Solar-Diurnal Variations of Terrestrial Magnetism, 441 



and two meridians of longitude likewise near each other, and 

 imagine our pole to be carried round this boundary. On the 

 supposition that there is no vertical current in the centre of 

 the path, the work done upon the pole will be equal to that 

 done by it. This will at once give us an equation between 

 X and Y i that is to say, between the components of the solar- 

 diurnal periodic force towards the geographical north and 

 west respectively. From this equation, and by means of 

 certain legitimate assumptions regarding the type of Y, which 

 represents very nearly the force influencing the declination 

 (and this we know to be of a simple uniform type), Dr. 

 Schuster is able to obtain certain relations between the solar- 

 diurnal variations of the declination and of the horizontal 

 force which ought to hold, if there be no vertical currents 

 between the upper regions and the Earth, or, rather, if such 

 vertical currents are inoperative as an element in producing 

 the solar-diurnal variation. One of these is that the northerly 

 or horizontal-force component of the daily variation ought to 

 have a maximum or a minimum at the time when the westerly 

 or declination component vanishes, that is to say attains its 

 mean position. 



Now at Greenwich, X or the horizontal-force component 

 has a maximum at 7 o'clock in the evening and a minimum 

 at noon ; while Y or the declination component vanishes a 

 little after 7 o'clock and between 12 and 1 in the afternoon. 

 At Bombay the declination-needle is at its mean value on an 

 average a little after 10 in the morning and about 10 in 

 the evening. The horizontal force has its maximum a little 

 after 11 in the morning and the minimum at a quarter past 9 

 in the evening. 



The observations at Lisbon and Hobarton show an equally 

 good agreement, those at St. Helena and the Cape of Good 

 Hope less so ; but in these two latter places the observations 

 taken at different months show a considerable difference of 

 behaviour. On the whole, Dr. Schuster regards the agreement 

 as satisfactory, and as so far tending to disprove the existence 

 of vertical currents through the Earth's surface. Pursuing 

 his analysis, he comes to the conclusion, from certain ap- 

 proximate expressions which he obtains, that the northerly 

 force ought to be a maximum in the morning and a minimum 

 in the afternoon in the equatorial regions ; while in latitudes 

 above 45° the minimum ought to take place in the morning. 

 Now this is almost exactly what happens. At Bombay, for 

 instance, the maximum of horizontal force takes place at 11 

 a.m., while at Greenwich the minimum takes place a little 

 after that time. 



Having thus obtained results which lead him to infer that 



