Terrestrial Magnetic Intensity. 59 



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 oscillations in a horizontal plane is mea- 

 sured, — whilst a graduated circle in the bottom of the box in- 

 dicates its arc of vibration. 



3. The needles which accompanied the instrument, when 

 originally sent from Norway, are two in number, one a cylin- 

 der 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. Dun- 

 lop, in the experiments made in Scotland at Sir T. M. Bris- 

 bane's expense, and published in vol. xii. of the Society's 

 Transactions. A reference to that volume will show clearly 

 Professor Hansteen's and Mr. Dunlop's method of observa- 

 tion, which, essentially, I have always followed. 



4. If we assume the magnetism'of a needle to remain in- 

 variable, the intensity of the earth's magnetism at different 

 places, or at the same place at different times, will be (on the 

 principle of the pendulum) inversely as the square of the time 

 required to perform a given number of vibrations in infinitely 

 small arcs, under the different circumstances. But various 

 adjustments have to be attended to, and corrections applied. 



5. Nothing more portable or more simple than the instru- 

 ment in its present form can be desired. These requisites are 

 no doubt obtained at the expense of some accuracy. Mr. Snow 

 Harris has shown* that the influence of the surrounding air 

 upon the needle gives rise to considerable errors, especially 

 when the needle is so small and light, as in Hansteen's ap- 

 paratus. But greatly to enlarge the needle, and to connect 

 an air-pump with the apparatus, is nearly equivalent to de- 

 priving the traveller of its use altogether. Hansteen's instru- 

 ment was the constant companion of my pedestrian excur- 

 sions, and had it been in any other form, the present obser- 

 vations would probably never have been made. Besides all 

 this, there are sources of error arising from imperfectly known 

 and irregular variations of the earth's intensity, and equally 

 or more important ones from changes of magnetic intensity in 



* Edinburgh Transactions, vol. xii. p. 1.— See also the Observations of 

 Professor Bache ; American Phil. Trans., vol. v. 



12 



