Page:The story of the comets.djvu/97

VI. of this resistance? Though the catastrophe may be averted for many ages by the powerful attraction of the larger planets, especially Jupiter, will not the comet be at last precipitated on the Sun? The question is full of interest, though altogether open to conjecture."

The following table, published by Encke, will more clearly illustrate the changes in the comet's periodic time:—

So far as it goes this table seems conclusive in its facts, but observations made at a return 10 years later than the last in the above table, namely in 1868, showed a sudden diminution in the retardation by nearly one-half the previously-noticed amount. And both the reality and also the permanence of this alteration were made clear in 1885. Some physical alteration in the comet has been suggested as the necessary explanation, but there is no visual evidence to lend colour to this idea.

The soundness of the explanation which assumes the existence of a Resisting Medium has been long and warmly canvassed, and it does not command the assent of astronomers generally. One strong point against it is that, with the exception perhaps of Winnecke's Comet (1858, ii.), none of the other short-period comets (all of them of small size and presumably slight mass) yield any indications of being subject to a like influence. On the other hand Von Asten, who