320 Report on Terrestrial Magnetism. 



stor-in" appear to succeed the changes which the mean or ordi- 

 nary tension of terrestrial magnetism undergoes. 



M. de Humboldt considers that it deeply interests the ad- 

 vancement of mathematical and physical sciences that, under the 

 auspices of His Royal Highness the President, the Royal So- 

 ciety should exert its influence in extending the line of simulta- 

 neous observations, and in establishing permanent magnetic 

 stations in the tropical regions on both sides of the magnetic 

 equator, in high southern latitudes, and in Canada. He pro- 

 poses this last station because the observations of horary variation 

 in the vast extent of the United States are yet extremely rare. 

 Those at Salem, calculated by Mr Bowdich, and compared by 

 Arago with the observations of Cassini, Gilpin, and Beaufoy, 

 may, he remarks, guide the observers in Canada, in examining 

 whether there, contrary to what takes place in Western Europe, 

 the (diurnal ?) variation dees not decrease in the interval between 

 the vernal equinox and the summer solstice. 



In a memoir published five years ago, M. de Humboldt states 

 that he has indicated as stations extremely favourable for the 

 advancement of our knowledge, New Holland, Ceylon, the 

 Mauritius, the Cape of Good Hope, the island of St Helena, 

 some point on the eastern coast of South America, and Quebec. 

 In order, he observes, to advance rapidly the theory of the phe- 

 nomena of terrestrial magnetism, or at least to establish with 

 more precision empirical laws, we ought to extend, and at the 

 same time to vary, the lines of corresponding observations ; to 

 distinguish, in the observations, of the horary variations, what is 

 due to the influence of the seasons, to a clear or a cloudy atmo- 

 sphere, to abundant rains, to the hour of the day or night, solar 

 time ; that is, to the influence of the sun, and what is isochronous 

 under different meridians : we ought, in addition to these observa- 

 tions of the horary variation, to observe the annual course of the 

 absolute variation of the inclination of the needle, and of the in- 

 tensity of the magnetic forces, of which the increase from the mag- 

 netic equator to the poles is unequal in the American or Western, 

 and in the Asiatic or Eastern hemispheres. All these data, the 

 indispensable basis of a future theory, can acquire certainty and 

 importance only by means of fixed establishments, which are per- 

 manent for a great number of years, observatories in which are 



