454 



NATURE 



\Oct. 3, 1872 



eastward velocity which it possessed when it left the equatorial 

 regions. It is a matter of indifference in what way this energy 

 is consumed by the molecules of the water, whetlier it be in 

 friction ui rotation, or whether it liecomes potential in the raised 

 water tliiough whicli the current Hows ; for in either case it is tlie 

 resistance offered by the stationary molecules which causes the 

 moving molecules to lose tlieir velocity. The resistance being 

 molecular, that which holds true of eastward holds equally true 

 of westward motion. This is proved also by the fact that a 

 current flowing from a higher to a lower latitude has its westerly 

 motion due to rotation as eftectually checked and diminished as 

 a cm rent flowing from a lower to a higher latitude has of easterly. 

 And what holds true of motion to the east or to the west, holds 

 equally true of motion to the south or north, for there is no 

 reason wiry the resistance should be less in one direction than in 

 another. 



It therefore follows that it is impossible that 6 foot-pounds 

 could impel a pound of water from the Equator to latitude 60° 

 against the molecular resistances to its motion, when during the 

 passage of the ])Ound of water it requires 9,000 foot-pounds to 

 overcome the resistance to the easterly detlections wliich take 

 place. Or if the molecular resistance of water be so infinitesimal 

 that 6 foot-pounds is sufficient, then it is impossible that molecular 

 resistance could consume 9,000 foot-pounds during tl\e easterly 

 deflection which takes place. 



I respectfully submit that this is a clear and obvious demon- 

 stration of the mechanical impossibility of the gravitation theory 

 of oceanic circulation. 



Prof. Everett says that Mr. Ferrel's argument from the tides is 

 quite conclusive in showing that the forces arising from difference 

 of temperature are of sufficient magnitude to keep u)5 an oceanic 

 circulation. If. Prof Everett, like Mr. Ferrel, really supposes 

 tliat a slope produced by the moon is the same as one produced 

 by difference of density, and that the process by which the water 

 tends to regain its level is the same in both cases, I am not sur- 

 prised he should consider Mr. Ferrel's argument conclusive. 



I beg to refer Mr. Wallace to the Philosophical Mai^asinc for 

 October 1871, p. 244, for an explanation of the fallacy of Dr. 

 Carpenter's famous experiment to which he alludes. 



Had the present state of my health permitted, I should have 

 entered somewhat more fully into some of the above points, but 

 in the meantime I mu=t withdraw from any further discussion. 



Edinburgh, August 27 James Croll 



The Aurora of Feb. 4 



The Hon. Rawson Rawson, Governor of Piarljadoes, has 

 favoured us with the following note : — 



" Memo, for his Excellency the Governor-in-Chief. 



" The aurora of the 4th of February last, to which you allude, 

 and notices of which you kindly sent me in N.-VTUiiE, was seen 

 he;e, and caused much concern. 



" I was not myself an eye-witness, but I have descriptions 01 

 it from trustworthy persons. I was first informed of it by a 

 .■servant of mine, who has the overlook of Mont Grace.* He 

 was reluming to his home near the Fort after 7 p.m., when, 

 about a mile away from Mont Grace, his attention was arrested 

 by what he imagined was a 'great fire' — the trees at Mont 

 Grace and all about the yard were lighted up and clearly seen. 

 My brother, who leased a sugar plantation in the neighbourhood 

 of Government House, and Mr. Gordon, who owns one in that 

 immediate neighbourhood, happened to be both in Scarborough 

 at the time, and, seeing the 'great glare,' were both seized with 

 the notion that their respective properties were on fire, and 

 hastened out to them to find that the supposed fire was farther 

 away. 



" Mr. Taylor, master of the barque Tobago, was rtdmg at 

 anchor at Courland Bay,t and was a witness of this aurora. He 

 described it to me as of ' a dark-red colour, extending half way 

 up to the zenith, and very brilliant, its situation being about 

 N. W. by \V. ' The labourers exclaimed that St. Vincent was on 

 fire. 



" This aurora lasted till half-past nine. Such a phenomenon, 

 if not altogether unknown in this latitnde, is at any rate very 

 rare. 



" Tobago." " DuG.^LD Ye.wes " 



^ Mr, Yeates' property, three miles from Scarborough, Island of Tobago. 

 ( I'obago is in 11° 3 ' N. lat. . and 60" 12' W. long.) rtfeu* 



+ On the opposite side of the island, / ■•., the iK>rthern side. 



The Solar Spectrum 

 I HAVE lately obtained and read " .Schellen's Spectrum 

 Analysis," translated by the Misses Lassell, and edited by Mr. 

 1 luggins, and feel at length constrained to dissent from a statement 

 which I there find — in this the present standard work on the sub- 

 ject — distinctly and repeatedly made, as I have seen it made else- 

 where before, a statement belief in which has tended and must 

 always tend to deter many from prosecuting independently a 

 most interesting study. I refer to the passage beginning 

 "The Possibility of Observing" (p. 3S2) to end of para- 

 graph, italicising the words "ordinary" in 1. 7 of p. 3S3 ' by 

 increasing the number of prisms " three lines below, " highly 

 dispersive power" inline 22.] The italics are mine, and are in- 

 tended to indicate that to which I object, not that the particular 

 passages in which they occur are explicitly incorrect, but that 

 they implicitly convey the incorrect notion, that the " highly 

 dispersive power" is essential to the primary success of the 

 observation " of the lines of the prominences in bright sun- 

 shine." 



The reason of my objection will be found in the following ex- 

 tract from an unpublished letter dated May 3, 1S69 : — 



" I think it will surprise you to hear that I havejust seen Mr. 

 Lockyer's three bright solar lines at several parts of the sun's 

 circumference with tlie Royal Society's telescope and spectro- 

 scope without any appliances or dnnces whatsoncr (sic), and that 

 with the greatest ease and certainty. Had I merely looked for 

 them, or for anything of the kind, a twelvemonth ago, I do not 

 see how I could have failed to see them ! 



" When the slit is placed parallel to the limb, the red line is 

 vivid across a bright solar spectrum, and the line near D (there 

 is no doubt about its position when seen in connection with the 

 solar spectrum) which is less prominent, as [also the line at or 

 near F, are easily seen." 



Also in another letter of the same date : — " Before I went into 

 camp last December (while still rumours only of Janssen's obser- 

 vations were current) I resolved to try with coloured glasses. 

 They were not received till too late ; the instrument was packed 

 up, and I was away, lly the time I returned the question had 

 passed on ; but I still wished to carry out original intentions, and 

 prepared accordingly, and was proceeding to direct the telescope 

 this morning when I saw the red line in the undefended part of 

 the slit, where I was fiicussing on the sun's limb. Of course, I 

 saw at once that I could do without my coloured glass, which, 

 practically, limited my field of view, and accordingly removed 

 it, and examined various parts of the limb with no screen at all. 

 At nearly all I could see tlie same three lines. At one place the 

 red was so bright that an outsider looking in at the spectrum 

 would certainly have carried away the impression of a coloured 

 riljbon with a bright line of red near the end." As a matter of 

 fact my wife had no difficulty in seeing at any rate the red line. 



Now, the application of this is to be found in the fact that 

 the spectroscope in question contained a single equilateral prism 

 and no more. 



Were there any merit in a fortuitous discovery of the kind, it 

 would suffice for me to declare that I was in complete ignorance 

 of the methods and appliances by which MM. Janssen and 

 Lockyer had succeeded in doing long before that which I now 

 found so obvious. What I do insist upon is that the visibiUty 

 not of the prominence lines only (see also Proc. R. S., No. 113, 

 1S69, in this connection), but of the prominences themselves 

 does not require a high dispersion. I have a fair acquaintance 

 with prominence forms ; but it has been derived- almost en- 

 tiiely from a study of them with an open slit, the use of which 

 1 had learnt long before, and a single prism spectroscope. 



Of course, I do not contest that the power of extended exami- 

 nation depends directly upon increase of dispersive power, only 

 that the lo'ioer limit is fully attained by a single 5o" prism. 



That high telescopic power is not essential either is proved by 

 the fact that I have examined prominences by applying a if in. 

 object glass to the end of the sliding tube of the spectroscope at 

 solar-focal distance, and using the instrument on its "soiree" 

 stand on a table — by way of experiment — with an amount of 

 success which, in 186S, would have made some sensation. 



It is obvious to remark in reply that probably the explanation 

 of the ready visibility asserted is to be found in more favourable 

 climatic conditions. 1 cannot admit it. By night, it is true, 

 there is often a remarkable translucency ; but the dusty, agitated 

 state of the atmosphere (in May) under a tropical sun, and at an 

 inland station, can certainly not be considered favourable for 

 observations of this particular character. And even were such 



