September 1, 1892.] 



KNOWLEDGE 



171 



type of spectrum seems to have been presented only during 

 a compai-atively short stage in the development of this 

 remarkable nebulous variable. 



The Auriga Nova is evidently not diminishing uniformly 

 in brightness. According to an article by Dr. Copeland, 

 published in the August number of Professor Hale's 

 Jiiiiniiil (if Astroni'iiii/ and Astro- Plii/sics, p. 600, the Nova 

 exhibited continual and irregular changes of brightness 

 throughout the month of February. On the •20th of March 

 it had dropped to the 9-1 magnitude, and by March 28th 

 to the ll'O magnitude.* Its present estimated magnitude, 

 92, would indicate a rise of more than 2t magnitudes. 

 These irregular changes in brightness and in spectrum 

 favour the theory, suggested in the June number of Know- 

 ledge, that the light of the Nova is due to its passage 

 through an irregular nebula rather than to a single collision 

 or near iipproach of two stars moving in nearly opposite 

 directions, as suggested by Dr. Huggins. — A. C. Hanyard.] 



To the Editor of Kno^xedge. 



Sir, — An active aurora of' great brilliancy was visible 

 here on the evening of Friday last from 9 till 10 p.m. 



The whole of the sky, fi-om N.'\\'. to N.E. and from 

 horizon to zenith, was filled with a mass of streamers. 

 The light was sufficient for reading moderately large type. 

 The streamers and rays were projected from the upper 

 edge of a low arch of dark-coloured vapour resting on the 

 northern horizon. The space occupied by the points of 

 the streamers covered the constellations Ursa Major on 

 the west and Cassiopceia on the east, and the intermediate 

 region. Among the brilliant sheaf of white streamers, an 

 occasional dark ray shot upwards from the generating 

 arch. J. Lloyd Bozward. 



Worcester, 17th August, 1892. 



AEE THE LIXAR CLEFTS ■ RIVER BEDS," OR 

 FRACTURES OF THE CRUST? 



To the Editor of Knowledge. 



Dear Sir, — One of the most remarkable geological 

 discoveries of modern times is the fact that the floors of 

 our great oceans are vast areas of slow permanent sub- 

 sidence, by the steady sinking "in of which, through 

 shrinkage due to secular cooling, the persistent degradation 

 of the continents is counteracted. Mr. J. Murray tells us 

 that " the result of many lines of investigation seems to 

 show that in .the abysmal regions we have the most 

 pennanent areas of the earth's surface ' ; and M. Faye 

 points out that " under the oceans the globe cools down 

 more rapidly and to a greater depth than beneath the 

 surface of the continents — at a depth of 4000 metres the 

 ocean will still have a temperature not remote from O^C, 

 while at a similar depth beneath the earth's crust the 

 temperature will be not far from 150° C." This greater 

 relative density of the crust beneath the oceans compared 

 to the continents has been amply proved by a long series 

 of pendulum experiments, and is accepted as an axiom. 

 According to Mr. Murray, •' the general aspect of the 

 abysmal regions must be that of vast undulating plains, 

 interrupted here and there by huge volcanic cones," which 

 rising to the surface occasionally, form the oceanic 

 (volcanic) islands. Lying at an average depth of three 

 miles, these areas of permanent subsidence cover i-oths of 

 the globe's surface, and the proofs that they have never 

 formed part of a continental surface are very clear in 

 several ways, one being the entire absence of stratified rock 



* According to Professor E. C. Pickering's measures, it liacl dropped 

 to the 14'3 magnitude on April 13tli. See Astroaomii and Astro- 

 Phiisiis. May, 1892, p. 417. 



fragments (sandstones, &c.') from the ejecta of oceanic 

 volcanoes, while so common in those existing on continents. 

 The area of the shallow and enclosed seas is put at f 'jths of 

 the earth's surface, that of the land being t'b'^Iis, the mean 

 elevation of the latter being 900 feet. Both of these con- 

 trast strongly with the abysmal plains, inasmuch as they 

 are marked by great inequalities of level, the fluctuations 

 being revealed in the strata. The fact for us to note 

 specially in the foregoing is, that the slow shrinkage of 

 our globe — due to secular cooling — is mainly taken up 

 in and by the steady subsidence of our ocean floors, which 

 are the coldest, densest, and hea\-iest portions of the 

 earth's crust. This slow, persistent, and invariable sinking 

 in of the sea bottoms is such a momentous feature in 

 terrestrial surfacing that we may well keep it in view when 

 endeavouring to solve the mystery of lunar details. The 

 question arises, Can we trace on the moon any evidence of 

 subsidence in the marea, or seas ? To begin with, we 

 may legitimately assume that whether the surfaces we see 

 are composed of sand or alluvium — the sea beds laid bare — 

 or whether they are the actual seas, solidified and floe- 

 covered, the absence of vegetation, and of all atmospheric 

 and pluvial erosion, would render all fractures of the crust 

 due to mareal subsidence both more permanent and con- 

 spicuous than they would be on our earth. 'We may also 

 assume Ijeforehand that repeated (if slow) subsidence of 

 any large circular area, such as Mare Serenitatis, would 

 eventually result in the formation of a series of marginal 

 tangential fissures, visible or invisible. Now, on examin- 

 ing the borders of this sea for evidences of subsidence, we 

 at once find tlie most remarkable confirmation of the above. 

 Beginning at the southern margin near Plinius, we find on 

 the great incline a triple system of tangential clefts, 

 one of which, 8, extends in a straight line from Taquet B 

 to Dawes, while another, s, of still greater length, runs on 

 the north towards Mount Argieus. From thence, an 

 almost continuous series of clefts runs tangentiallv around 



Saulk 







the western border of the mare, past Littiow, Le Monnier, 

 and Chacornac, to, and across the walled plain Posidon, on 

 the north-west, taken up faintly in crossing Lacus Som- 

 uiorum, and at the foot of the cliffs east of E, in longitude 

 20' W. — the northern coast line. Again, on the eastern 

 border at N, we have another cleft, tangentially placed, 

 like two others, N and E, at the Sulpicius Gallus on the 

 southern margin, thus completing the circuit of the mare, 



