Mar. 19, 1874J 



NATURE 



383 



the winds, temperatures, and barometrical indications might 

 have been added for the like reason. 



13. Table XI. also requires a note of the season of the year 

 and the number of days of observations. It may here be asked, 

 how is the discrepancy to be reconciled between the lessening of 

 ozone as you sail to t/tu Polar Regions, and the increase of ozone 

 as you ascend in the air, when the temperature as regularly falls 

 in tlae one case as in the other. 



14 The records showing the connection between phosphoy- 

 t-siiiice and manifestation of ozone are very satisfactorily drawn 

 out, and may probably become of much value in a new investi- 

 gation. 



15. The less prevalence of ozone in the higher cxtratropical 

 latiludcs may be due, as suggested in another case, to the dry- 

 ness of the atmosphere impairing the sensitiveness of the test 

 papers, so that for the present such deductions are under sus- 

 picion. 



16. Tlie idea that tire prevalence of ozone is coincident gene- 

 rally with a loiKf barometer seems well supported by the observa- 

 tions recorded, but some explanation will be required to account 

 for its maximum occurrence with south-east winds in Tables IX. 

 and XL, if one should accept the theory of its connection only 

 witli equatorial winds. 



17. That its presence may be connected with ?(«;v« tempera- 

 Inres of the air seems better established at sea than on land, as 

 also its coincidence with huiniJily of the air, though this is some- 

 what vitiated by the conscientious suggestion that its manifesta- 

 tion may be due to the increased susceptibility of the test-papers 

 when moist. 



18. In the statement that ozone increases as you ascend moun- 

 tainous elevations, it is not stated what winds were blowing at 

 the time, which would appear to be necessary, if the idea of its 

 prevalence with any particular wind were considered essential. 



19. The key to the origin and prevalence of ozone in the at- 

 mosphere seems still undiscovered, and we do not yet appear to 

 have determined if it belongs to aqueous vapour or a special 

 wind, or whether it be an additional constituent of the air, like 

 carbonic acid, or a floating entity, like a cloud. 



NUBIBUS 



The Limits of the Gulf Stream 



As one of those engaged in the compilation of the Atlantic 

 pilot'Charts published by the Admiralty, on which are given the 

 limits, velocity, and general features of the Gulf Stream, as well 

 as the boundaries of the regions in which ice and icebergs may 

 be fallen in with in the North Atlantic, I cannot allow the letter 

 in N.\TURE (vol. ix. p. 343), by W. W. Kiddle, of the White 

 Star Mail steamship Oeeanie, to remain unchallenged. 



The Gulf Stream and ice boundaries, delineated on the North 

 Atlantic chart, referred to in that letter, are in their details 

 transcripts from the Atlantic pilot-charts. 



These details were the result of much patient investigation, 

 and obtained from many sources probably unknown to Captain 

 Kiddle ; among the most valuable were the painstaking and 

 sound observations made by members of the United States Coast 

 Survey, and to be found embodied in the annual reports between 

 1S43 and 1859 ; and especially from the exhaustive and learned 

 work on currents, so well known to cultivators of nautical science, 

 by the late Major Rennell. 



If the average boundaries of the Gulf Stream cannot be laid 

 down within reasonable limits from the authorities I have 

 quoted, aided, too, by the many observations of ships of war, 

 extending over the present century, I fear that Captain Kiddle's 

 results wiU not assist us in a more accurate delineation. 



It is, however, to be hoped that Captain Kiddle's informa- 

 tion on the currents may be more reliable than that he has ven- 

 tured upon giving with regard to the limits of iceherg-drift ; 

 here recorded facts are irresistibly against him. He has only to 

 consult any North Atlantic memoir on the subject, and he will 

 find that icebergs have been fnllen in with so far south as 

 36° 10' N., or 7° south of the high authority he quotes. I would 

 refer him on this interesting suliject, .is well as how icebergs are 

 found on the southern edge of the Gulf Stream, and why it is 

 possible "that bergs could drift square across the heated waters 

 of the Gulf Stream to lat. 39^ N.," to a paper by the well-known 

 W. C. Kerlfield, of the United States (reprinted in the Nautical 

 Maqaziiietor 1S45), '"'ho gathered the facts that have simply been 

 utilised in the Admiralty charts. 



London, March 1 1 Thomas A. Hull 



The Great Ice-Age 



Mr. Green, reviewing Mr. J. Geikie's work on the "Great 

 Ice-Age" (Nature, vol. ix. p. 318), expresses the opinion that 

 a glacial period must have been one of intense cold. This is the 

 general opinion, and yet I think it can be shown to rest on a 

 misconception. If the climate at any given elevation is cold 

 enough to form glaciers, no decrease of the winter temperature 

 will increase their magnitude ; while on the other hand a low 

 summer temperature is shown by the facts of physical geography 

 to be eminently favourable to glacialion. This last may almost 

 be called an identical proposition, for permanent snow means 

 snow whith lasts through the summer. 



As Mr. CroU has pointed out, there have been periods where 

 the sun's greatest and least distances were respectively greater 

 and less than now. He thinks that a glacial period occurred 

 when, in the course of the precession of the equinoxes, the sun's 

 greatest distance occurred in the winter, so as to cause a cold 

 -itfinter. I think the true theory of the glacial climate is exactly 

 the reverse of this : that is to say, it was caused by the cold 

 summer which occurred when the sun's greatest distance was in 

 the summer. 



I have stated these views at greater length in the Journal of 

 the Geological Society of London, 1869, p. 350. 



Old Forge, Dunmurry, Co. Antrim, J. J. MuKPHY 



March 8 



Mars 



Iix a most interesting article on the planet Mars, in your issue 

 of N.ATURE for Feb. 19, which has just been shown to me, the 

 Rev. T. W. Webb directs attention to the question of the colours 

 of Mars being due to effects of contrast or not, and says — "Nor 

 does it seem to have been noticed that no effect of contrast has 

 been traced in the Polar snows." 



Kindly permit me to inform Mr. Webb that, in a paper on 

 Mars in the last volume of the " Monthly Notices of the Royal 

 Astronomical Society," I expressly state that, "on May 14, 

 1873, the south Polar ice appeared (in an S^-inch silvered glass 

 reflector, by Browning) of quite a pale sky-blue colour, evidently 

 by contrast," and I may add that this effect I noticed also on 

 two or three subsequent occasions. 



Burton-on-Trent, March 12 Edward B. Knobel 



POLARISATION OF LIGHT* 

 VI. 

 ■jV/r ENTION was made in the previous article of the 

 ■'•*■'■ bands produced in the spectra of polarised light. 

 Beside the fact of the existence of these bands it has 

 been found upon examination that the state of polarisa- 

 tion at different parts of the interval between two suc- 

 cessive bands varies ; and such an examination may be 

 made by means of a quarter-undulation plate or a Fres- 

 nel's rhomb. 



If we carefully examine the spectrum of light which has 

 passed through a selenite, or other ordinary crystal, we 

 shall find on turning the analyser that, commencing with 

 two consecutive bands in position, the parts occupied by 

 the bands and those midway between them are plane- 

 polarised, for they become alternately dark and bright ; 

 while the intermediate parts, i.e. the parts at one-fourth 

 of the distance from one band to the next, remain per- 

 manently bright. These are, in fact, circularly polarised. 

 But it would be incorrect to conclude from this experi- 

 ment alone that such is really the case, because the same 

 appearance would be seen if those parts were unpo- 

 larised, i.e. in the condition of ordinary light. And on 

 such a supposition we should conclude, with equal justice, 

 that the parts on either side of the parts last mentioned 

 (i.e. the parts separated by one-eighth of the interval be- 

 tween two bands) were partially polarised. But if we in- 

 troduce a quarter-undulation plate between the selenite 

 and analyser, with its axis inclined at 45° to that of the 

 selenite, circular polarisation will be converted into plane 

 and plane into circular. This being so, the parts which 



* Continued from p. 326. 



