COMETS' TAILS. 275 



But though we cannot observe aurora3 in daylight, we are not with- 

 out resource, for even when invisible, they give notice of their presence 

 by disturbing the ordinary course of the records photographically taken 

 in our magnetic observatories. In 1899 van Bemmelen discussed the 

 records of such magnetic storms taken in Batavia. He found that they 

 show maxima in March and September, minima in January and June, 

 and a daily maximum at 3 p. m., and minimum at 1 a. m. 



3. Monthly Variations. 



It is only recently (1898) that the collection of statistics of aurorge 

 published by Eckholm and Arrhenius has brought to light two curious 

 monthly variations in their number. 



One of these, with a variation of 20% on each side of the mean, 

 depends on the revolution of the moon in her orbit, showing in the 

 northern hemisphere a maximum when the moon is farthest south of 

 the equator, a minimum when she is farthest north: and vice versa for 

 the southern hemisphere. 



The explanation appears highly ingenious. It is as follows: The 

 moon, being unprotected by an atmosphere, is charged by the streams 

 of particles that reach her much as the outer layers of our own atmos- 

 phere are charged, and therefore, as we have good reason to believe, to a 

 far higher negative potential than is observed at the surface of the earth. 

 If so, she will seriously affect the number of aurorae at any place over 

 wliich she stands, by lowering the potential gradient, and thus re- 

 ducing the number of negative discharges in the highest regions of 

 our atmosphere. 



The other variation of some 10% each way has a period of 25.93 

 days and affects both hemispheres alike. At first sight it is natural 

 to refer this to the synodical time of revolution of the sun on his axis 

 as determined by observations of sunspots. But this is 27.3 days. 

 Remembering that the earth never departs more than 7° from the sun's 

 equator, we should rather take the time of revolution of the equatorial 

 belt for comparison. As estimated by the motion of the faculae, this 

 is 2G,06 days, the equator moving faster than the sunspot belts, and 

 probably the time of revolution of the outermost layers, from which the 

 particles stream, is yet a little shorter. The agreement with the 

 period of the aurora (25.93 days) would thus be within the limits of 

 error of the observations. 



Atmospheric Electricity. 



Let us now trace the effect of the aurorae on the earth's atmosphere. 

 If they are really kathode rays on a grand scale, they must ionize the 

 air, the negative ions will form centers for condensation, and sinking 

 to the earth by gravitation, will charge it negatively, leaving the layers 



