212 



KNOWLEDGE 



[NO^-EMBER 1, 1892. 



is in aphelion the earth's distance from the planet is 

 411 million miles. Consequently, at an October oppo- 

 sition, when Jupiter is in perihelion, he appears nearly 

 40 per cent, brighter than at an April opposition when he 

 is iu aphelion. Jupiter presents a great many analogies 

 with the sun. Its density compared with water is 1-378, 

 whereas the density of the sun is slightly greater, viz., 

 1-444. As in the case of the sun's photosphere, the 

 equatorial regions of Jupiter revolve a little more rapidly 

 than the regions more to the north and south, thus pro\ing 

 Jupiter to be, hke the sun, mainly a gaseous body. Its 

 surface, however, presents a very different appearance 

 from the sun's surface. The disc of the planet is crossed 

 by dark belts of cloud which are continually changing 

 their form. On the belts and between them are seen black, 

 white, and reddish spots. Miss Gierke gives an interesting 

 account of one of the largest and most permanent of 

 these, known as the " Great Eed Spot.'' It was first 

 observed by Prof. Pritchett, of Glasgow, Missouri, in 

 1868. and soon attracted considerable attention. After 

 three years of conspicuous brightness its colour began to 

 fade, till in 1882-83 it had almost ceased to be visible. 

 In IS'^S it had begim to recover in brightness and then 

 showed as a faint pink oval ring, with its centre occupied 

 by a white cloud, which in the following year so extended as 

 almost to obliterate its outline. The veil has, however, since 

 cleared away, and left it as an elongated brick-red spot 

 which moves somewhat slower than other markings in the 

 same latitude. The different velocities gave occasion for 

 an interesting observation by Mr. Stanley Williams on the 

 conjimction of the red spot with a dark one, which is a 

 fairly persistent feature of the planet. Its size was such 

 that it would cover about half the shorter diameter of the 

 greater spot if it should pass above it, and its relative speed 

 was suflieieut to carry it across its length in two months. 

 The question how it would behave under these circum- 

 stances was one of considerable interest, for, its transit 

 above or below the red spot would show which of these 

 classes of objects occupied the higher level in the planet's 

 atmosphere. Miss Clarke remarks that the black spot 

 contrived to baffle expectant astronomers by doing neither. 

 It took a third course and went round the obstacle, 

 swerving away to the south and coasting the southern 

 rim of the red spot. We can heartily recommend Miss 

 Clerke's little shilling monograph, and hope that it will 

 start many a reader in the pleasant paths of observational 

 astronomy. 



Mr. Barnard has kindly sent for reproduction in Know- 

 ledge some beautiful photographs of Swift's comet, show- 

 ing remarkable structure in the coma and tail, and 

 proving that the structure changed very rapidly from day 

 to day. 



— » • t — 



One of the results of Mr. Burnham's departure 

 from the Lick Observatory is that a Chicago millionaire, 

 Mr. Charles T. Yerkes, has undertaken to present the 

 University of Chicago with a refractor of 45 inches in 

 diameter — that is, the object-glass will have a diameter 

 greater by 9 inches than that of the great Lick telescope. 

 He has commissioned Mr. Burnham and Prof. Geo. E. Hale 

 to order the instrument, the object-glass of which will be 

 made by Messrs. Alvan Clark. It has been truly said that 

 with these large instruments the man at the small end of 

 the telescope is the most important part of the equipment. 

 With such a telescope, and such an observer as Mr. Burn- 

 ham to use it, Chicago ought to make its mark in the 

 history of astronomy. 



Utttcr 



[The Editor does not hold himself responsible for the opinions or 



statements of correspondents.] 



1 » I 



To the Editor of Knowledge. 



Sir, — In the interesting article in the last number of 

 Knowledge on " A^'hat is a Nebula? " Mr. Ranyard draws 

 a vivid conception of the extreme tenuity of nebular matter. 

 It seems to involve, however, a question which, to my 

 uninformed mind, is very mysterious. I assume the 

 following statements to be substantially correct : — 



1st. The spectrum of such nebular matter shows it to 

 be, in a great number of cases, gaseous. 



2nd. Its luminosity means also high temperature. 



3rd. The temperature of ethereal space is immensely 

 lower than anything we have experience of. 



Under such conditions, how is it possible that this nebular 

 matter can maintain its luminosity and its temperature for 

 a period indefinitely long ? If it is true that all substances 

 cool do^^Ti Ijy radiation into colder space, one would think 

 that a nebula ought to disappear almost as soon as it is 

 formed, instead of streaming out its light for ages. 



Middlesbro', Oct. 22nd, 1892. R. H. 



[We are hardly in a position to say that luminosity 

 necessarily indicates a high temperature. The light of the 

 glow-worm is evidently produced at a low temperature — 

 and the auroral light is given out by matter in the cold 

 regions of our upper atmosphere. It may be that the 

 nebular matter is not all glowing, but that only the matter in 

 isolated and widely separated regions is caused to glow by 

 electrical discharges, or the impact of meteors. Personally, 

 I do not incline to the latter theory, or to the theory of 

 illumination by electrical discharges, but I will explain my 

 ideas in a further paper on nebuln?, for which there is not 

 room in the present number. 



I have to thank Mr. Walter Sang for pointing out a 

 numerical mistake in my last article on nebulae. The 

 density of atmospheric air at the sea-level at standard 

 temperature and pressure is ysVs ^^ ^^^ density of water, 

 not j;^^. And the velocity of our sun in a circular orbit 

 about the Orion nebula at the distance of a. Centauri, 

 assuming the conditions as to density and volume of 

 the nebula mentioned iu the last number, would be 7'5 

 miles, not 18 miles a second. If the nebula had a millionth 

 of the density of atmospheric air, the velocity of our sun 

 in a circular orbit about the nebula, if it were situated 

 at the distance of a Centauri from us, would be 75-08 

 miles a second, and whatever the distance of the nebula 

 under the conditions assumed, the angular velocity of our 

 sun in a circular orbit, as seen from the nebula, would be 

 constant. Similarly, the angular velocity in any elliptic or 

 paraboUc orbit would be constant, and the apparent velocities 

 of stars moving under the control of gravity in the neigh- 

 bourhood of the nebula, as seen from the earth, would be 

 independent of the distance of the nebula. — A. C. Rany.vrd.] 



THE OLDEST MAMMALS. 



By R. Lydekker, B.A.Cantab. 



P to the year 1818 it was a generally received 

 axiom of geology that mammals were totally 

 unknown before the tertiary period ; and that 

 period was consequently designated the age of 

 mammals — a name, by the way, which is still 

 perfectly appropriate, if taken to imply that these animals 

 then, and then only, became the dominant inhabitants of 

 the world. In that vcar, however, the illustrious Cuvier, 



u 



