FOR THE ADVANCEMENT OF SCIENCE, 1857. 493 



change ; while the other is strictly abnormal, and simultaneous. 

 If this be so, it would follow that we are not justified in separat- 

 ing the larger changes from the rest, merely on the ground of 

 their magnitude ; and that a different analysis of the phenomenon 

 will be required. 



The effects hitherto considered are all referrible to the Sun as 

 their cause. Professor Kreil discovered, however, that another 

 body of our System namely, our own satellite exerted an effect 

 upon the magnetic needle ; and that the magnetic decimation under- 

 went a small and very regular variation, whose amount was 

 dependent on the lunar hour-angle, and whose period was there- 

 fore a lunar day. This singular result was subsequently confirmed 

 by Mr. Broun, in his discussion of the Makerstoun observations ; 

 and its laws have since been fully traced, for all the magnetic 

 elements, by General Sabine, in the results obtained at the 

 Colonial Magnetic Observatories. 



The foregoing facts bear closely upon the debated question of 

 the causes of the magnetic variations. It has been usual to ascribe 

 the periodical changes of the earth's magnetic force to the thermic 

 action of the Sun, operating either directly upon the magnetism 

 of the earth, or affecting it indirectly by the induction of thermo- 

 electric currents. Here, however, we have a distinct case of 

 magnetic action, unaccompanied by heat ; and the question is 

 naturally suggested, whether the solar diurnal change may not 

 also be independent of temperature. 



The most important fact, in its bearing upon this question, 

 is the existence of an annual inequality in the diurnal variation, 

 dependent on the Sun's declination, recently pointed out by 

 General Sabine. If we deduct the ordinate of the curve, which 

 represents the mean diurnal variation for the entire year, from 

 those for the summer and winter half-yearly curves respectively, 

 the differences are found to be equal and opposite ; and the curves 

 which represent them are, consequently, similar, but oppositely 

 placed with respect to the axis of abscissae. From this, General 

 Sabine draws the inference, that the diurnal variation is a direct 

 fft'cct of solar action, and not a result of its thermic agency. 



The most important step which has been recently taken in 

 this country, to advance the science of Meteorology, has been tin- 

 formation of a department connected with the Board of Trade, for 



