772 



SCIENCE 



[N. S. Vol. XXXI. No. 803 



body's velocity. In this last case we have 

 stuck to one point of view and obtained a 

 correct result. 



This feature connected with the so-called 

 "addition of velocities" is what Professor 

 Michelson and others so strongly object to 

 in the relativity principle, but the result is 

 a perfectly natural one as soon as we have 

 seen the admissibility of more than one 

 point of view and the difference in esti- 

 mates caused thereby. 



D. P. COMSTOCK 

 Massachusetts Institute 

 OF Technology 



SOME CONSIDERATIONS AS TO THE NATURE 

 OF COMETS AND THEIR PROBABLE 

 RELATION TO THE SVN 



The ideas herein put forward are not all 

 original with the author, though it is be- 

 lieved some of them may be. It is hoped that 

 the considerations may, however, help to a 

 simple rational understanding of the major 

 facts regarding the behavior of comets. 



The exceedingly high temperature of the 

 sun causes it to be surrounded by an atmos- 

 phere of vapors. Some of the vaporized mat- 

 ter condenses in the outermost layers and 

 eruptions are constantly occurring which 

 partly fill the space around it with very fine 

 particles, the smaller of which are repelled by 

 the pressure of the sun's radiation, which 

 pressure even overcomes the gravitative 

 force of the sun itself. These ejected par- 

 ticles probably constitute the streamers which 

 are visible during total eclipses as extending 

 from the sun to immense distances. "What 

 we see is the effect of innumerable overlapping 

 streams. Their extreme tenuity is evidenced 

 by the comparatively feeble luminosity in 

 spite of the great depth of the flux which we 

 are at any time observing. This depth is, of 

 course, greater than the diameter of the sun. 

 Such coronal streamers are by no means uni- 

 formly distributed about the sun, but in cer- 

 tain directions, varying continually, may be 

 more dense than in others, coinciding perhaps 

 with great eruptive areas of the sun's surface. 



It probably happens that when the outbreak 

 is unusually violent, and when the earth 

 happens to be passing through that part of 

 space occupied by an abnormally extended 

 streamer, an aurora of greater or less inten- 

 sity or duration may attend the sweeping of 

 the earth by such a streamer. The particles 

 are probably ions or carry electric charges, 

 and induced auroral streamers in the earth's 

 atmosphere are for the time being visible on 

 its dark side away from the sun. 



It has been thought that comets may act in 

 a somewhat similar way to disclose the con- 

 dition of the ejected material of ^the sun, or, 

 as may be conceived, to disclose a stratifica- 

 tion or unevenness of distribution of the 

 ejected matter from the sun. Since there is 

 reason to believe that much of this matter is 

 in a highly electrified state, it is not to be 

 doubted that electrical phenomena are at the 

 same time produced, with accompanying evo- 

 lution of light. Indeed, in the free space 

 around the sun, there must be a great inten- 

 sity of ultra-violet radiation which of itself 

 would cause emission of negative ions from 

 matter in its path and produce electrical dis- 

 turbances. But aside from this possibility, 

 the comet is recognized as an assemblage of 

 particles larger or smaller, moving in an 

 orbit which involves great variations of its 

 distance from the sun. In passing through 

 the depths of space far away from the sun, 

 these parts or particles may tend, by their 

 very feeble gravitative effect, to gather up any 

 finer particles which, on account of the in- 

 tense cold of space, are substantially solid, 

 even though at ordinary temperatures they 

 would be gaseous. The parts of the comet's 

 nucleus more or less porous would in this 

 way accumulate upon their surfaces and in 

 their pores occluded gases, condensed mate- 

 rial and fine dust, and there would be a period 

 of many years in which this gathering-up 

 process, as in the ease of Donati's and other 

 long-period comets, could occur. Let a comet 

 as an assemblage of such small masses after 

 its long course through remote space, during 

 which it has gathered fine particles ejected 

 from the sun or from other bodies, reach, in 



