302 



NA TURE 



\_July 24, 1884 



notice ; so we may conclude that the hour of its arrival there is 

 "very accurately determined, which gives a mean daily velocity of 

 2162 miles ; and taking the velocity from its journey and a half 

 round the world, from India to lat. 24 N., long. 140J W., we 

 find it to be 2192 miles per day, or 30 miles only in excess of 

 the other computation. But if we take the whole journey from 

 Krakatoa to that locality, about 2| revolutions round the globe, 

 we find the mean to be 30 miles less than the first, or 2132 miles, 

 and that will be accounted for through the diminished value of 

 the degree in longitude at the mean latitude between Java and 

 lat. 24 N., long. 140^° W. 



The mean diurnal velocities obtained from the intermediate 

 stations, between Java and India, agree very closely ; when we 

 consider that at those several places the phenomenon was wholly 

 unexpected, and thus in most instances the dates and times given 

 appear to be somewhat late, it is quite possible and natural that 

 it escaped notice at least once ; in India, however, we may con- 

 clude that they were on the alert, and consequently the mean 

 velocity deduced from that place ought to bear great weight. 

 There is another thing that ought not to be lost sight of, viz. that 

 without this list of stations, more than encircling the globe, one 

 might suppose that the cloud after leaving Krakatoa stretched 

 away westward, and as I gather from Lockyer's paper by his 

 north-south line, to have extended to the north and south, forming 

 a letter V with the apex at tin- Straits of Sunda. Now Lockyer 

 tracks it to Panama, to which place we see it to have had a 

 diurnal velocity of 2059 miles, and from Panama to India I made 

 it 2200 miles per day, which makes me believe that the cloud 

 was performing a spiral path northward round the globe. 



Before proceeding I will now turn to the observations in the 

 Southern Hemisphere, in order to see whether the same has 

 taken place there. This table has been prepared in like manner 

 to the former, viz. the dates and times are reduced to that of 

 Krakatoa, and the distances in English miles obtained from the 

 difference in degrees of longitude reduced to the value of the 

 mean latitude of the two places. 



Table II. — Showing the Mean Diurnal Velocity in English 

 Miles of the Phenomena of Coloured Suns and Brilliant 

 Sunsets in the Southern Hemisphere 



1 for Southern Hemisphe 



The marked similarity between these two tables is most 

 striking, and, as in the first table, the greatest discrepancy is 

 found between Krakatoa. and Mauritius, where the data are 

 reckoned in so many hours, in which case an hour or two makes 

 a material difference in the diurnal velocity. At present I cannot 

 find any station reporting the phenomenon between Mauritius 

 and Adelaide, but we may conclude that after it passed Mauritius 

 it crossed Africa, the South Atlantic, and South America, whence 

 we may expect to hear of it, as there are many competent ob- 

 servers in that part of the world ; it then traversed the great 

 South Pacific (Ocean and North Australia, and after performing 

 another such journey round the world, but in a higher latitude, 

 -was seen at Adelaide in South Australia about September 17. 

 I conclude, as Mr. Todd, the Government Astronomer, there says 

 in his report to Nature, ''that it was visible during the last 

 fortnight of September." We next hear of it at the Cape of 

 Good Hope on September 20. It again crossed the South 

 Atlantic and South America about the latitude of Buenos Ayres, 

 and a third time traversed the South Pacific, striking the coast 

 of New Zealand on September 25, the date of my first seeing 

 it : Mi, which occasion the western sky at sunset presented all 

 the colours seen in the pearl-shell. Since then the western and 

 eastern skies have presented those beautiful crimson tints that 

 have delighted the world, and on many occasions I have seen 

 it almost in the zenith two hours after sunset. During some 



evenings it has quite illuminated the western face of buildings 

 with a bright glare as from a fire, whilst on others it has been 

 very faint and sometimes not discernible : giving to my mind the 

 idea of its not being a continuous band but a series of dust clouds 

 with clear spaces between. 



From an investigation of the two tables it will be seen that the 

 mean diurnal velocity in the Northern Hemisphere was, during the 

 first revolution, about 2162 miles, and during the second it in- 

 creased to 2192, or 30 miles per diem extra. And the same 

 increased velocity is observed in the Southern Hemisphere, 

 where we find the approximate velocity during the first two revo- 

 lutions, viz. on its reaching Adelaide, to be 2041, whereas during 

 the next revolution from Adelaide round to New Zealand it was 

 2120 miles, or an increase of So miles per day. It will be 

 further noticed that in the Northern Hemisphere the time occu- 

 pied in its first revolution was about eleven days, and the same 

 rate is observed during the next revolution and three-quarters, or, 

 in other words, within the tropics it encircled the world in eleven 

 days. It is the same within the southern tropics, where it took 

 21 \ days to reach Adelaide in its second revolution, but it per- 

 fumed the next revolution in about 9^ days, reaching New 

 Zealand in 29J days after the eruption. Thus it performed 2f 

 revolutions in the Northern Hemisphere in 293 days, and in the 

 Southern Hemisphere it performed 2§ revolutions in 29 \ days, 

 showing that the initial velocity at starting has only very slightly 

 fallen off in even latitude 45 S. So in the following discussion 

 I will adopt a mean diurnal velocity for the dust cloud of 2083 

 miles, or 87 miles per hour to the westward. 



As I showed at the beginning that, if the atmosphere be con- 

 sidered as part and parcel of the earth, a particle of it at a certain 

 height will cover a greater distance in a certain time than that 

 part of the earth immediately beneath would, so if we know 

 the rate per hour that a certain thing apparently moves to the 

 westward, or seems to lag behind the diurnal revolution, we can 

 ascertain the height. We know that it lags behind at the rate 

 of 20S3 miles per day, which, added to the circumference of the 

 world, gives a circle of 26,90s miles, and this divided by3'I4l6 

 gives a diameter of S565 miles, or 664 miles greater than that of 

 the earth, or a height of 332 miles above the surface. Or, 

 putting it this way, we may assume that at the latitude of 

 Krakatoa the earth has an hourly velocity of 1034 miles, and 

 that any matter ejected thence into the upper regions of the 

 atmosphere, would retain the same rotary velocity as it had 

 before, viz. 1034 per hour to the eastward ; but we have material 

 under our observation which cannot keep its zenithal position 

 at starling, by 87 miles per hour, showing it to be at an ele- 

 vation of 332 miles. 



Now the spectroscope tells us that the red colour is pro- 

 duced through dust of almost ultra microscopic fineness, and 

 in some specimens of this dust that have already fallen the 

 microscope shows the existence of salt crystals, which fact in 

 itself almost proves it to be of volcanic origin, and not meteoric 

 or cosmic dust. Now Prof. Helmholtz states that "the re- 

 flecting medium, whatever it was, over Berlin on the last three 

 nights of November, was about 40 miles above the earth ; " 

 ami if we work on this data we have a circle whose diameter 

 is So miles greater than that of the earth, or a circle of 7981 

 miles, which, multiplied by 3'i4i6 gives a circumference of 

 25,073, or 248 miles more than that of the earth, which, 

 divided by 24, shows an excess of about 10^ miles per hour 

 above the surface velocity of rotation. But we want to account 

 for an excess of S7 miles per hour ; so if we accept Prof. Helm- 

 holtz's statement we must only suppose that at the altitude of 40 

 miles there is an easterly current, or one moving to the west- 

 ward, of 77 miles per hour ; for, assuming as we do from 

 the foregoing tables and calculations that the earth rolls from 

 under the cloud at the rate of 87 miles per hour, unless we 

 admit of an easterly current we cannot stop short of that 

 enormous height of 332 miles unless we suppose that the power 

 of gravitation has only a feeble hold on those most minute 

 dust particles at the altitude of 40 miles, where the atmosphere 

 has not the many thousandth part of the density it has on the 

 surface of the globe. 



Mr. W. H. Preece writes stating his opinion that the mass of 

 matter ejected retained the same electric sign as that of the 

 earth, and as long as that was the case the repulsion force 

 would be sufficient to keep the matter afloat ; and in reference 

 to that theory Mr. Crookes writes to state that with a rarefaction 

 of one-millionth of the atmosphere, two pieces of electrified 

 gold leaf repelled each other at a considerable angle for thirteen 

 months, and goes on to state that that rarefaction is attained at 



