512 
PROFESSOR A. SCHUSTER ON THE DIURNAL 
altogether satisfactory. Mr. Whipple, in the ‘ British Association Eeport’ (Birming¬ 
ham, 1886, page 71), says :— 
“ Contrasting the Kew results with those of Greenwich, we may fairly consider the 
difference to he due in some measure to instrumental causes, the construction of the 
magnetographs being dissimilar at the two observatories. The slight difference in 
position of the two observatories may likewise have some influence.” 
The difference amounts to about 15 per cent., and it seems as if the question 
whether such a difference can be due to instrumental causes deserves a careful 
examination. Mr. Chambers, at Bombay, has found similarly that the results of the 
magnetographs differ from those obtained by the old magnetometers ; and he seems to 
ascribe the difference to an “ influence of height above or below the ground level.” 
The height of the magnetometer was 6 feet above ground, and that of the magneto- 
graph ’l\ feet below ; the former gives ranges greater by 7 per cent, for the declination 
variation, and the difference is greater still for the horizontal force component. That 
there should be a real difterence of that magnitude in the two positions seems exces¬ 
sively unlikely, and we must conclude that at present the results given by magneto¬ 
graphs are doubtful to the extent of about 10 per cent. 
In conclusion, we may sum up the principal results obtained in this paper as 
follows ; — 
1 . The principal part of the diurnal variation is due to causes outside the Earth’s 
surface, and probably to electric currents in our atmosphere. 
2 . Currents are induced in the Earth by the diurnal variation which produce a 
sensible effect chiefly in reducing the amplitude of the vertical component and 
increasing the amplitude of the horizontal components. 
3. As regards the currents induced by the diurnal variation, the Earth does not 
behave as a uniformly-conducting sphere, but the upper layers must conduct less 
than the inner layers. 
4. The horizontal movements in the atmosphere which must accompany a tidal 
action of the Sun or Moon or any periodic variation of the barometer such as is 
actually observed would produce electric currents in the atmosjihere having magnetic 
effects similar in character to the observed daily variation. 
5. If the variation is actually produced by the suggested cause, the atmosphere 
must be in that sensitive state in which, according to the author’s experiments, there 
is no lower limit to the electromotive force producing a current. 
In conclusion, the author begs to return his thanks to Mr. William Ellis for help 
given in some of the calculations, and also to his assistant, Mr. Arthur Stanton, for 
much labour bestowed on making and checking numerical calculations. 
