1886. J NEW YORK ACADEMY OF SCIENCES. 255 



from the planet at some past time by sometliing like volcanic 

 action. 



On the whole, however, the most important work relating to 

 comets has been that of the Russian astronomer, Bredichin. He 

 has brought the meclianical and matlienuitical portion of the 

 theory of comets' tails to a high degree of perfection; following 

 out the lines laid down by Bessel, but improving and correcting 

 Bessel's formuU?, and determining their constants by a most 

 thorough discussion of all tlie accurate observations available. 



It it hardly possible to doubt any longer that all the facts can 

 be represented on the hypothesis that the tails are composed of 

 minute particles of matter, first driven off by the comet and then 

 repelled by the sun. Bredichin's most interesting result, arrived 

 at in 1878, is that the tails appear to be of tliree, and only three, 

 distinct types — the long straight streamers which are due to a 

 repulsive acceleration about twelve times as great as the sun's 

 attraction; the second and most ordinary class of broad-curved 

 tails for which tiie repulsive force ranges between one and two 

 and a half times that of the attraction; and, finally, the short, 

 stubby brushes which are found in a few cases, and correspond to 

 a repulsive force not more than one-fourth the sun's attraction. 

 Supposing, as he does, that the real repulsion is the same for 

 each atom, the apparent repulsion, or repulsive acceleration, 

 would be greater for the lighter atoms and nearly inversely pro- 

 portional to their molecular weights ; and so he concludes that 

 probably tails of the first type are composed of hydrogen, those of 

 the second type of hydro-carbons, like coal gas, and those of the 

 third, of iron and its kindred metals. As to the second type, the 

 spectroscope speaks distinctly in confirmation. Tails of the firsts 

 and third types are not common, and are usually faint, and since 

 Bredichin's result was announced, there has been no opportunity 

 for spectroscopic verification in their case. 



I said his investigations had given a mathematical and mechan- 

 ical explanation of comets' tails ; but the physical question as 

 to the nature of the force which causes the observed repulsion, 

 remains unsettled, though I think there is no doubt that general 

 opinion is crystallizing into a settled belief that it is electrical; 

 that the sun is not at the same electric potential as surrounding 

 space, and that, in consequence, semi-conducting masses of pul- 

 verulent matter, such as comets seem to be, are subject to power- 

 ful electric forces as they approach and recede from the central 

 body. At the same time there are those — Mr. Ranyard, for in- 

 stance — who forcibly urge that the direct action of the solar heat 

 might produce a similar repulsive effect by causing rapid evapo- 

 ration from the front surface of minute particles, charged with 

 gases and vapors, frozen by the cold of outer space. 



