92 



KNOWLEDGE 



[Mat 1, 1898. 



cluster the prominence forms do not appear to be wholly 

 gaseous or nebulous, they are marked out by a series of 

 stellar points. 



The scale of the annexed block is not sufficiently large 

 to show the detaUs which are visible in the Lick and 

 Henry photographs, and I wiU endeavour 

 on some future occasion to give the readers 

 of Knowxedge a tint block showing the 

 details of this remarkable structure on a 

 larger scale. To my mind the form of the 

 Fig. 6. — The structure indicates the existence of a stream 

 Prominence-like of matter on a colossal scale rushing 

 Structure. outward from the central region of the 



cluster, and the broadened summit of the structure seems 

 to me to indicate the passage of the stream through a 

 resisting medium, the density of which is not negligible 

 compared with the density of the out-rushing stream. 

 This would seem to indicate that the density of the stars 

 associated with the out-rushing stream cannot be very 

 considerable, or their motion would not be retarded by the 

 resistance of a nebulous medium. Any assumption in- 

 volving the existence of an extensive resisting medium, 

 with a density equal to a ten thousand millionth part of 

 the density of our own atmosphere at the sea level, lands 

 us in difficulties similar to those which we pointed out in 

 discussing the density of the Orion nebula," that is, the 

 mass of the nebula would be so great that it would become 

 recognizable to us by reason of the very rapid proper 

 motions of stars in its neighbourhood, whatever were 

 the distance of the nebula. In fact, the greater the 

 distance of the nebula (its angular diameter remaining the 

 same), the greater would be the attracting mass of the 

 nebula, and the apparent motion of stars in its neighbour- 

 hood would remain unchanged. 



Our own sun — if it were expanded so as to fill a sphere 

 with a diameter equal to the diameter of the orbit of 

 Neptune — would have its average density reduced more 

 than two hundred thousand million times ; but since the 

 average density of the sim is 1-444 times that of water, the 

 density of the sun, when expanded so as to fill the orbit of 

 Neptune, would still be considerably greater than a ten 

 thousand millionth of the density of atmospheric air at the 

 sea level. It may easily be shown that a star, similar in 

 density and magnitude to our sun as it now is, would lose 

 an inappreciable fraction f of its velocity if it plunged 

 diametrically through the matter of the solar system 

 when heated up so as uniformly to fill such a Neptunian 

 sphere, even if we assume that in passing through such a 

 Neptunian mass the star cut a tunnel of ten times its own 

 diameter, and gave to all the matter within the tunnel a 

 velocity similar to its own initial velocity on entering. It 

 seems to me, therefore, that we may conclude that the 

 stellar masses which lie along the gigantic structure 

 shown in Fig. 6 must have very small masses compared 

 with our sun, for their density must be very small indeed 

 compared with the density of our sun. In fact, they must 

 be quite a difl'ereut type of body from the stars we know, 

 if their density is comparable with the average density of 

 the resisting medium which fills the interspaces between 

 the stars of such a cluster. 



Science Notes. 



— ..♦-I — 



An enormous meteorite, weighing nearly one ton, from 

 Western Australia, has been received by Mr. .J. R. Gregory, 

 of Charlotte Street, Filzroy Square. It is 4 feet 2 inches 

 long by 2 feet 3 inches wide, and 2 feet 6 inches thick. 



* " old and JVevv Astronomy," p. 793. 

 t Less than a three hundred thousandth part. 



From observations of the minor planet Victoria, made at 

 twenty-one meridian observatories in 1889, Dr. Gill has 

 obtained the value 8-809" for the solar parallax, with a 

 probable error of i 0-0066". An incidental result brought 

 out by the discussion of the observations is that Leverrier's 

 determination of the perturbation of the moon's action on 

 the earth is too high. If this be so, then the value 

 generally accepted for the mass of the moon will have to 

 be decreased by about one per cent. 



The Astronomical Journal, No. 292, contains some notes 

 by Prof. Asaph Hall on the masses of Mars, Saturn, 

 Uranus, and Neptune, determined by observations of 

 their satellites. The definitive values obtained are as 

 follows :— 



Mass of Mars 

 Mass of Saturn 

 Mass of Uranus ■ 

 Mass of Neptune - 



3.104,7 00 



2 2,7 6 5 



1 



1 9,14 9 



of the sun's mass. 



There is still a good deal of speculation with regard to 

 the substance which Prof. Dewar lately exhibited as solid 

 air. The frozen mass might contain both solid oxygen and 

 solid nitrogen, or solid nitrogen alone with liquid oxygen 

 interspersed. Substances which possess the property of 

 retaining liquids in an apparently solid form — like gelatine 

 — are known as " colloids," but it would be hazardous at 

 present to assert that nitrogen belongs to this class. Air, 

 however, becomes solid at a temperature and pressure at 

 which pure oxygen retains its gaseous state. 



v-*^ 



In a review of " The Old and New Astronomy," in the 

 current number of the Edinbitri/h Beview, the writer, whilst 

 admitting the existence of "singular vacuities" and 

 "star-fenced lanes" in the Milky Way, advocates the 

 theory that " such vacancies are undoubtedly what they 

 seem ; they are obscure simply because they are destitute 

 of stars. They are negative entities." Though the 

 evidence for the actual existence of dark structures is not 

 accepted by the reviewer, he freely admits the actual 

 existence of bright tree-like forms springing from dark 

 areas. It is remarkable how differently various minds 

 estimate evidence from probability, which cannot con- 

 veniently be expressed in numbers. 



« ♦ » 



Two Akka girls, representatives of the pigmy race of 

 Africa, who were rescued from Arab captors by Dr. 

 Stuhlmann and his companions, have been brought to 

 Europe, and will remain in Germany for some months. In 

 the summer they will be taken back to Africa, where they 

 will be placed in some mission house, or otherwise provided 

 for. Though probably from seventeen to twenty yearsof age, 

 they are only as tall as an ordinary boy of eight. According 

 to a correspondent of the / 'a ih/ Nrw.i, who saw them at Naples, 

 their behaviour is " infantile, wild, and shy, but without 

 timidity .... They showed neither wonder nor 

 admiration of the people and things around them." 



The London Geological Field Class commences its 

 summer aftei-noon excursions this year on the 29th April. 

 The excursions are planned by Prof. H. G. Seeley, who 

 accompanies the class every Saturday afternoon, whether 

 the weather is wet or fine, from the end of April till the 

 end of July. The class visits classical geological sections 

 and places of interest in the neighbourhood of London. 

 The railway journey generally occupies from half an hour 

 to an hour, and the distance walked is from three to six 

 miles. The class is open to ladies and gentlemen, and 

 the fee for the twelve excursions is one guinea. 



