999 



optic axis of the crystal and the magnetic axis of the field are 

 parallel. 



Advancing to the consideration of atmospheric magnetism, the 

 author first refers to the earth as a source of magnetic power from 

 which emanate lines of magnetic force passing into space accord- 

 ing to a particular but recognized distribution, and in obedience 

 to the general laws which govern the distribution of power about a 

 given irregular magnet. In pure space the magnetic power is con- 

 sidered as transmitted onwards with a certain degree of facility 

 which is constantjbut may be increased or diminished by the presence 

 of paramagnetic or diamagnetic matter within that space. The atmo- 

 sphere is a portion of such matter, and can affect the magnetic lines 

 which pass from the earth into space, and affects them differently 

 according to variations which continually occur in it under natural 

 circumstances. Four-fifths nearly by volume of the air is nitrogen, 

 which is a gas that neither under any difference of temperature or 

 of expansion shows any alteration in its power of affecting the trans- 

 ference of the magnetic force; whether added to space therefore in 

 one state or another, or when undergoing changes of a corresponding 

 kind by natural cause, it has no influence on the magnetic force. 

 The perfect identity in magnetic action of hot and cold nitrogen, 

 the author proves by new and delicate experiments. Oxygen forms 

 the remaining fifth of the atmosphere. Its great magnetic changes 

 by expansion have been described in the Twenty-fifth Series. Those 

 produced by difference of temperature were described in the Philo- 

 sophical Magazine for 184-7> but are nov/ resumed with more care, 

 and found to belong to it alone, and not to nitrogen or to carbonic 

 acid : as its temperature is raised its paramagnetic force diminishes, 

 being resumed as the temperature falls again. These properties it 

 carries into the atmosphere, so that the latter is in reality a mag- 

 netic medium ever varying, from the influence of natural circum- 

 stances, in its magnetic power. If a mass of the air be cooled it 

 becomes more paramagnetic, if heated it becomes less paramagnetic 

 (or diamagnetic), as compared with the air in a mean or normal 

 condition. 



The effect of the approach and retreat of the sun in his daily 

 course is to produce such variations of changes in the temperature 

 and expansion of the atmosphere as to influence the lines of force 

 emanating from the earth, both in their direction and intensity ; and 

 the manner in which this influence will be developed is by means of 

 figures and descriptions stated by the author in relation to Ihe 

 annual and daily variation, and the irregular perturbations of the 

 magnetic force, which he thinks are consequences of it. He then 

 applies the result of the magnetic observations at Hobarton as a 

 test of the probable truth of the hypothesis, and considers that it 

 affords strong confirmation. The upper or north end of the needle 

 there goes v/est until about twenty-one o'clock, whilst the dip in- 

 creases ; the dip still increasing until noon, the upper end returns 

 rapidly eastward, as the sun passes by, until two o'clock, the dip then 

 decreasing ; after which the needle goes west again, following the 



