510 
PROFESSOR A. SCHUSTER OH THE DIURHAL 
to the assumed cause, the vertical component of the magnetic force is the unportant 
factor, as that component changes sign on crossing the magnetic equator. In order 
that electric currents should be induced which could account for the observed move¬ 
ment of the magnetic needle, it is only necessary to imagine convection currents m the 
upper regions from East to West during certain parts of the day, and from West to 
East at other times. Judging from the analogy of the theory of waves in shallow 
water, a horizontal motion of considerable velocity might be produced by a tidal action 
due to solar and lunar attraction. It is true that no periodic change of the barometer 
has been traced with certainty to a tidal action ; but I suppose that a tidal w'ave 
must nevertheless exist, and that its horizontal flow might be considerable, while the 
changes of pressure might escape our attention. As regards the effect of the Sun we 
have, indeed, a daily jieriod of the barometer which is probably due to thermal 
effects. It is curious and suggestive that the horizontal motion which must accom¬ 
pany the change in pressure is just such as would account for the daily variation of the 
magnetic needle. In the tropics the principal minimum of the barometer takes place 
about 3.40 o’clock in the afternoon, and the principal maximum about 9 o’clock 
in the morning. According to the theory of waves, there would be a horizontal 
movement from West to East in the afternoon, and from East to West in the mornino-. 
The direction of the induced electric currents would be away from the equator in both 
hemispheres in the afternoon, and towards the equator in the morning. This is 
exactly the system of currents we have been led to, starting from the observed 
magnetic variation. The only difficulty I feel in suggesting that the cause of the 
diurnal variation of the magnetic needle is the diurnal variation of the barometer lies 
in the fact that it would oblige us to place the electric currents into the lower regions 
of the atmosphere, as these only will be much affected by the thermal radiation of the 
Sun. The phase of the barometric oscillation has been found to be reversed on the 
top of mountains, and it wordd be interesting to see whether the magnetic variations 
show any peculiarities at great heights.^" 
The region of the atmosphere which other considerations lead me to consider as the 
most sensitive to electromotive forces is that of the cirrus clouds, and I should be 
inclined to look to that region for a solution of the question. The lunar action 
seems, according to the researches of Mr. Chambers, to be a modification of the solar 
action rather than an independent effect. This might be accounted for if we suppose 
that the conductivity of the air depends on the position of the Sun, while the electro¬ 
motive forces depend on the combined positions of the Sun and Moon. 
* [Hote added October 11, 1889.—Since writing tbe above I have become acquainted with Hann’s 
recent work on the diurnal oscillation of the bai’ometer (‘ Wien, Deukschriften,’ vol. 55, 1889.) It 
appears from the regularity of the semidiurnal period in different altitudes and latitudes that its cause must 
lie in atmospheric movements in higher regions of the atmosphere. The reversal of phase mentioned in 
the text is due to local effects and has nothing to do with the regular oscillation. It seems to be 
exceedingly probable in the light of these researches that the daily variation of the magnetic needle is 
connected with the daily oscillation of the barometer in the way described in the text.] 
