2 50 THE POPULAR SCIENCE MONTHLY. 



tance of 1,250,000 miles ; for it is to be remembered that the comet 

 as well as the earth would have an enormously rapid motion, and the 

 disturbing power of the earth would therefore only act for a short 

 time. But a minor planet — even the largest of the family — would not 

 have the twenty-thousandth part of the earth's power to disturb a pass- 

 ing comet. At a distance of 200,000 miles a comet would pass such 

 an asteroid without any marked disturbance of its motions," and at a 

 distance of 1,250,000 miles there would practically be no disturbance 

 at all. " It is, of course, not absolutely impossible that one of the 

 comets of the pair should have been encountered by one minor planet, 

 and the other by another, but the probability against such a contin- 

 gency is so great, that we need scarcely entertain the idea even as a 

 bare possibility." 



On the other hand, the supposition that the comet was destroyed 

 or dissipated by meteor - streams, though not altogether untenable, 

 seems little likely to be correct. I was disposed, when I wrote the 

 article from which I have quoted the above passages, to think other- 

 wise. " The comet," I said, " had been seen to divide into two parts 

 in a portion of the solar system where certainly no bodies but meteor- 

 ites can be supposed to travel. It seems reasonable to suppose that on 

 that occasion the head of the comet had come upon some group of 

 meteors, and so had divided, as a stream of water divides against a 

 rock. Assuming this, we find reason for believing that the track of 

 this comet crosses a rich meteor region. The particular group which 

 had caused the division of the comet would of course pass away, and 

 would not probably come again in the comet's way for many years or 

 even centuries ; but another group belonging to the same system 

 might in its turn encounter the comet, and complete the process of 

 dissipation which the former had commenced. On this theory the 

 distance between the companion comets would introduce no difficulty. 

 For not only is it quite a common circumstance for meteoric systems 

 to have a range of several millions of miles, but — a much more impor- 

 tant consideration — both the comets would be bound to return to the 

 scene of the former encounter. It was there that each had been sent 

 off on a new track, but each new track started from there, and there- 

 fore each new track must pass through there." The reasoning here is 

 correct enough as far as it goes, but it does not duly take into con- 

 sideration the extreme sparsity of meteoric distribution and the ex- 

 treme tenuity of the heads and even of the nuclei of comets. As I 

 pointed out in an essay which appeared in the "Popular Science 

 Review " two months only after the essay from which I have quoted 

 above had ajspeared in the "St. Paul's Magazine" (if I remember 

 rightly), the meteors of even one of those comparatively rich clusters 

 which produce an important display are strewed so sparsely that each 

 occupies on the average a space corresponding in volume to a cube one 

 hundred miles in length, breadth, and height. The largest meteor in 



