534 THE .sciEyriFic MUNTIILY
positions of comet tails were studied estensively by Bredit-liiii, who fouod that there were three classes of tails, corresponding to three fairly defi- nite ratios of repulsive to attractive forces, as indicated by three different degrees of lagging behind the line joining the sun and the head (Fig. 7) . Bredichin determined that the long slender tails, observed in a few comets, which lag l)ehind only slightly are the result of a repulsive force twelve to fifteen times as intense as the attractive force. He found another clasc of comet tails, of medium lag, for which the repulsive fowes were from S.8 to 0.5 times the attractive forces. Another class of tails, short and bushy, with very strong lag, were explainable on the a<Kuniption that the repulsive forces were relatively weak, from 0.3 to 0.1 i,f tliG attractive forces.
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