TRANSACTIONS. 



L Account of some Experiments made in Different Parts of Europe, on Terrestrial 

 Magnetic Intensity, particularly with reference to the Effect of Height. By 

 James D. Forbes, Esq., F. R. SS. L. Sf E. 8fC., Professor of Natural Philosophy 

 in the University of Edinhu7'gh. 



Read 19th December 1886. 



1. The Council of the Royal Society of Edinburgh having, on my application 

 in 1832, entrusted me with Hansteen's Magnetic Intensity Apparatus, in their 

 possession, I feel it to be my duty to communicate to the Society the results then 

 and subsequently obtained with it. 



2. The instrument consists of a mahogany box 5 inches long, 4 broad, and 

 2 deep, with sides and top of glass, having also a wooden tube, screwing into 

 the top, for containing a silk- worm's fibre about 5 inches long, by which the 

 magnetic needle is suspended so as to place itself horizontally, and after being 

 caused to deviate from its point of rest, the time of any given number of oscil- 

 lations in a horizontal plane is measured, — whilst a graduated circle in the bot- 

 tom of the box indicates its arc of vibration. 



3. The needles which accompanied the instrument, when originally sent 

 from Norway, are two in number, one a cylinder 3 inches long and 0.1 inch in 

 diameter, is marked on its case " No. 1." The other is shorter, thicker, and 

 heavier, and from its form has always been called the " Flat" needle. These 

 were the needles used with this apparatus by Mr Dunlop, in the experiments 

 made in Scotland at Sir T. M. Brisbane's expense, and published in Vol. XII. of 

 the Society's Transactions. A reference to that volume will shew clearly Profes- 

 sor Hansteen's and Mr Dunlop' s method of observation, which, essentially, I 

 have always followed. 



4. If we assume the magnetism of a needle to remain invariable, the inten- 



VOL. XIV. PART I. A 



