PART II. POLAR MAGNETIC PHENOMENA AND TKRRKLLA EXPERIMENTS. CHAP. V. 



633 



In this connection it would be natural first to find out whether the length of a comet's tail has any 

 special relation to the distance of the comet from the plane of the sun's equator, since we have seen, 

 in treating of the zodiacal light, that it must be assumed that there is a ring of radiant matter round the 

 sun in that plane. 



Table CVIII gives the results of a series of calculations that have been made in order to 

 make this matter clear. Here fi is the comet's heliocentric latitude, x its distance from the plane of the 



J* I* 4G it IJO. 



Fig. 229. 



sun's equator, r its distance from the sun, / the length of its tail, and a the angle between the radii 

 vcrtores of the comet and the earth, r, x, and / are measured in radii of the earth's orbit, / being only 

 approximate, as the tail is imagined to extend radially out from the sun, which in this connection is 

 sufficiently accurate. 



The orbit-elements employed in the calculations are taken from PH. CARL'S "Repertorium der Cometen 

 Astronomic". 



The angle between the plane of the sun's equator and the ecliptic is put at 7, in accordance with 

 Arrhenius' "Lehrbuch der cosmischen Physik " (p. 153); and the angle between the line of intersection 

 of these two planes, and the line of equinox is put at 70, also in accordance with the last-named authority. 



