Royal Society. 73 



Hccoiding to varintions which continually occur in it under natural 

 circumstances. Four-tifths 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 intinence 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-tiflh Series. Tiiose 

 produced by difference of temperature were described in the Philo- 

 sophical Magazine for 1847, but are now 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 

 Cor 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 the 

 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 west 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 

 sun. On examining the results at Toronto, corresponding effects 

 were found to occur, when the upper or south end of the needle was 

 considered, and therefore in accordance with the hypothesis. The 

 examination of the observations made at Greenwich, Washington, 

 Lake Athabasca, Fort Simpson, and St. Petersburgh, are considered 

 as further adding confirmation. By the aid of these obser\ations the 

 author restates his principles more minutely, endeavouring to indi- 

 cate what difference, changes in the inclination, declination, place 

 of the sun, land, and sea, &c. will produce. 



Though the sun is the cause of those changes in the atmosphere 

 which affect the lines of force of the earth, he is not assumed as the 

 centre of action as regards those lines; that is considered to exist 

 bomewhere in the atmosphere. It appears to be in the upper re- 

 gions and not on the surface of the earth, because it increases the 

 dip of places north and south of the tropics which have u certain 



