C. GENERAL PHYSICS. 73 



current of heat continually ascending from the interior, and 

 from the rotation of the sun. Such currents, Professor Zoll- 

 ner maintains, exist in all rotating cosmical bodies, even after 

 the surface, cooled by radiation, has become rigid to a certain 

 extent. This is the case with the earth, and the continuous 

 regular currents of the interior liquid mass produce different 

 effects upon the outer shell, mechanical, thermal, and also 

 magnetical, the latter as a necessary consequence of the elec- 

 tricity originated by the currents. The professor further 

 maintains that by this hypothesis the general phenomena of 

 terrestrial magnetism may be satisfactorily explained, and 

 that they are related to the currents of the inner liquid mass, 

 and whatever affects these currents, as, for instance, volca- 

 noes, reacts immediately upon the magnetism of the earth. 

 Whenever a cosmical body becomes entirely solid, no in- 

 duced magnetism can exist, etc. 19 C, xvi., 123. 



CONCURRENT MAGNETIC OBSERVATIONS. 



Mr. Diamilla-Miiller, of Milan, invites the concurrence of 

 physicists in making a series of simultaneous observations in 

 terrestrial magnetism on the 15th of October, 1872. He re- 

 marks that the simultaneous observations of the 20th of Au- 

 gust, 1870, have furnished a long and rich series of data in 

 reference to the diurnal variation of the needle, taken as a 

 whole, and over the entire surface of the earth. Among the 

 most important results of this series is that the secular varia- 

 tion of the horizontal needle, on the surface of the globe, in- 

 creases or diminishes in proportion to the value of the angle 

 formed by the needle with the astronomical meridian, this 

 variation being two minutes per annum near the line of zero, 

 or' in declination, and seven minutes where the declination is 

 equal to fourteen degrees, such proportion being exhibited 

 symmetrically on either side of the line of no declination. 



The special object of the second series is to ascertain the 

 absolute mean daily declination, for the purpose of determin- 

 ing the secular variation of the isogonal lines in other words, 

 the increase or diminution of the declination. They will also 

 serve to assist in the construction of magnetic charts, as with 

 their aid it will be possible to resolve many pending ques- 

 tions relative to the real position of certain isogonal lines, and 

 to the proportional value of their secular displacement. The 



D 



