176 



NA TURE 



\Dec. 20, 1883 



Ecce itcruin. Here we come back to Krakatoa. Grant- 

 ing the distance to vvliich tlie vapoar and dust were ejected 

 from tlie bowels of Krakatoa to have been so great that 

 the more rapidly rotating surface of the carlli brought 

 Panama under this vapour and dujt in the space of less 

 than a week, we have a gigantic pepper-box capable of 

 condensing and congealing vapour which had long re- 

 mained undisturbed in its serene heights. We do not 

 need to call in the known currents of the atmosphere to 

 explain the dispersion Poleward and therefore eastward 

 of the volcanic matter, gravitation alone accounting for 

 the transmission of the particles down the inclined isobaric 

 planes. 



To my theory of ice spiculse it has been objected that 

 these ought to produce halos. So, whenever the recent 

 phenomena have been most strikingly developed, they 

 have done. Yesterday was the third occasion during this 

 period when, from 2.15 to 2.50 p.m. the sun was sur- 

 rounded by a remarkable halo, the sky at the time being 

 totally devoid (in the neighbourhood of the halo) of any 

 visible upper clouds whatsoever. Cumuli passing the 

 halo appeared green. The halo was followed by a splendid 

 glow in the evening, and again this morning. 



December 15 W. Clement Ley 



Ik you are not yet suffering from a plethora of letters on 

 this subject, I should like to add a few remarks to those 

 which have been already made. 



On Thursday, December 6, I witnessed one of these 

 gorgeous sunsets in company with a friend, from the top 

 of Rusthall Common, near Tunbridge Wells. Like Mr. 

 Rollo Russell, I noticed that the peculiar lasting glow 

 came from a lofty stratum of pale, tibrous, nearly trans- 

 parent cirriform haze, which was almost invisible as the 

 sun set, but afterwards came gradually into view, at first 

 white in colour, and then gradually changing to orange, 

 pink, and iinally red, the change to pink occurring at 

 4.25 and to red at 4 45. 



We also observe! a strange reactionary effect produced 

 by this glow, viz., that long after the red tints had faded 

 from the ordinary cirrus in the western sky and from some 

 snow-shower cumuli in the east, ihey were both rfelighted 

 by the glow which had meanwhile increased in the west. 



On Friday, this retlection on to low clouds all over the 

 sky from the undoubtedly lofty stratum in the west was 

 more noticeable, and it at once stru:k me that persons 

 who had not observed the entire process of the extinction 

 of the real reflection of the sun by these clouds, and their 

 subsequent reillumination by reflection from the upper 

 glow (as Miss Ley terms it), might erroneously be led to 

 attribute this secondary illumination to their reflection of 

 direct sunlight. On this ground alone, I should be rather 

 inclined to accept with a little hesitation the observation 

 on which Prof. Helmholtz bases his calculation, viz., 

 that the clouds which were illuminated by the sun were 

 45^ above the horizon two hours alter sunset. 



Nothing that I saw on either Thursday or Friday at 

 all favoured such a fact. On the contrary, there was some 

 positive evidence in favour of the reflecting medium being 

 situated at a much more moderate altitude. In the first 

 place, judging by an eye often engaged of late in taking 

 vertical angles with a theodolite, 1 should say that on both 

 days (when the sky was very clear and the stratum which 

 ernitted the glow was unusually well defined) the maximum 

 height of the glow-stratum was not more than from 10° 

 to 12° above the horizon. 



Moreover the interval between when the ordinary cirrus 

 ceased to glow and this upper stratum began to glow 

 corresponded very much more with a height of from ten 

 to thirteen miles than with such an enormous height as 

 forty miles. 



Miss Ley has, I believe, already calculated the height of 

 the stratum to be thirteen miles, and I think this height is far 

 more probable than one of forty miles. Besides, can we 



imagine either vapour, or volcanic dust, or a mixture of 

 both, to be capable of remaining in suspension in air of 

 such tenuity as must exist at such an altitude ? More- 

 over, I think it must be admitted that whatever be the 

 cause, whether meteoric dust, or impalpable pumice carried 

 over by the upper anti-trade currents from the Java eruption, 

 the reflection arises from a definite stratum and not merely 

 from an atmosphere filled throughout with such dust. 

 Possibly, as Mr. Edmund Clark suggests, the dust may act 

 as a nucleus for the condensation of any vapour that may 

 exist at such a high level, and it is possible that just as we 

 findcertaindefinite positions at which condensationoccurs, 

 and therefore clouds float, at lower altitudes, so there may 

 be some particular height at which condensation is 

 determined in these upper regions, thus accounting for 

 the definiteness of the reflection and the presence of the 

 cirrus haze to which it app.irently belongs. 



Thus, Dr. V'ettin of Berlin has recently shown that the 

 clouds have a marked tendency to float at certain defined 

 levels, which can only be supposed to result from the 

 action of certain physical causes regarding whose nature 

 we are at present entirely ignorant. 



The name of the cloud and the corresponding elevation 

 in feet are as follows : — 



Name of stratum Height in feet 



Lower cloud 1,600 



Cloud 3,800 



Cloudlets 7,200 



Under cirrus 12,800 



Upper cirrus 23,000 



Now we see that these heights increase very nearly in a 

 geometrical ratio, with 2 as the common factor, so that we 

 might anticipate a tendency for cloud to be formed 

 (assuming that the empirical relation held good) at an 

 elevation of about 46,000 feet, or a height of nearly nine 

 miles. It would be at least interesting to find that the 

 average height of the reflecting layer in these recent 

 sunsets lay at about this elevation. 



Another circumstance which favours the notion that the 

 dust would be carried from the tropics, and float above, 

 and not below, this level is that, while at all lower eleva- 

 tions the polar currents predominate, it is just about this 

 same level that the equatorial or southerly air-currents 

 begin to exceed those which have a northerly component 

 in strength and frequency. Thus, according to Vettin, 

 the following figures represent the relative volumes (.') ' of 

 air carried by the equatorial and polar currents at different 

 altitudes over Berlin : — 



Equatorial Polar Height in feet 



I From 41,000 feet up 

 305 226 ' to the extreme limits 



( of the atmosphere. 

 253 228 41,000 



206 222 23,000 



164 212 i2,Soo 



loS 131 7,200 



92 118 3,800 



83 158 1,600 



This table, I think, makes it easier to understand how 

 the dust should have been transported over to extra-tropical 

 regions from the neighbourhood of Java, and why it 

 should appear only in the very high strata. 



E. Douglas Archibald 



Gilbert White of Selborne, in one of his letters 

 (Ixv., to the Hon. Daines Barrington), describes the 

 "amazing and portentous phenomena" observed in the 

 summer of 17S3. " The sun at noon looked as blank as 

 a clouded moon, and shed a rust-coloured ferruginous 

 light on the ground, particularly lurid and blood-coloured 

 at rising and setting. The country people began to look 



^ I have not the copy of the Zeitschri/t by me just now, and am only 

 quoting from memory. I cannot therefore be sure whether it is volumes or 

 frequencies. For the purpose in hand either would do equally well. 



