476 



NATURE 



[March 20, 1890 



this dust ocean was clearly red, and it stretched in every direc- 

 tion as far as the eye could reach, although there was no special 

 wind or local cause for it. It was evidently like the dust seen in 

 mid-ocean from the Peak of Tenerife — something present all the 

 time, and a permanent ingredient of the earthy atmosphere." 



Dust Storms. — These storms, as suggested by Dr. Henry 

 Cook, from whose paper to the Quarterly Journal of the Royal 

 Meteorological Society, in 1880, I am now quoting, may be con- 

 sidered under three heads, according to their intensity — atmo- 

 spheric dust, dust columns, and dust storms. Dr. Cook, allud- 

 ing to occurrences in India, observes that there are some days 

 on which, however hard and violently the wind may blow, little 

 or no dust accompanies it ; while on others, every little puff of 

 air or current of wind forms or carries with it clouds of dust. If 

 the wind which raises the dust is strong, nothing will be visible 

 at the distance of a few yards, the sun at noon being obscured. 

 The dust penetrates everywhere, and cannot be excluded from 

 houses, boxes, and even watches, however carefully guarded. 

 The individual particles of sand appear to be in such an elec- 

 trical condition that they are ever ready to repel each other, and 

 are consequently disturbed from their position and carried up 

 into the air. 



Dust columns are considered by Dr. Cook as due to electrical 

 causes. On calm, quiet days, when hardly a breath of air is 

 stirring, and the sun pours down its heated rays with full force, 

 little eddies arise in the atmosphere near the surface of the 

 ground. These increase in force and diameter, catching up and 

 whirling round bits of sticks, grass, dust, and, lastly, sand, until 

 a column is formed of great height and considerable diameter, 

 which usually, after remaining stationary for some time, sweeps 

 away across country at great speed. Ultimately it loses gradu- 

 ally the velocity of its circular movement and disappears. In 

 the valley of Mingochar, which is only a few miles in width, 

 and surrounded by high hills. Dr. Cook, on a day when not a 

 breath of air stirred, counted upwards of twenty of these 

 columns. They seldom changed their places, and, when they 

 did so, moved but slowly across the level tract. They never 

 interfered with each other, and appeared to have an entirely 

 independent existence. , 



Dr. Cook describes as follows a dust storm which took place 

 at Jacobabad : — "The weather had been hot and oppressive, 

 with little or no breeze, and a tendency for dust to accumulate 

 in the atmosphere. On the evening of the storm heavy clouds 

 gathered and covered the sky. About 9 p.m. the sky had 

 cleared somewhat, and the moon shone. A breeze sprang up 

 from the west, which increased and bore along with it light 

 clouds of sand. At 9.30 p.m. the storm commenced in all its 

 fury. Vast bodies of sand were drifted violently along. The 

 stars and moon were totally obscured. It became pitch dark, 

 and it was impossible to see the hand held close to the face. 

 The wind blew furiously in gusts, and heaped the sand on the 

 windward side of obstacles in its course. Lightning and thunder 

 accompanied it, and were succeeded by heavy rain. The storm 

 lasted about an hour, when the dust gradually subsided. The 

 sky again became clear, and the moon shone brightly. The 

 storm appeared to have entirely relieved the electrical condition 

 of the atmosphere. A pleasant freshness followed, and the 

 oppressive sensation before mentioned was no longer experienced. 

 This, indeed, is the general effect of storms in Upper Scind. 

 The air is cooled, the atmosphere cleared, and the dusty con- 

 dition of the atmosphere which usually precedes them for several 

 days completely disappears." 



In the case of a memorable sand storm which occurred at 

 Aden on July i6, 1878, and recorded by Lieutenant Herbert 

 Russell, there was a remarkable play of light on the objects 

 which remained within sight. The sudden darkness from the 

 storm gave a peculiar and ghastly tint to the white sand and 

 neighbouring plain, while the curling masses of sand drifted 

 before the gale, resembling a dark yellow smoke. The varied 

 lights, quickly changing, were curious and most grand ; the sea 

 a clear green, and Slave Island and Shum-Shum, usually of an 

 arid brown colour, became of an ashy white. 



In a dust storm I experienced myself at Luxor, on the Nile, 

 the suffocating effect of the sand as it drove into the lungs and 

 air passages was very trying. People rushed to the immediate 

 river side, where some relief was found. 



A book on " Whirlwinds and Dust Storms in India," by 

 P. L. H. Baddeley, Surgeon, Bengal Army, i860, gives some 

 interesting information on the electrical character of dust storms 

 and dust pillars. When at Lahore in 1847, this gentleman was 



desirous of experimenting on the electrical state of the atmo- 

 sphere in a dust storm, and with this object he projected into 

 the air, on the top of his house, an insulated copper wire fixed 

 to a bamboo ; the wire was brought through the roof into his room, 

 and connected with a gold-leaf electrometer, a detached wire com- 

 municating with the earth. A day or two after, during the 

 passage of a small dust storm, he observed the occurrence of 

 vivid sparks from one wire to the other, and, of course, strongly 

 affecting the electrometer. He subsequently witnessed at least 

 sixty dust storms of various sizes, all presenting the same kind 

 of phenomena. 



Volcanic Dust. — This dust consists mainly of powdered vitri- 

 fied substances, produced by the action of intense heat. It is 

 interesting in many respects. The so-called ashes or scories 

 shot out in a volcanic eruption are mostly pounded pumice, but 

 they also originate from stones and fragments of rocks which, 

 striking against each other, are reduced into powder or dust. 

 Volcanic dust has a whitish-grey colour, and is sometimes 

 nearly quite white. Thus it is that, in summer, the terminal 

 cone of the Peak of Tenerife appears from a distance as if 

 covered with snow ; but there is no snow on the mountain at 

 that season of the year ; the white cap on the Peak is entirely 

 due to pumice ejected centuries ago. It is probably to this 

 circumstance that the island and Peak owe their name, as in the 

 Guelph language the words Tener Ifa mean zvhite mountain. 



The friction caused by volcanic stones and rocks as they are 

 crushed in their collision develops a mass of electricity which 

 shows itself in brilliant displays of branch lightning darting from 

 the edges of the dense ascending column. During the great 

 eruption of Vesuvius, in 1822, they were continually visible, and 

 added much to the grandeur of the spectacle. It not unfre- 

 quently happens that dust emitted from Vesuvius falls into the 

 streets of Naples ; but this is nothing in comparison with the 

 mass of finely-powdered material which covered and buritd the 

 towns of Pompeii, Herculaneum, and Stabiae in the year 79. 



On this occasion, according to the younger Pliny, total dark- 

 ness from the clouds of volcanic ashes continued for three day-, 

 during which time ashes fell like a mantle of snow all over the 

 surrounding country. When the darkness cleared away, the 

 calamity was revealed in all its awful extent, the three towns 

 having disappeared under the showers of dust. 



The eruption of Krakatab, a mountain situated on an island 

 in the Straits of Sunda, exceeded, in all probability, in its 

 deadly effects, and as a wonderful phenomenon of Nature, the 

 outburst of Vesuvius in the year 69. The KrakatjTo Committee 

 of the Royal Society have collected and published in their inter- 

 esting Report particulars of that memorable eruption, all of 

 them thoroughly authenticated and reliable. The following is 

 extracted from a communication to the Report by Prof. Judd: — 

 " On August 26, 1883, it was evident that the long-continued 

 moderate eruptions of Krakatab had passed into the paroxysmal 

 stage. That day, about i p.m., the detonations caused by the 

 explosive action attained such a violence as to be heard at 

 Batavia and Buitzsenborg, about 100 English miles away. At 

 2 p.m. Captain Thompson, of the Medea, then sailing at a point 

 76 miles eastnorth-east of KrakatsTo, siw a black mass like 

 smoke rising into the clouds to an altitude which has been 

 estimated at no less than seventeen miles (nearly six times the 

 height of Mont Blanc)." 



If this surmise be correct, some idea of the violence of the 

 outburst can be formed from the fact that during the eruption of 

 Vesuvius in 1872 the column of steam and dust was propelled to 

 a height of from 4 to 5 miles only. 



At 3 p.m. the explosions were loud enough to be heard 150 

 miles away. At Batavia and Buitzsenborg the noise is described 

 as being like the discharge of artillery close at hand. Windows 

 rattled, pictures shook, but there was nothing in the nature of 

 earthquake shocks — only strong air vibrations. 



Captain Wooldridge, of the Sir R. ^'a/^, viewing the volcano 

 at sunset on the 26th, describes the sky as presenting a most 

 terrible appearance, the dense mass of cloud of a murky tinge 

 being rent with tierce flashes of lightning. At 7 p.m., when 

 from the vapour and dust clouds intense darkness prevailed, the 

 whole scene was lighted up by electrical discharges, and at one 

 time the cloud above the mountain presented the appearance of 

 an immense pine-tree, with the stem and branches formed of 

 volcanic lightning. The air was loaded with excessively fine 

 ashes, and there was a strong sulphurous smell. The steamer 

 G. G. Loudon, within 20 or 30 miles of the eruption, passed 

 through a rain of ashes and small bits of stone. 



