TRANSACTIONS OP THE SECTIONS. 557 



city could be traced. But in felspar, epidote, and pyroxene, 

 according to the author's own observations, and in chromate of 

 lead according to the observations of Professor Norrenberg, 

 one of the axes of elasticity in question coincides very accurately 

 with the axis of one of the principal zones of the crystal. 



An Account of a new Phcenomenon of sonorous Interference. 

 By R. Addams, Lecturer on Chemistry and Natural Philo- 

 sophy. 



Two systems of sound waves, simultaneously generated by a 

 tuning-fork, in tubes, interfere and neutrahze each other when 

 the axes, or lines of direction in which the two systems are 

 propagated, are at right angles to each other. 



The apparatus which the author of this communication em- 

 ploys to demonstrate the foregoing case of interference, consists 

 of two glass tubes, one inch in diameter, each furnished with 

 a piston, in order to adjust the length of the included column 

 of air, so as to make it unisonant with a tuning-fork (according 

 to the method first devised by Mr. Wheatstone). These tubes 

 are placed rectangularly, one vertically, the other horizontally, 

 and with their mouths in contact, edge to edge. 



When a tuning-fork is vibrated, and held so that the medial 

 line between its branches coincides with the intersecting point 

 of the axes of the tubes, there will be no sound heard ; but 

 upon covering the mouth of either tube with a card, an audible 

 sound is reciprocated by the air in the open one. 



Variations in the intensity of the sound occur by altering the 

 angle of position of the tubes. 



Account of Magnetical Observations in Ireland, and of a New 

 Method of observing the Dip and the Force with the same 

 Instrument. By the Rev. Professor Lloyd, F. T. CD. 



In the last Report of the Transactions of the British Asso- 

 ciation for the Advancement of Science it was recommended, 

 " that a series of observations upon the intensity of terrestrial 

 magnetism be executed in various parts of the kingdom, similar 

 to those which have been carried on in Scotland by Mr. Dun- 

 lop ; and that observations should be made in various places 

 with the dipping-needle, in order to reduce the horizontal to 

 the true magnetic intensity." After alluding to the time occu- 

 pied in the preliminary observations required in such a task, — 

 such as those made to ascertain the magnetic condition of the 

 needles used for the force, and the changes of this condition 



