86 TERRESTRIAL MAGNETISM. 



unsearchable reciprocal action of molecules hi the interior 

 of substances, has led to important numerical laws ; and 

 the admirable sagacity of experimenting physicists has 

 succeeded in discovering, in solid and gaseous substances, 

 previously wholly unsuspected polar properties peculiarly 

 connected with temperature and with atmospheric pressure. 

 Important and undoubted as are these discoveries, they 

 cannot, in the present state of our knowledge, be yet 

 regarded as affording satisfactory bases of explanation of 

 the laws derived from the observations of the terrestrial 

 magnetic phsenomena. The surest means of exhausting the 

 measurable variations as respects space, and of enlarging 

 and completing the mathematical theory of terrestrial mag- 

 netism, so grandly sketched by Gauss, is to prosecute 

 continuously successive determinations of the three mag- 

 netic elements at well-chosen points of the globe, each such 

 determination referring to a definite epoch. The anticipa- 

 tions which I have myself formed of the great results which 

 will follow hereafter from the combination of experiment 

 with mathematical reasoning, have been already touched 

 upon in the earlier pages of the present volume. ( 76 ) 



We look in all the physical phsenomena which take place 

 on our planet, for cosmical connection. The word Planet 

 of itself leads us to dependence on a central body, and to 

 connection with a group of heavenly bodies of very various 

 magnitudes, probably having all the same origin. The 

 influence of the Sun's position in the heavens on the 

 manifestation of the Earth's magnetic force was very early 

 recognised; most clearly, in the discovery of an horary 

 variation in the declination of the needle ; more obscurely, 

 in the conjecture formed by Kepler, a century before, of the 



