V. OBSERVATIONS ON THE DIRECTION AND INTENSITY 

 OF THE TERRESTRIAL MAGNETIC FORCE IN IRE- 

 LAND, MADE BY THE REV. HUMPHREY LLOYD, M.A., 

 F.R.S., &c.; BY CAPTAIN EDWARD SABINE, R.A., F.R.S., 

 &c.; AND BY CAPTAIN JAMES CLARKE ROSS, R.N., 

 F.R.S., &c. 



Fifth Report of the British Association for the Advancement of Science. 



THE observations which form the subject of the present communi- 

 cation were made during the years 1834 and 1835, in compliance 

 with the recommendation of the British Association urged in the 

 first and second Reports of its proceedings. Their main object 

 has been to determine the direction of the lines of magnetic dip 

 and intensity in Ireland, and to make a small, but it was hoped 

 exact, addition to our knowledge of the laws of distribution of the 

 earth's magnetism. The observations are threefold : first, observa- 

 tions of the horizontal part of the earth's magnetic force, as de- 

 termined by the time of vibration of a needle suspended horizontally, 

 after the method of Professor Hansteen ; secondly, observations of 

 dip, made in the usual manner ; and thirdly, observations oi dip 

 and intensity at the same time, and with the same instrument, 

 according to the method adopted by Professor Lloyd, and already 

 submitted to the Association.* 



I. HORIZONTAL INTENSITY. 



The instruments employed in the first series of observations 

 were constructed after the model of that of Professor Hansteen. 

 The needles are cylinders 2| inches long, and -13 of an inch in 

 diameter, suspended by a few filaments of the silkworm's thread. 



* Fourth Report, p. 557. Transactions of the Royal Irish Academy, fW. XJ'IL 



