696 TERRESTKIAL MAGNETISM. 



A comparison of the changes of intensity of the horizontal 

 force with the changes of intensity of the total force seems to show 

 that the change of intensity in the horizontal force is due principally 

 to a change in the dip, rather than to a real change of intensity in 

 the Earth's magnetic force. 



Lieutenant Foster believes he accounts for the diurnal variations 

 of direction and intensity by supposing that the magnetic axis 

 of the earth revolves round and is inclined to the earth's axis at 

 an angle of 2' or 2^'. 



5. Magnetic Observations by Sir John Ross. 



The magnetic observatory at Felix Harbour was built of snow, 

 200 yards from any metal. 



Observations for Variation were made every two hours in April 

 and May. 



In the Diurnal Variation instrument (by Dollond) the needle 

 was nicely suspended by a single fibre of New Zealand flax. Sir 

 John Ross says, " It was materially disturbed by the approach of 

 ** any metallic substance, by any sudden increase or decrease of 

 " light, and by alteration in the temperature." The increase 

 of temperature caused by looking through the microscopes had a 

 considerable effect, and the needle had a " constant tremulous 

 motion." 



Sir John Ross also says, " When too dark to observe without 

 " a light, I was obliged to take a paper lantern, and even then, 

 *' when the lighted candle shining through the paper was held 

 " for the purpose of reading off the arc, it produced a horizontal 

 *' motion in the needle corresponding to the direction and 

 " strength of the light." 



The position was near the magnetic pole, in latitude 69° 59' and 

 longitude 92° W., where the variation was 89° 45' W. and the dip 

 89° 55'. " During the winter it was proved that the needle was 

 " disturbed by and followed the light of a candle ; that it was 

 " materially disturbed by a brilliant Aurora Borealis, particularly 

 " when that was of a deep red." 



Note. — This passage shows the delicacy of suspension of the 

 needle, and may possibly have some bearing on the beautiful 

 experiments of Mr. Crookes on delicately suspended bodies 

 subject to the action of a candle, although the effects may 

 probably be due to the fact that some kinds of paper are strongly 

 magnetic. 



The dip was determined by three dipping needles, and was the 

 same with each, viz., 89° o5\ 



At Spence Bay the dip was 89° 56% and the variation 

 68° 35' W. 



On the shores of the sea to the west of Boothia Felix, Sir 

 James Ross made observations of declination and dip, and calcu- 

 lated the position of the magnetic pole. 



On reaching the calculated position, attempts were made to 

 determine the magnetic meridian by suspending a dipping needle 

 first by means of a single fibre of silk, and afterwards by a single 



