228 



SCIENCE 



[N. S. Vol. XXXI. No. 789 



request telegraph and telephone companies to 

 report any such disturbances as may occur 

 during our passage through the comet, should 

 this happen. However, such currents should 

 be considered only in connection with other 

 phenomena, since alone they can have but 

 little meaning. 



5. Diurnal Variation of the Earth's Mag- 

 netism. — It has been known for a long time 

 that there is regularly both a diurnal and a 

 semidiurnal variation in all the elements of 

 terrestrial magnetism; and it has been shown 

 by Schuster" that the origin of these daily dis- 

 turbances is outside the surface of the earth. 

 The origin of this variation is, probably, the 

 Foucault currents caused by the sweep of the 

 ionized, and therefore conducting, air across 

 the lines of magnetic force. The more ion- 

 ized, or the better conducting the air, other 

 things being equal, the greater these currents 

 and, if this theory is correct, the greater the 

 resulting diurnal variation in the records ob- 

 tained at magnetic observatories. 



If then the particles of a comet's tail are 

 highly electrified, or should in any way pro- 

 duce, on our coming into them, an ionizing 

 action on the atmosphere, there must result 

 corresponding changes in the diurnal varia- 

 tions. The action of the cometary particles, 

 presumably, would be on the outer layers of 

 the atmosphere where any change in the con- 

 ductivity is most efl^ective. Also since, in 

 general, the winds increase with latitude and 

 the lines of magnetic force become more con- 

 centrated and more nearly vertical, therefore 

 any change in the diurnal variation, especially 

 of the declination, that may be due to the 

 action of a comet's tail probably would be 

 most marked in the higher latitudes. 



It seems, therefore, that it would be espe- 

 cially well to study and compare the diurnal 

 variations obtained at the many excellent 

 magnetic observatories just before, during and 

 just after the coming passage of the earth 

 through the tail of Halley's comet — assuming, 

 of course, this event to take place. 



6. Auroral Displays. — Auroras serve as 



Thil. Trans., A, Vol. 180, p. 467, 1889; Vol. 

 208, pp. 163-204, 1908. 



rather delicate indicators of the electrical 

 state of the outer atmosphere, and therefore 

 should be carefully watched for and minutely 

 noted during a continuous period of several 

 days equally overlapping the supposed epoch 

 of our intersection with the material of the 

 comet. 



7. Line and Band Absorption. — The atmos- 

 pheric absorption lines and bands furnish 

 about the best means we have for detecting 

 changes in the composition of the atmosphere, 

 especially of the outer portions. Therefore it 

 may be desirable to compare the atmospheric 

 lines and bands during the passage of the 

 earth across the comet's tail with the lines and 

 bands obtained at other times. 



If the electrification of the outer air is 

 materially changed during this passage there 

 may result a corresponding temporary change 

 in the amount of ozone in that region, that 

 perhaps could best be detected through the 

 great ozone absorption band^ at wave-lengths 

 9 /x to 10 /ji. 



8. Atmospheric Transmission. — In reducing 

 the data obtained with integrating pyrheli- 

 ometers it is customary to use, with certain 

 corrections, the simple Bouguer equation, 



I = /„(!"!, 



in which I is the observed solar intensity 

 through the air mass m, 7„ the intensity out- 

 side the atmosphere, and a the coefficient of 

 transmission. This latter varies from day to 

 day, but, assuming it to remain constant for a 

 few hours, can be determined by observations 

 taken with different values of m, or, as Kim- 

 ball* has shown, by a single observation of the 

 intensity, together with a simultaneous meas- 

 urement of sky polarization. 



Since a is such a variable quantity its de- 

 termination while, perhaps, of some value in 

 this connection, can not be regarded as very 

 promising of definite information concerning 

 the material of a comet through which we 

 might be passing. 



9. Meteoric Trails. — Since the particles com- 



^ Angstrom, Arkiv fiir Matematik, Astronomie 

 och Fysik, 1, 395, 1904. 

 * Mount Weather Bulletin, 2, pp. 55-65, 1909. 



