324 



KNOWLEDGE. 



September, 1913. 



that the Sun might long continue in this way to heat 

 and illuminate the planets. But the insufficiency of 

 this process is a fatal objection, except as possibly 

 affording a minute portion of the solar radiation ; 

 for, were the Sun's heat thus maintained at its pre- 

 sent rate of emission, calculation shows that our own 

 Earth and the nearer planets would be subject to a 

 bombardment also, sufficient to add about fifty tons 

 of meteoric matter on each square mile of the Earth's 

 surface per day ; whereas in reality the actual amount 

 falling is not one-millionth of that quantity. The 

 fall of sufficient material in such vast quantity 

 implies the presence of a yet greater amount in the 

 surrounding regions, and very evident effects upon 

 the motions of the planets would also be perceived 

 which are not observed. Nevertheless, the idea that 

 a balance between expenditure and receipts is kept 

 up in this way seems so beautiful an one that we may 

 perhaps be permitted to regret that it is not true. 



Long-continued observations of the light were 

 made by the Rev. G. Jones, chaplain of the 

 American steamship " Mississippi," during a cruise 

 of two years round the world, and his observations 

 and results were published in a beautiful volume 

 during the year 1858. The final conclusion at 

 which he arrived from the totality of his observa- 

 tions was that the zodiacal light is produced by a 

 nebulous ring having for its centre the Earth, which 

 lies within the orbit of the Moon ; in other words, 

 a kind of terrestrial ring like one of the rings of 

 Saturn. Captain Wilkes, a well-known American 

 navigator, objected to Jones's views and proposed an 

 alternative theory that the zodiacal light is simply a 

 terrestrial meteorological phenomenon, the result of 

 the illumination of that part of the Earth's atmo- 

 sphere upon which the sun falls vertically within 

 the tropics. The Dutch observer Gronemann 

 similarly considered the light to be of terrestrial 

 origin, and denied that the relation asserted to exist 

 between the luminous cones seen in the east in 

 the morning and those seen in the west during the 

 evening had been established ; he also declared that 

 it had not even been proved that they shared the 

 apparent diurnal motion of the heavenly bodies. In 

 1876 Serpieri, of Urbino, presented to the Italian 

 Spectroscopic Society a memoir dealing with the 

 results obtained by various previous observers, and 

 gave as his opinion that the light was of electrical 

 origin, perhaps analogous to the aurora. Thus we 

 see that there is still much to be done before we can 

 be said satisfactorily to understand this mysterious 

 phenomenon, although, as we shall point out later 

 on, there are good reasons for thinking that the 

 " auroral " theory of the zodiacal light arises from a 

 confusion of ideas. 



Long ago Humboldt, whose interest had been 

 excited by finding the intensity of the light much 

 greater in Spain than in Germany, expressed his 

 astonishment at the variability of this phenomenon 

 under the tropical skies of South America. Even 

 whilst its luminosity was greatest it would for some 



minutes diminish appreciably, and then without 

 warning brighten up again to its former intensity. 

 "The strength of the light increased in a remarkable 

 manner as I approached the Equator in Southern 

 America. In the constantly clear and dry atmo- 

 sphere of Cumana, in the grassy llanos of Caracas, 

 on the elevated plains of Quito, and in the Gulf of 

 Mexico, especially on the plateaux (of ten thousand 

 to twelve thousand feet above sea level), where I 

 stayed for some time, I noticed that its brilliancy 

 often surpassed that of the brightest part of the 

 Milky Way between Argus and Sagittarius, or, 

 speaking of the part more familiar to us in the 

 northern hemisphere, that part between Aquila 

 and Cygnus " (" Cosmos," Vol. IV). On the whole, 

 however, Humboldt considered that the brilliancy 

 of the zodiacal light did not increase with the 

 altitude of the region where it was observed. He 

 insisted especially upon " the intrinsic variability 

 of the phenomenon itself," but he did not dispute 

 the possibility of the influence of varying atmo- 

 spheric conditions, the greater or less transparency 

 of the upper and lower strata of the air upon its 

 brightness, as we have already noticed with regard 

 to the coloration perceived in extra-tropical regions, 

 scintillation, and so on. 



Amongst others who have studied the various 

 phenomena, Professor Searle has also devoted much 

 time and thought to the different theories pro- 

 posed. Dealing with the observations of Jones and 

 others he concludes that the zodiacal light varies 

 in position during the course of the year, being 

 more to the north in autumn than in spring (of our 

 hemisphere) ; atmospheric absorption has a great 

 influence also upon its apparent position ; the zone 

 of the sky occupied by the minor planets presents 

 peculiarities like those of the zone occupied by the 

 light, and thus one is led to think that the latter 

 also is due, in part at least, to small bodies moving in 

 planetary orbits. The light does not interfere with 

 the visibility of even faint stars, and it disappears 

 by setting, and not by growing fainter. This last 

 movement, due to the Earth's diurnal motion, as we 

 have seen, was denied by Gronemann. 



From observations made at sea whilst in the 

 Tropics during 1862 Helis concluded that the 

 change of position of the light depends more on the 

 time of year than on the situation of the observer. 

 Colonel Tupman made a number of observations 

 from 1869 to 1871 in Southern Europe, and gave an 

 account of his work in The Monthly Notices of the 

 Royal Astronomical Society, Vol. XXXII. He stated 

 that the axis of symmetry of the light with regard 

 to the Ecliptic varied considerably during the course 

 of the year, being almost parallel to that plane in 

 winter and making an angle of as much as 20° 

 with it in August and September. He also 

 asserted that the plane of the light did not pass 

 through the Sun. If this be indeed the case it would 

 seem that the matter composing it does not turn 

 round the Sun, but his observations differ very 



