44 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



and the matter of which it consisted was evi- 

 dently in the state of ignited gas. The coma 

 probably shone by reflected solar light the 

 spectrum of the light of the coma differing en- 

 tirely from that characterizing the light emitted 

 by the nucleus. The prism gives no informa- 

 tion whether the matter forming the coma be 

 solid, liquid, or gaseous, though terrestrial 

 phenomena would suggest that the parts of a 

 comet, which are bright by reflecting the sun's 

 light, are probably in the condition of fog or 

 cloud. The nitrogen line is the only one de- 

 tected in the nucleus. We must wait for a 

 comet of sufficient splendor to permit the satis- 

 factory prismatic examination of its physical 

 state during various changes of its perihelion 

 passage. 



Father Secchi, at Rome, in January, 1866, 

 made a spectrum analysis of Tempel's comet, 

 and found the light monochromatic (green), 

 similar to that of the nebula in Orion. The 

 monochromatic condition of the light he attrib- 

 utes to a molecular constitution different from 

 that which forms the planets and stars. He 

 infers that not all the light which comes from 

 the comet is reflected from the sun, or, if it is 

 so, that it suffers a singular sort of absorption. 

 The spectrum of the comet exhibited three 

 principal lines on a faintly-shaded ground. The 

 latter is probably due to light reflected from 

 the sun; the residue, the light of the comet 

 itself. 



Influence of the Tidal Wave on the Moon's 

 Motion. Mr. Adams and M. Delaunay, have 

 shown by their recent investigations that the 

 change in the eccentricity of the earth's orbit 

 accounts for only about one-half of the secular 

 acceleration of the moon's motion, viz., about 

 six seconds in a century. Prof. Harrison sug- 

 gests that the other half might be accounted 

 for simply by assuming that the length of the 

 sidereal day has increased by merely .01197 of 

 a second in the course of 2,000 years. M. 

 Delaunay subsequently showed that the re- 

 tarding effect of the tidal wave, originally 

 pointed out by M. Mayer, twenty years ago, 

 sufficed to account for the increase required in 

 the longth of the day. He concluded that the 

 six seconds of acceleration resulting from the 

 change of eccentricity in the earth's orbit are 

 real, but that the other six seconds are only 

 apparent. At a still later date M. Delaunay 

 admitted that the tides produce a real accelera- 

 tion of the moon's motion. In a paper, pub- 

 lished in the "Philosophical Magazine" last 

 August, by Mr. Croll (the second on that sub- 

 ject by the author), he offered the following 

 considerations to show that the solar wave 

 must diminish the earth's motion around the 

 common centre of gravity of the earth and 

 moon, and must therefore accelerate the an- 

 gular motion of the moon. Suppose the rota- 

 tion of the earth to be reduced to that of the 

 moon, viz., once a month. In that case the 

 earth would always present the same side to the 

 moon. The lunar wave would of course exist 



the same as at present, but would remain sta 

 tionary on the earth's surface. The solar wave 

 would also exist the same as it does now, bu 

 would move round the earth once a month, 

 instead of once in twenty-four hours as at pres- 

 ent. However slow the motion, a considerable 

 amount of heat would be generated by friction, 

 The source from which the energy lost in the 

 form of heat would be derived, would evidently 

 be the rotation of the earth round the common 

 centre of gravity ; for it is to this source that 

 the motion of the water is due. Now the ef- 

 fects which would take place under these cir- 

 cumstances do actually take place under the 

 present order of things. The two sets of effects 

 caused by the lunar and the solar waves do not 

 interfere with each other ; . consequently the 

 solar wave must be slowly consuming the vis 

 viva of the earth's rotation round the common 

 centre of gravity. It is this vis viva which 

 keeps the earth and moon separate from each 

 other. As the vfa viva is consumed, the two 

 must approach each other, and thus the angular 

 motion of the moon be accelerated. The solar 

 wave does not consume the vis viva of the 

 moon's motion around the common centre, but 

 only that of the earth. Since the earth is gradu- 

 ally approaching nearer to the moon in conse- 

 quence of the consumption of centrifugal force, 

 which keeps it separate from that orb, the moon 

 must therefore be moving with all its original 

 vis viva in an orbit which is gradually becom- 

 ing less and less, and the period of its revolu- 

 tion is consequently diminishing in length. 

 According to M. Croll's calculation, it is there- 

 fore merely a question of time though the 

 possible date of the catastrophe is incalculably 

 remote when the earth and moon shall come 

 together. 



The Zodiacal Light. M. Liandier, in the 

 Comptes Rendus, says that he has watched the 

 zodiacal light for several years, during the even- 

 ings of February and March. In 1866 he ob- 

 served it from the 19th of January to the 5th 

 of May. He reports that it has the shape of a 

 perfect cone, varing in luminosity and color 

 from a dull gray to a silver white, its changing 

 aspect probably being caused by the condition 

 of our atmosphere. In February the summit of 

 the cone reached Pleiades, and the Twins in 

 May. Between January and May he found it 

 to follow the zodiacal movements of the sun. 

 M. Liandier believes the luminous cone to be a 

 fragment of an immense atmosphere, which 

 envelops the sun on all sides. If so, he says it 

 may be expected to exercise an enormous 

 pressure on the sun, with a great development 

 of heat, and, if local variations occur, may ex- 

 plain the phenomena of sun-spots, through the 

 reduction of temperature that would follow its 

 diminished pressure. 



Nebulm. About sixty nebulas, examined by 

 Mr. Huggins during the past year, reveal a spec- 

 trum of one, two, or three bright lines. Their 

 elements cannot be determined, and the mate- 

 rial of the nebulas i supposed to be luminous 



