstone made some remarks, and described experi- 

 ments which seemed to reconcile the anomalies alleged. 



Bb2 



