Aug. 26, 1875] 



NATURE 



329 



The first two sections, both of them on Ancient | 

 Bristol, are by Mr. J. Taylor, of the Bristol Library. 

 Section 3, on Modern Bristol, is by Mr. J. F. Nicholls, of 

 the City Library. The fourth section, on Local Govern- 

 ment and Taxation, is by Mr. H. Naish : and then follows 

 a section on Educational Organisations, to which there are 

 several contributors. Mr. D. Davies, the medical officer 

 of health, has supplied the section on Sanitary Condi- 

 tion and Arrangements, after which comes Section 7, on 

 Physical Geography and Geology. This occupies sixty- 

 four pages, and would perhaps have been of more prac- 

 tical use if printed as a separate pamphlet that could be 

 conveniently carried in the pocket. Mr. Tawney has 

 written the Introduction ; the Silurian, the Carboniferous, 

 and Millstone Grit is by Mr. Stoddart ; the part on the 

 Coal Measures and " New Red Period " is written by Mr. 

 Tawney ; that on the Rhgetic and Liassic by Mr. Ralph 

 Tate, and the concluding part on the Inferior Oolite is 

 again by Mr. Tawney. 



Bristol is better off for geological maps than any other i 

 part of the country, for not only are there the sheets of 

 the Geological Survey, but there is Mr. Sanders' splendid 

 map of six inches to the mile, which includes the whole 

 of the Bristol coal-field. 



It is a pity there was not a sketch map introduced in the 

 guide, with just the names given of the places referred to 

 and an indication of the spots where the sections are 

 taken from. As it is, strangers to the district will expe- 

 rience some difficulty in following the text, as many of 

 the names are not on the published maps. With regard 

 to the sections, too, there is no indication of the direction 

 in which they are taken, nor of the scale to which they 

 are drawn. One of the most useful features of the geolo- 

 gical portion is that which gives the localities where the 

 sections of the strata can be seen ; and, as the district 

 within a short distance contains from the Silurian up to 

 the Oolites, omitting the Permian, is of interest. There 

 are many references to the more important papers that 

 have been printed, and in cases of difference of opinion the 

 writer has added his own views. The much vexed ques- 

 tion of the age of the " dolomitic," " triassic," " magne- 

 sian," or " reptilian " conglomerate, is duly referred to. 



The notes on anthropology have reference to the tumuli 

 and chambered barrows, and to the present condition of 

 Bristolians. " A certain amount of physical degeneration 

 has taken place among the native Bristolians, as among 

 the natives of other British cities ; 300 of them yielded 

 to me an average stature and weight of 5 feet 5-8 inches 

 and 132I lbs., after deductions made for shoes and cloth- 

 ing. The average height of men in the surrounding 

 counties may fairly be put at half an inch more." 



The book has one serious defect, for which the compiler 

 and not the authors are responsible ; there is no index. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the v/riters of, rejected manuscripts. 

 No ttotice is taken of anonymous communications,'^ 

 "Climate and Time" 



The review of "Climate and Time" in Nature, vol. xii. 

 p. 121, contains some remarks in reference to my tables of the 

 eccentricity of the earth's orbit, to which, in justice to myself, I 

 I must refer, the more so as they relate to points which compara- 

 1 lively few of your readers have it within their power to deter- 

 mine whether or not the reviewer was justified in making the 

 rema rks in question. 



" We have repeated," says the reviewer, " the calculations for 



two of the most remarkable dates, viz., 850,000 and 900,000 



years ago respectively, and find that at the former date the 



eccentricity was '0697 instead of '0747, and at the latter date 



L was "0278 instead oi '0102 as expressed in the table." 



What proof does the reviewer give that his results are correc 

 and mine incorrect ? The following is the reason he assigns : — 

 "To satisfy ourselves," he says, "that the mistakes are Mr. 

 CroU's and not ours, we have recalculated also one of Mr. 

 Stone's and one of M. Leverrier's, and in both instances have 

 exactly verified them." This can hardly be accepted as sufficient 

 evidence, for I had myself recalculated one of Mr. Stone's and 

 no fewer than five of M. Leverrier's, " and exactly verified 

 them." 



I suspect that the reviewer has made his calculations somevi'hat 

 too hastily ; for if he will go over them a little more carefully, 

 he will, I have no doubt, find that after all my results are per- 

 fectly correct, excepting only a trifling typographical error, to 

 which I shall presently refer. 



The value for 900,000 years ago ought to be '0109 instead of 

 •0102, as stated in the table. This mistake arose out of the 

 curious circumstance of a small speck of ink having been 

 dropped on the tail of the 9, which led to its having been substi- 

 tuted for a 2, ten years ago when the tables were first published — a 

 fact of which I was not aware till a week or two ago, when looking 

 over the manuscripts of my original calculations, all of which I 

 have preserved. Since my calculations were called in question 

 by your reviewer, I have had them examined by three expe- 

 rienced mathematicians, and the conclusion at which each of 

 them has arrived is that they are perfectly correct. 



The reviewer continues : — "The fact that the eccentricity was 

 large when he represents it so, and small when he makes it 

 small, seems to indicate that some approximating progress [pro- 

 cess ?] has been followed, and that possibly his diagram may 

 give a rough idea of the changes of eccentricity for past time." 



I can assure the reviewer that nothing could be further from 

 the truth than this assumption. I have computed the eccen- 

 tricity and longitude of the perihelion for no fewer than 129 

 separate periods, and in every case Leverrier's formulae have 

 been rigidly followed, and I have every reason to believe that 

 the diagram gives not a rough but an accurate idea of the changes 

 of eccentricity. The values given in the tables will, I trust, be 

 found to be perfectly accurate up to at least the fourth place of 

 decimals, which is as far as these fonnulae can be relied upon to 

 yield correct results. 



The following are the results which, considering the trouble 

 that has been given to their verification, I think will stand the 

 most severe scrutiny : — 



Period 850,000 years ago. Period 900,000 years ago. 



h^= -00413927 h^— -000059858 



/*= 00144124 /**= -000059812 



h* + /» = -00558^51 h'* + P =-000119670 



J h'^ + l^ — -0747 = Eccentricity | sjh"^ + /* = -010939 = Eccentricity 



Edinburgh, August 10 James Croll 



A Lunar Rainbow, or an Intra-lunar convergence of 

 Streams of slightly illuminated Cosmic Dust? 



About 8.30 p.m. yesterday a large zone of the sky, from the 

 horizon at W.N.W. to the horizon at E. by S., was illuminated 

 in a very remarkable manner, and this illumination lasted about 

 three-quarters of an hour, when it gradually died out. 



During all this time the sky was very clear and cloudless, 

 thereby forming a dark back-ground, on which the phenomenon, 

 whether lunar rainbow, or many rainbows, or intra-lunar con- 

 verging streams of cosmic matter, was splendidly projected. 



This exhibition consisted of one grand central feather springing 

 out of the horizon at W. N. W. and crossing this meridian at about 

 20° north ot the zeniih. The width of this stream, with little 

 variation throughout its length, was 7° or 8°. Its light was that 

 of a vtry bright white cloud, its edges most beautifully defined ; 

 its iorm that of a very elouf^ated feather, but without any shaft. 

 On either side of this main feather was a system of seven or eight 

 minor and fainter streams, threads, or beams of fight, all more 

 or less extending from the western to the eastern horizon, SBb- 

 tending a chord common to themselves and to the main stream 

 of light, and converging towards the axis of the central stream so 

 as apparently to intersect it at a point about 30° or 40" below the 

 western horizon, at which the whole system subtended an azimuth 

 of about 20°; and near the zeniih, where its iraiisverse section 

 was a maximum, that section subtended an angle of about 40*. 

 At this time the moon was about 15° east of the meridian, and 

 her declination about 9° S. IJoth systems ol the minor streams 

 of light on the sides of the m.iin stream appeared to haT« a. slight 



