Dec. 2, 1886] 



NATURE 



I laid the theory before the late Prebendary Webb a few years 

 ago, and some selections from it were published in the Journal 

 of the Liverpool Astronomical Society, and, being nece-sarily in- 

 complete, the extracts were not very intelligible. I have never 

 attempted the settlement of the lunar surface temperature, which 

 is quite beyond me, leaving the same in the hands of Prof. 

 Langley, and have confined myself to the solution of the peculiar 

 and unearthly surfacing we see. This I find best explained by 

 glaciation, under conditions of intense cold, say -60° or 80° C, 

 and absence of all gaseous atmosphere. 



I quite indorse Capt. Ericsson's conclusions as to the extreme 

 unlikelihood of such a small globe being finally surfaced by 

 igneous agencies, after it had seas of water, atmosphere, and 

 probably polar caps. 



Neison, in his "Moon," page 41, line 7i distinctly implies 

 that this took place, i.e. "that this high temperature could only 

 arise after the practical disappearance of bodies of water from 

 the lunar surface," the rise in lunar temperature being due to 

 solar heat. 



I caimot follow Neison in this, and, on the contrary, believe 

 that the temperature has steadily, if slowly, declined, from a 

 period when there was erosion, with air and water. Polar caps 

 then formed, as on our earth and Mars, and extended as the 

 temperature fell, until at last the entire globe was cased in ice, 

 the last portions to glaciate being what we call the equatorial 

 seas. 



Like Capt. Ericsson, I look on the craters and walled plains 

 as bavin;;; been lagoons of water, left here and there as glacia- 

 tion extended, at places of greater depth, or more likely as 

 submarine volcanic vents, for we see their sites as craterlets and 

 cones after final glaciation. 



The aqueous vapjur given off from these lagoons would form 

 a local dome-shaped atmosphere that would retard explosive 

 ebullition, and on its reaching the outer limit of critical tem- 

 perature, would condense and fall as snow ; what fell beyond 

 the lagoon margin would pile to form the ring, and the lagoon 

 surface or flow be gradually lowered by its removal. 



But I cannot follow Capt. Ericsson in supposing that the 

 water had a centrifugal motion, and acted as a gigantic carving- 

 tool, that sculptured the enormous terraces in Tycho, Theo- 

 philus, lie. On the contrary, I look on it as a quiet process, 

 and that all the circular for j.s, from small craterlets to even such 

 forms as Mare Crisium or Imbrium, with its huge maritime 

 ranges, are due to one cause. The series is complete. 



I quite agree with Mr. Darwin that a layer of water vapour 

 would exist (and be visible) over the ice on the moon if only the 

 temperature be high enough ; but, at very low temperatures, ice 

 practically doe^ not vaporise even in vacuo (see Ganot's 

 " Physics "). Aqueous vapour not being seen, I conclude the 

 temperature is below (say) - 80° C. But the most potent argu- 

 ment in f.ivO(.ir of my theory is that it reasonably and consistently 

 explains a// the peculiar features of lunar surfacing, i.e. :— 



The absence of distinct Polar caps ; 



The absence of water and aqueous vapour (now) ; 



The absence of distinct colour in details ; 



The brightness of all raised, rugged surfaces, mountains, 

 cliffs, peaks ; 



The relative darkness of levels whereon meteoric dust can lie ; 



The extraordinary circularity of forms, large and small, in- 

 complete, or overlapped ; 



The cones, whether central or isolated ; 



The clefts or rills, also strings of craterlets ; 



The maritime zones, ridges, and banks ; 



The haze or cloud, and nimbus or rayed brightness ; 



The da'k points seen by Dr. Klein ; 



Lastly, if not least, the long bright rays. 



I do not think I overstate the case when I say that seleno- 

 graphers will find these features consistently solved by the one 

 hypotheis, and no enigmas left. 



I cann A a^k for space to go into details here, but will forward 

 a shirt synopsis of the leading features, in case they may be 

 required, arranging them as nearly as may be as in the pre- 

 ceding lit. S. E. Peal 



Sibsagar, .\s3am, October 13 



The Astronomical Theory of the Great Ice Age 



The lecture and the letter of Sir Robert Ball, however lucid, 

 do not appear to carry this question further than where Dr Croll 

 left it. It is easy to understand that when the shape of the 



earth's orbit was different, winter days might be colder and 

 summer days hotter than now. What the theory at present 

 wants is an exposition of the successive series of effects by which 

 this state of climates would transform the Emerald Isle into a 

 mere Greenland. It is scarcely an explanation to say that " vast 

 fluctuations like these must correspond to vast climatic changes 

 of the kind postulated." We desire to be shown that they will 

 correspond, and that the correspondence will be of the kind 

 required. Taking .Sir Robert Ball's own illustration, I am quite 

 ready to admit that his horse alternately starved and crammed 

 will not run a dead heat with one itniformly fed ; but in default 

 of experience I should not feel certain that his animal would die 

 of accumulated fat. 



We know that there have been past periods of heat-supply 

 more uniform than at pre-ent, and periods of wider fluctuation. 

 We see also in geological records ages of vast snow accumula- 

 tion and ages of rich vegetation near the Pole. We need a 

 demonstration that such wider fluctuations do tend to the one 

 and not to the other ; towards snow-accumulation and not 

 towards snow-dissipation. Attempts in this direction have been 

 made, but much seems needed yet. E. Hill 



St. John's College, Cambridge, November 23 



Meteor 



The large meteor described in Nature by Mr. P. \j. Sclater, 

 was observed here as follows : — 



Nov. 17, 7h. 1 8m. — Fireball many times brighter than Venus. 

 Path from 32|° + 45° to IsS'-t-SS". Motion very slow, duration 

 7 seconds. Train, but no enduring streak. The fireball, as it 

 gradually descended to the northern horizon, varied greatly in 

 brilliancy, and gave a series of flashes lighting up the sky with 

 great effect. I have occasionally seen larger fireballs, but never 

 observed one more satisfactorily. This meteor was observed at 

 Handswortb, Birmingham ; at Crawshaw Booth, Lancashire ; 

 and at many other parts of the country. Its unusual brightness 

 seems to have attracted wide notice. 



Fireballs from Taurus are often seen at about this epoch ; but 

 that of November 17 appears to have belonged to a radiant- 

 point in Aries. W. F. Denning 



Bristol 



Freshwater Diatoms in the Bagshot Beds 



Will you kindly favour me with space to ask any of your 

 numerous readers, who may be specially interested, if they can 

 furnish me with any references to published records of fresh- 

 water Diat'jms being observed in the carbonaceous earthy sands 

 of the Middle and Lower Bagshot Beds of the London Basin? 

 In conjunction with one of my pupils, I have lately subjected 

 many of these green and dark-grey sands and earths to micro- 

 scopic examination ; and our labours have been rewarded by the 

 discovery of a rather extensive unicellular flora, particulars of 

 which will be shortly laid before the Geological Society. Mean- 

 while, I shall be happy to have the co-operation of other 

 workers in the same field. A. Irving 



Wellington College, Berks, November 28 



THE MATHEMATICAL TRIPOS^ 

 I. 



IT is with the greatest pleasure that I avail myself this 

 evening of the already well-established custom which 

 permits one of our members, once in two years, to address 

 to his colleagues a few general remarks connected with 

 the science that forms our common bond of union. 

 It is not often that a mathematician has an opportunity 

 of laying before his fellow workers, by word of mouth, any 

 views of his except such as relate to the actual mathe- 

 matical investigations upon which he is engaged, which, 

 from their very nature, can appeal directly only to the 

 few who have laboured in the same field ; and I feel it to 

 be a high privilege to be permitted, in this room, and sur- 

 rounded by familiar faces, to give expression to my thoughts 

 and hopes upon subjects that are of common interest to 

 us all as mathematicians. 



* Address delivered before the London Mathematical Society by the 

 President, Mr. J. W. L. Glaisher, M.A., F.R.S., on vacating the chair 



