DISCOVERY 



303 



the spiral nebulae — that they are composed of clouds of 

 dust expelled from the galactic system by radiation- 

 pressure and that they shine by reflected light — has been 

 fruitful in provoking much discussion. Professor Perrine, 

 the distinguished American astronomer who directs the 

 Argentine National Observatory, has been working at 

 the same problem and summarises his views in a recent 

 article. There are se^•eral points of agreement between 

 Professor Perrine and Professor Lindemann. According 

 to both h\-potheses the spirals have had their origin in 

 dust-particles driven out of the galactic system by light- 

 pressure. Dr. Lindemann, however, believes that the 

 spirals shine by reflected light and are non-stellar, i.e. 

 truly nebulous bodies ; while Dr. Perrine maintains that 

 these conclusions are quite untenable. " In one of these 

 objects emission has been observed, and in at least one 

 case known to me (N.G.C. i,o68) it is clearly established 

 that both emission and absorption give the same radial 

 ^•elocity. This could not be the case if the absorption 

 spectrum was due to reflected light." In the main. 

 Professor Perrine holds, the spirals are composed of stars 

 and the cosmical matter in them is merely incidental. 



^^'hat, then, are the spirals ? They are not external 

 galaxies, but are " relatively small yet more or less 

 autonomous sj-tems, comparable to the globular clusters, 

 which for the most part have been ejected from our stellar 

 system chiefly in the form of cosmical clouds by light- 

 pressure." They have, however, now condensed to form 

 stars, though nebulous matter is also present in them. 



The great velocities indicate that they are beyond the 

 gravitational control of our system, and are gradually 

 becoming autonomous svstems. " If the spirals," Dr. 

 Perrine asks, " are so certainly finding their way into the 

 great outer spaces, is it not logical to expect that there is 

 something there for them to go to ? " The bearing of 

 this on Einstein's view of a finite universe is obvious. 

 .\lso, one is tempted to ask, ha\e we here the secret of 

 the renewal of the universe ? Willmanvof these compara- 

 tively' small systems which we call the spirals come together 

 and form a galaxy or galaxies similar to our own, which 

 has, on Dr. Shapley's cosmogonic hypothesis, been formed 

 by the union of many star-clusters ? Dr. Perrine truly 

 remarks that the study of the spirals " appears to be one 

 of the most promising fields in the science." 



The New Astronomer at the Cape 



The post of His Majesty's Astronomer at the Cape, 

 vacant by the death of Mr. Hough, has been filled by the 

 appointment of Mr. Harold Spencer Jones, M.A., B.Sc, 

 F.R.A.S., chief assistant at Greenwich Observatory. Mr. 

 Jones, after a brilliant academic career, was appointed 

 to his present post in 1913, and has been a prominent 

 figure in the scientific world. Last year he was in charge 

 of the Greenwich expedition to Christmas Island for 

 the purpose of observing the total solar eclipse. Cloudy 

 weather, howe\er, prevented a view of the eclipse. Mr. 

 Jones is a co-editor of The Observatory and author of an 

 authoritative textbook on General Astronomy. 



Hector M.\cphersox. 



The Air Ministry and 

 Atmospheric Problems 



By R. J. V. Pulvertaft, B.A. 



I. Testing a City's Air 



LoXDOX, according to a French critic, is a town which 

 is only inhabitable because the natives spend the 

 summer in Scotland, the winter in the South of France, 

 and the remaining months of the year in bed. The 

 criticism is unkind, and Londoners can at least proudly 

 boast of one of the world's lowest death-rates. A 

 large city, with the resources of modern sanitation at 

 its command, has advantages unknown to the Riviera 

 or the Highlands. But even the most patriotic Lon- 

 doner cannot feel altogether happy about the smoke 

 and impure air which he is forced to breathe. Quite 

 apart from fogs, with their constant sequel of illnesses 

 and deaths due to chest complaints, the pollution of 

 the atmosphere in towns has effects which are everj'- 

 where to be noticed. 



For example, man\^ moths, which in purer air wear 

 a coat of modest grey, are forced in London to clothe 

 themselves in funereal colours. The pepper-and-salt 

 moth, within living memory, has almost disappeared, 

 in its normal variety, from the London suburbs, and 

 has been replaced b\' a completely black and much 

 smaller variety, formerly rarely seen. The spring 

 usher moth — a delicate insect which justifies its poetic 

 name by emerging " before the swallow dares " — takes 

 the winds of March with its beauty, nowadays, dis- 

 guised on the oaks of X'l'imbledon Common in a cloak 

 of deeply banded black. Again, the Elgin marbles, 

 which saw many a century die in Greece without 

 losing the white gleam of their marble, have acquired 

 in London the prevailing dingy colour of their new 

 land. The repairs of Westminster Abbey have largely 

 been made necessary by the decaj' of its stone under 

 the influence of the impure atmosphere. 



" They dreamt not of a perishable home 

 Who thus could build " — 



sang Wordsworth ; and they built it, therefore, of a 

 stone very perishable in the smoke-ridden air of which, 

 also, they never dreamed. 



We are getting better, it is true. London has not 

 nearly as unlo\-ely a sk\' as it had twent\' years ago ; 

 gas and electric heating have made great improve- 

 ments. Though London can never hope for the 

 gleaming whiteness of towns such as Paris, Turin, or 

 Alexandria, her atmosphere is brighter and cleaner 

 than many other large towns and cities. The Advisory 

 Committee of the Air Ministry has recently published 

 a report on Atmospheric Pollution which contains 



