Page:The New International Encyclopædia 1st ed. v. 20.djvu/252

* VOLCANO. 204 VOLCANO. lightning, and lacked but little of tlie latter's blinding brilliancy. It has so far been impossi- ble to determine the exact or absolute nature of these vivid lights traversing the volcanic cloud, but there can be no question as to their electric character, even if it be assumed that a certain distinctive condition had possibly been prear- ranged in the distribution of a field of inflam- mable or other gases. Perhaps the most remarkable feature of the great Pelee eruption of May 8. 1902, was the violent disturbance in the magnetic field which it occasioned. While minor disturbances of the magnetic needle had akeady been noted in earlier eruptions, these had generally been in close proximity to the scene of action, and in some instances had been referred to as an eflfect of the ejection of rock material containing magnetic iron. But at the time of the Pelee eruption the disturbance seems to have been of world-wide penetration, as it was noted by careful observers at most widely removed stations of meteorologi- cal and magnetic research, such as Cheltenham (Maryland). Baldwin (Kansas), Toronto (Can- ada ), Stony burst ( England ) , Val Joyeux ( France ) , Athens, Potsdam. Pola. Vienna, and Zi-ka-wei (China), the last-named station being situated almost exactl,y on the meridian of longitude opposite to that of Pelee — otherwise, on the oppo- site side of the globe. At most of these stations, the disturbance was noted almost exactly two minutes after the time of the destruction of Saint Pierre, and in some places its continuance was extended over several hours. No satisfac- tory interpretation lias up to this time been given regarding the nature of these magnetic phenomena. The Krakatoa eruption of 1883 seems to have made itself felt as a distinct non-seismic shock or tremor through the entire diameter of the earth, and perhaps the same is true of the May eruption of Pelee. At any rate, a baro- graph record made at Zi-ka-wei indicates a dis- tinct coneussional wave arriving at the station four hours after the magnetic disturbance was initiated. There is liardly room to doubt that the more rapid of (he shoek-pulsations were transmitted directly through the earth, and were followed at a later time by the surface waves, which, traveling in opposite directions over the globe, gave interference areas as well as areas of most pronounced concentration. Tlie sound-waves following volcanic eruptions travel, doubtless, both through the earth's inlerior and over its surface. Eruptions are frequently heard at very great distances, and yet, anomalous though it may appear, need not be audilile near by. Hum- boldt called attention to this singular fact in connection with the eruption of the Soufriftre in 1812, the noise of which wa.q well noted in the valley of the Orinoco and beyond, at a distance in direct line of perhaps T.'iO or 800 miles. The May eruption of Pelee was likewise transmitted by .sound to the most distant parts of the Orinoco Valley, and to the northern coast of Venezuela (Maraeaibo. SCO miles distant), and yet all evi- dence poinls to the conclusion that it was not heard at Fort-de-France, the capital of Marti- nique, at a distance of 14 miles froTn the scene of the great disaster. Similarly, no true shock of the eruption of August 30, 1902, of which the writer was witncsn, was detected on the slope of the volcano itself at a distance of four miles in a direct line from the crater, although it was re- marked at Port-of-Spain, on Trinidad, and at Carupano, on the Venezuelan coast, with the dis- tinctness and intensity of the discharge of siege guns. These anomalies in sound-carriage still await their explanation, although the facts bear- ing upon the explanation are not unlikely to be similar to those which underlie the ordinary forms of acoustic opacity — alternating layers, with different degrees of tension, density, and temperature of the atmosphere. Atmcspheric DISTURBANCE.S. These appear mainly in winds of great violence, frequently of tornadic intensity, which pass in advance of the clouds of eruption ( in this sense compar- able with the wind that is forced ahead of a snow avalanche ), and others whose course is di- rected toward the volcano, and which are seem- ingly indraughts into vacual areas made by the sudden explosion of steam. These so-called con- trary currents liave been noted particularly in the eruptions of Tarawera, in 1886, of Bandai- San, Japan, in 1888, and of Pelee. They are also at times very destructive, almost as much so as the direct winds which are scattered about by the outgoing blasts. It is not unlikely that most of the destruction that followed the explosion of Bandai-San, in 1888, and which resulted in the annihilation of the greater part of seven vil- lages, was the work of this form of wind, ilany attempts have l)een made to associate volcanic outbursts with particular phases of the moon and sun ( sun-spot periods ) and with special meteoro- logical conditions of our atmosphere ; and on these assumed relations special prophecies as to future volcanic happenings have been built. But no relation of this kind has so far been found, nor is there well-founded reason for believing that it in any way exists. 5Iany of the most violent eruptions have taken place during storm periods ; others of equal violence at times of almost absolute atmospheric stability. For at least six days preceding the May eruption of Pelee the barometric variation at noon was only one millimeter, the barometer marking steadily at this time 702 millimeters. This condition also existed at and before the time of Pelfe's sec- ond death-dealing eruption (August 30. 1902), and it was noted in the great eruption of Vesuvius in 1794. The Geographical Distribxition of Vol- c.xoE.s. With hardly an exception all volcanoes are situated in wliat can reasonably be assumed to be areas of weakness in the earth's crust, such areas being the ocean basins themselves, the border-lands of these basins, or the mountain ranges which flank or construct the continental outlines. It follows then that volcanoes are citlier insular (oceanic) or sub-oceanic — i.e. placed on the conlinents close to the oceanic border. No dis(antly inland (active) volcano is known unless it can be shown to be in a region of crustal instability (Southern :Mexieo) or one in which a comparatively recent disruiUion has taken place (the volcanoes of the Gieat Rift Val- ley, of East Central Afri<'a — Jlfnmbiro (iroup). Mount Wrangcl, in .Maska. is a possilde exccplion to (liis rule. The main line of areas of volcanic dislribulion are: (1) The Pacilic-Carililiean belt following the .Andes of South. ierica. with nniny dormant, extinct, or active sinnniits (.-Veen-