28 



NATURE 



[May 8, 1879 



May I trespass upon your space so far as to ask Mr. Christie 

 to explain a little more clearly what the precise carbon bands 

 are with which the bands in the comet-spectmm are coincident. 

 He speaks of the less-refrangible edge of the brightest comet 

 band coinciding with the corresponding edge of the green 

 carbon-band at 5200 — in the spectrum of an alcohol vacuum- 

 tube — and then proceeds to remark that the bands in the 

 spectrum of alcohol are identical with those in the spectrum of 

 olefiaut gas and of carbon-dioxide and carbon-monoxide. If we 

 are to understand that all the gases are inclosed in vacuum-tubes, 

 then it must be remembered that the spectrum they give is the 

 second spectrum of carbon in which the brightest bands have the 

 wave-lengths S6l0"5, 5I98"4, 4834, and 4505. But it appears 

 from Prof, Young's comparison of the comet spectrum with the 

 blue flame of a Bunsen burner, that the brighter band agrees 

 with that of the first spectrum of carbon whose wave-length is 

 5i6S'5 within about the interval of the * lines 



(*i - *< = 5'83'o - S'667 = 16-3). 

 The spectrum with which Huggins compared the spectrum of 

 Comet II. 1868, and of Coggia's comet in 1874, was obtained 

 by taking the spark in olefiant gas at the ordinary pressure, and 

 is therefore again the first spectrum of carbon. 



The following comparison will exhibit the uncertainty which 

 I wish to have explained : — 



Fositloa of least refrangible Edges of Band:>. 



Brorsen's comet 5600 ... 5200 ... — 



Carbon spectrum 1 5^34'7 ■■■ 5i6S'3 ... 4739'8 



Carbon spectrum II S^lo'S ••• 5I98'4 ... 4834 



May I suggest the following comparison : to bring up the 

 occulting bar from the blue end of the spectrum till it just covers 

 the brightest comet band, then to introduce into the telescope 

 first the light from a Bunsen burner or blowpipe flame, and then 

 that from a vacuum tube inclosing carbonic oxide. 



If the comet spectrum is that of Carbon I., light will be seen 

 in the second case, but none in the first ; if it be that of Carbon 

 II., no light will be seen in either case. 



Giggleswick, May 5 William Marshall Watts 



I CAN fully confirm Young's observation that the spectrum of 

 Brorsen's comet is not now the same as that observed by Huggins 

 in 18681 as figured in Roscoe's "Spectrum Analysis," p. 251. 

 On the 28th and 30th I observed the spectrum with a Browning's 

 " miniature spectroscope " on a 4i-inch refractor, and compared 

 it with the carbon spectrum of a low gas flame, and found the 

 three usual bands of the latter to coincide with the three bands 

 of the comet as completely as my instruments would show. It 

 was needful to use a wide slit. T. W. Backhouse 



Sunderland, May 6 



Temperature Equilibrium in the Universe in Relation to 

 the Kinetic Theory 



I AM inclined to think I shall best answer Mr. W. Muir's 

 letter by not disputing the vague charges of unsoundness he has 

 brought against me, but in endeavouring to make more clear 

 the position for which I contend. 



The object of my paper (Nature, vol. xix. p. 460) was to 

 contest the necessity of supposing that existing physical prin- 

 ciples must have been violated in past time. I sought to prove 

 that there was no reality in this necessity, by showing that even 

 from our present imperfect knowledge, an explanation for the 

 existing state of things might be evolved cortsistent with prin- 

 ciples which at present prevail. Perhaps I may do well to add 

 a few words in order to elucidate a point which was not too 

 clearly expressed. 



For mere sake of illustration, let us imagine a spherical enve- 

 lope which permits neither change of volume nor passage of 

 heat, to inclose a space of diameter, say 10'° times the distance 

 between the sun and Sirius. First, let all the matter within this 

 space be at the zero of temperature. Second, let all the matter 

 within our envelope be at such a temperature that it is entirely 

 dissociated into discrete molecules. Between these two extremes 

 there is room for any number of mean states in which matter might 

 be more or less aggregated, or discrete, and my point was that the 

 universe might actually be in one of these intermediate states. 

 It should be scarcely necessary to observe that we have limited 

 our space merely for the sake of fixing our ideas. All that we 

 require is gained, if, instead of using the impermeable envelope, 

 we surround our sphere with infinite space filled with matter in 

 va similar condition to that which the sphere inclosed. It is 



important to note that the volumes must be taken large enough 

 to form a fair sample of the general state of the universe. 

 Prof. Clerk Maxwell has shown ("Theory of Heat," p. 328) 

 that a demon existing inside a gas might find irregularity where 

 to us giants all appears uniform. With respect to the universe, 

 may we not be in the position of demons? 



London, April 29 S. Tolver Priston 



Barometric Pressure and Sun-Spots 



In his letter to Nature, toI. xviii. p. 567, on "Sun-Spots 

 and Weather," Mr. Fred. Chambers has shown that the curve 

 of mean barometric pressure at Bombay throughout the year 

 varies with the inverted sun-spot curve. Taking this fact to- 

 gether with the commonly-received idea that the annual varia- 

 tion of barometric pressure in Central Asia is due to the corre- 

 sponding annual variation of solar radiation, he thence con 

 eludes that "the sun is hottest about the time that the spots 

 are at a maximum, and coldest about the time when they are at 

 a minimum." Now, even if the validity of the logical process 

 by which "secular" is substituted for "annual" in this argu- 

 ment be admitted to hold in a general way, have we any reason 

 to suppose that the atmospheric conditions at Bombay, a marine 

 station on a peninsula, can be adequately taken to represent 

 those which prevail in the centre of the Asiatic continent, or 

 that they approximate to the latter to any greater extent, or 

 even as much as those at St. Petersburg, for example ? 



On the other hand, if conditions which presumably reach their 

 maximum intensity in the centre of the continent are so distinctly 

 marked at such a distance from it as Bombay, they should at 

 least be visible to some extent at St. Petersburg, which is cer- 

 tainly more continental in position, if not actually nearer the 

 centre of the continent than the former city. 



Such being the case, I should be glad to know how Mr. 

 Chambers would account for the following remarkable fact, viz., 

 that the mean annual barometric pressures at St. Petersburg from 

 1822 to 1871 show a well-defined relation to the sun-spots />re- 

 cisely the reverse of that evinced by the figures for Bombay. 



I might, if I had followed Mr. Chambers's example, have 

 concluded, with as good grounds for my opinion, that "the 

 sun is coldest when most spotted, " and vice versa, but I prefer 

 to wait until more extensive investigations have given us a 

 sounder basis for induction than at present exists. Meanwhile, 

 I place before your readers the figures on which my statement 

 regarding the St. Petersburg pressures is based. The employ- 

 ment of a variety o< methods of comparison has invariably given 

 the same results. 



In the following table the variations from the mean, expressed 

 in millimetres, are compared with the sun-spots according to the 

 plan recommended by Mr. Meldrum, and which for some pur- 

 poses is superior to those generally adopted hitherto. For 

 brevity's sake only the final columns are given : — 

 Mean Cyclet 



Max. years Ik 5th line. 



Mid. yeare in 7th Inie. 



It will be noticed that the pressure epochs lag behind the 

 sun-spot epochs in the same way as the air-temperature epochs 

 determined by Dr. Koppen. 



The figures for the pressure are taken from the Annals of the 

 Central Observatory. The above relation was first brought to 

 my notice by my friend, Mr. S. A. Hill, of Allahabad. 



Mr. Chambers's notion that "if the winter rainfall of 

 Northern India is really due to the cold of winter, we should 

 expect it to be greatest when the sun is coldest " is partly 



