XXvi SIXTH REPORT — 1836. 



by a sufficiently extended series of observations, namely, at Limerick 

 by Captain Sabine, and at Dublin by Professor Lloyd, and having com- 

 pared the results with those obtained by means of the same needles at 

 a spot out of Ireland, whose magnetic intensity had been previously 

 settled, by availing themselves of the obsen'ations of Captain James 

 Ross, at London, our authors proceeded to estimate the relative inten- 

 sity of the magnetic force at twenty-five different places within the 

 compass of Ireland, by observations made at each of these simultaneously 

 with others at Dublin or at Limerick. Thej'^ thus obtained data by which 

 to exhibit the law of Terrestrial Magnetism in Ireland, in a similar 

 manner to that by which Humboldt laid down the laws of the distribu- 

 tion of Terrestrial Heat. The same principle was adopted in deter- 

 mining the lines of dip as of intensity, and the general result was ob- 

 tained, that the angle which the lines of dip in Ireland make with the 

 meridian of Dublin is 56° 48', and that the dip increases one degree for 

 every distance of 101 miles in a direction perpendicular to these lines. 



The preceding method of estimating the intensity by the number of 

 vibrations in a given time only applies to that portion of the earth's 

 magnetic force which operates in a horizontal direction. In order, 

 therefore, to determine the whole amount of this force, observations of 

 the kind above alluded to must be combined with others on the dip. 

 TTiis third series accordingly was instituted at twenty-three different 

 stations in Ireland, and the result arrived at was, that the lines of abso- 

 lute intensity make an angle of 33° 40' with the meridian of Dublin, 

 and that the intensity increases in a direction perpendicular to these 

 lines by the 1-lOOth part for every 95 miles of distance. 



Tlie importance of these researches in extending our knowledge of 

 Terrestrial Magnetism, and affording the data on which a coirect theory 

 with respect to this subject may hereafter be based, will be felt even by 

 those who do not fully appreciate the skill and labour they required ; 

 and no better proof could be afforded of the substantial benefits arising 

 from such an institution as the British Association, than that of having 

 originated such an inquiry. 



On the subject of Heat, Dr. Hudson, of Dublin, has detailed some 

 experiments, the tenor of which he considers incompatible with the com- 

 monly received theory respecting its radiation, which we owe to Pro- 

 fessor Prevost, of Geneva, inasmuch as their tendency would be to esta- 

 blish that cold is equally radiated with heat — a result inconsistent 

 with the notion of the former being a negative quality. Without pro- 

 fessing himself a convert to the views of Professor Leslie, who supposed 

 heat to be radiated in consequence of the alternate expansion and con- 



