May 21, 1897.] 



SCIENCE. 



783 



the geologists tell us that the age of the 

 earth is to be reckoned by hundreds of 

 millions of years. Thus arises a question 

 to which physical science has not been able 

 to give an answer. 



The problems of which I have so far 

 spoken are those of what may be called the 

 older astronomy. If I apply this title it is 

 because that branch of the science to which 

 the spectroscope has given birth is often 

 called the new astronomy. It is commonly 

 to be expected that a new and vigorous 

 form of scientific research will supersede that 

 which is hoary with antiquity. But I am not 

 willing to admit that such is the case with 

 the old astronomy, if old we may call it. 

 It is more pregnant with future discoveries 

 to-day than it ever has been, and it is more 

 disposed to welcome the spectroscope as a 

 useful hand-maid, which may help it on 

 to new fields, than it is to give way to it. 

 How useful it may thus become has been 

 recently shown by a Dutch astronomer, 

 who finds that the stars having one type of 

 spectrum belong mostly to the Milky 

 Way, and are farther from us than the 

 others. 



In the field of the newer astronomy per- 

 haps the most interesting work is that asso- 

 ciated with comets. It must be confessed, 

 however, that the spectroscope has rather 

 increased than diminished the mystery 

 which, in some respects, surrounds the con- 

 stitution of these bodies. The older as- 

 tronomy has satisfactorily accounted for 

 their appearance, and we might also say for 

 their origin and their end, so far as ques- 

 tions of origin can come into the domain of 

 science. It is now known that comets are 

 not wanderers through the celestial spaces 

 from star to star, but must always have be- 

 longed to our system. But their orbits are 

 so very elongated that thousands, or even 

 hundreds of thousands, of years are required 

 for a revolution. Sometimes, however, a 

 comet passing near to Jupiter is so fasci- 



nated by that planet that, in its vain at- 

 tempts to follow it, it loses so much of its 

 primitive velocity as to circulate around the 

 sun in a period of a few years, and thus to 

 become, apparently, a new member of our 

 system. If the orbit of such a comet, or 

 in fact of any comet, chances to intersect 

 that of the earth, the latter in passing the 

 point of intersection encounters minute 

 particles which causes a meteoric shower. 

 The great showers of November, which 

 occur three times in a century and were 

 well known in the years 1866-67, may be 

 expected to reappear about 1900, after the 

 passage of a comet which, since 1866, has 

 been visiting the confines of our system^ 

 and is expected to return about two years 

 hence. 



But all this does not tell us much about 

 the nature and make-up of a comet. Does 

 it consist of nothing but isolated particles,- 

 or is there a solid nucleus, the attraction of 

 which tends to keep the mass together? 

 ISTo one yet knows. The spectroscope, if 

 we interpret its indications in the usual 

 way, tells us that a comet is simply a mass 

 of hydro-carbon vapor, shining by its own 

 light. But there must be something wrong 

 in this interpretation. That the light is re- 

 flected sunlight seems to follow necessarily 

 from the increased brilliancy of the comet as 

 it approaches the sun and its disappear- 

 ance as it passes away. 



Great attention has recently been be- 

 stowed upon the physical constitution of 

 the planets and the changes which the sur- 

 faces of those bodies may undergo. In this 

 department of research we must ieel grati- 

 fied by the energy of our countrymen who 

 have entered upon it. Should I seek to 

 even mention all the results thus made 

 known, I might be stepping on dangerous 

 ground, as many questions are still un- 

 settled. While every astronomer has enter- 

 tained the highest admiration for the energy 

 and enthusiasm shown by Mr. Percival 



