82 COSMOS. 



own times (and this difference of opinion is very remarkable) 

 two distinct views were promulgated in reference to the 

 nature of the influence exerted by the sun. 



Some physicists, as Canton, Ampere, Christie, Lloyd, and 

 Airey, have assumed that the sun, without being itself 

 magnetic, acts upon terrestrial magnetism merely by pro- 

 ducing changes of temperature, whilst others, as Coulomb, 

 believed the sun to be enveloped by a magnetic atmosphere 7 * 

 which exerts an action on terrestrial magnetism by distribu- 

 tion. Although Faraday's splendid discovery of the para- 

 magnetic property of oxygen gas has removed the great diffi- 

 culty of having to assume with Canton that the temperature 

 of the solid crust of the earth and of the sea must be rapidly 

 and considerably elevated from the immediate effect of the 

 sun's transit through the meridian of the place, the perfect 

 co-ordination and an ingenious analysis of all the measure- 

 ments and observations of General Sabine have yielded this 

 result : that the hitherto observed periodic variations of the 

 magnetic activity of the earth cannot be based upon periodic 

 changes of temperature in those parts of the atmosphere 

 which are accessible to us. Neither the principal epochs of 

 diurnal and annual alterations of declination at the different- 

 hours of the day and night, nor the periods of the mean 

 intensity of the terrestrial force 80 coincide with the periods of 



79 Memoires de Mathem. et de Phys. presentes a VAcad. Roy. des Sc. 

 t. ix, 1780, p. 262. 



30 "So far as these four stations (Toronto, Hobarton, St. Helena, 

 and the Cape), so widely separated from each other and so diversely 

 situated, justify a generalisation, we may arrive at the conclusion that 

 at the hour of 7 to 8 A.M. the magnetic declination is everywhere sub- 

 ject to a variation of which the period is a year, and which is every- 

 where similar in character and amount, consisting of a movement of 

 the north end of the magnet from east to west, between the northern 

 and the southern solstice, and a return from west to east between the 

 southern and the northern solstice, the amplitude being about 5 minute-s 

 of arc. The turning periods of the year are not, as many might be disposed 

 to anticipate, those months in which the temperature at the surface of our 

 planet, or of the subsoil, or of the atmosphere (as far as we possess the 

 jneans of judging of the temperature of the atmosphere) attains its 

 maximum and minimum. Stations so diversely situated would, indeed, 

 present in these respects thermic conditions of great variety ; whereas 

 uniformity in the epoch of the turning periods is a not less conspicuous 

 feature in the annual variation than similarity of character and nume- 

 rical value. At all the stations the solstices are the turning periods of 



