xiv CONTENTS. 



ART. P*G3 



73. Method of measuring the amount of magnetism produced in a 



steel magnet by terrestrial induction 162 



74. Correction of the formulae, used in the determination of the 



Earth's horizontal magnet-power, for effects of induction . 165 



SECTION IX. 



ON SUBPERMANENT MAGNETISM IN IRON SUBJECTED TO MECHANICAL 

 VIOLENCE. 



Articles 7577. Pages 168172. 



75. Primary experiment on Subpermanent Magnetism; a long 



plate or slender bar of iron is placed on a firm frame 

 (sometimes called the "Magnetic Anvil") with its length 

 parallel to the local dip, and is struck repeatedly with a 

 hammer; it becomes a magnet, with red magnetism in the 

 end which dipped (in northern magnetic latitudes), and this 

 magnetism does not change with change of the magnet's 

 position 168 



76. Variations of the experiment. All lead to the supposition 



that iron, in a state of tremor or jar, is peculiarly able to 

 .receive induced magnetism, and .to retain it firmly . . 1G9 



77. Eeversion or destruction of the magnetism. Origin of the 



term "Subpermanent." , 171 



SECTION X. 



ON THE MAGNETISM OF IRON SHIPS, AS AFFECTING THEIR COMPASSES. 

 Articles 7882. Pages 173188. 



78. Philosophical and Commercial Importance of this subject. 



Complication of the Magnetic considerations involved in it . 173 



79. Brief History of the first steps in this science . . . 174 



80. Reference to the causes of partial failure in the correction of 



the compass 178 



81. Continuation of the history. Investigation of the effect of the 



ship's heeling 181 



82. Examination of the heeling-disturbance, and remarks on the 



possibility of correcting it ....... 186 



