April 1, 1901." 



KNOWLEDGE. 



75 



from Prof. Pickering {Harvard Circular^ No. 56). It is 

 extremely fortunate that tlio spectrum was photographed 

 both before and at inaxiinum brilliancy. At. these 

 stages in the history of the star. Prof. Pickering finds the 

 spectrum to have been distinctly of the Orion type, with 

 a few bright lines, chiefly on the red sides of dark ones. 

 Bv the ■J4th, however, the spectrum had undergone a 

 most exlriuirdinan* change, and then resembled the 

 well-known spectrum of Xova Aurigie, as it did on the 

 25tb when photographed at Kensington. The full signi- 

 ficance of this transformation is not yet dear, but it was 

 evidently a most important event in the development 

 of the new star. 



The tinal stage, if Xova Persei behaves like Nova 

 Cygni. Xova Aui'iga>. and others, which it resenibled 

 soon after discoverv. will be that of a planetary nebula. 



Fio. 2.— Persons. 



It may be worth while to add that up to March 3rd 

 the principal features of the beautiful spectrum of Nova 

 Persei were observable with a McCIean spectroscope in 

 use with a two or three inch telescope. This form of 

 spectroscope is almost as easy to manipulate as an 

 ordinary eyepiece, and is especially useful when it is 

 not intended to take measurements of the lines. 



Numerous theories have been advanced to account for 

 .such outbursts as' that of Nova Persei, but judgment on 

 them should perhaps be suspended until the present nova 

 has been more completely studied. Still, a few remarks 

 on this question may not be out of place. Attempts to 

 explain a new star as the result of some action taking 

 place in a single body have not been considered success- 

 ful, chiefly on account of the diiBculty of explaining the 

 nebular state which most of them finally assume, and 

 the collision theory in one form or another 

 appears to be of much more general application. 

 On Sir Norman Lockyer's hypothesis, the fact that a 

 nebula remains is taken as an indication that a nebula 

 — previously unrecorded — was there to begin with, and 

 the outburst is attributed to the passage of a dense 

 swarm of meteorites with great velocity through the 

 sparser swarm composing the nebula; the denser swarm, 

 on this view, is responsible for the dark line spectrum 

 and the sparser one for the bright lines. Mr. Monck, 



in 1885, compared the action to that which takes place 

 in a shooting star, and suggested that "now stars aro 

 dark (or faintly luminous) bodies which acquire a short- 

 lived brilliancy by rushing through some of the gaseous 

 masses which exist in space " : a somewh.at similar view 

 has been adopted and extended by Prof. Sceligcr. Many 

 of the phenomena seem to bo explained on either of these 

 suppositions, but collections of small bodies appear to 

 be most consistent with the rapid cooling which takes 

 place. In either case, the original nebula or " cosmic 

 cloud'' might be expected to shine more brightly after 

 the collision than before, in consequence of the disturb- 

 ance to which it has been subjected. The fact that 

 gaseous nebulse, like the new stars, occur in greatest 

 abundance in the neighbourhood of the Milky Way 

 strengthens the collision theory, and it may be noted, 

 too, that on account of their greater dimensions nebulae 

 ai'e much more likely than stars to be struck by 

 wandering bodies. 



It may be remarked that there is another possible 

 explanation of the pairs of bright and dark lines which 

 are found in the spectra of new stars, besides that which 

 refers them to two bodies having a great relative velocity 

 in the line of sight. The lines in certain spectra are 

 displaced slightly when the vapours producing them 

 are under pressure, and Prof. Wilsing has found that 

 in spark discharges through liquids, pairs of lines some- 

 what resembling those of new stars appear in the 

 spectrum. The evidence on this point, however, is 

 incomplete. 



As the great brightness of Nova Persei allowed of 

 the application of powerful spectroscopes, it may bo 

 expected that a complete study of the spectra which 

 have been observed and photographed will throw 

 additional light on the origin of such celestial outbursts, 

 and doubtless also on various other questions relating to 

 the progress of events in stellar evolution. 



It is satisfactory to know that the new star was seen 

 independently by several observers, though Dr. Anderson 

 appears to have been the first in point of time, and it 

 was through him that the news was at once circulated. 

 This, it wHl be remembered, is the second discovery of 

 the kind with which Dr. Anderson's diligence has been 

 rewarded, the previous one being that of Nova Aurigae 

 in 1892. 



THE WHITE NILE-FROM KHARTOUM TO 

 KAWA. 



AN ORNITHOLOGISTS EXPERIENCES IN THE SOUDAN. 



By Harry F. Witherby, f.z.s., m.b.o.u. 



I._THE DESERT RAILWAY, KHARTOUM, AND 



OMDURMAN. 

 Shortly after the battle of Om Debreikat and the 

 death of the Khalifa Abdullahi, I found that it would 

 be possible to travel in the Soudan. For fifteen years 

 the country had been in the hands of powerful savage 

 tribes, and before that time English naturalists, and 

 espe9ially ornithologists, had greatly neglected the 

 Soudan, although some Germans, as for instance Brehm, 

 Hcuglin, and von Miiller, h;Kl done excellent work there. 

 The country being thus more or less new ground to 

 English ornithologists, I was the more anxious to visit 

 it, and study the birds of a small portion at all events of 

 what had become English territory, at least by right of 

 conquest. 



