42 



SCIENTIFIC NEWS. 



[April 1st, 1887. 



a mile from the magazine, and facing in the opposite direc- 

 tion, were seen to fall out of their frames before the noise of 

 the detonation reached the spectators. 



The extent of the region shaken is also of great moment, 

 as throwing a light upon the seat of the centre of action. 

 If a shock be felt simultaneously, or nearly so, in places 

 hundreds of miles apart, we know that its source must be 

 very deep. The great Lisbon earthquake in the last cen- 

 tury is supposed to have agitated one-eleventh part of the 

 earth's surface. That of Krakatoa was probably felt at 

 least over as great an extent. That of Charleston affected 

 a land-area of 774,000 square miles, and probably at least 

 half as much more of ocean-area. The epi-centre, that 

 is, the point supposed to be exactly above the source of 

 the shock, is not by any means the part most severely 

 shaken. 



In earthquakes which are localised within narrow limits, 

 the disturbance is often near the surface. Thus, in an 

 earthquake of no great intensity or extent, which occurred 

 a few years ago in a coal-mining district in the north of 

 France, not the slightest agitation was felt in the workings, 

 whilst on the surface of the earth buildings were shaken, 

 and movable objects displaced. Indeed, the number and 

 extent of mining accidents connected with earthquakes is 

 by no means great. We might have naturally expected 

 that in Chili, Peru, Mexico, etc., the mines must often 

 collapse upon the unfortunate workers. But such does not 

 seem to be the case. 



The first crude attempt to explain the phenomena of 

 earthquakes took its stand on their supposed connection 

 with volcanic eruptions. It was suggested that vast volumes 

 of gases and vapours were generated in the interior of the 

 earth and sought for free escape. Where such vent was 

 found the result was a volcano ; where there was no outlet, 

 earthquakes ensued. It was even asserted that where 

 mines were numerous they allowed of an escape for the 

 gases, and thus served as safety-valves. In support of the 

 volcano theory, it was shown that Asia Minor, Syria, the 

 Greek Islands, Spain, Portugal, northern Italy, and the eastern 

 side of North America, have no active volcanoes, and have 

 at all times suffered more or less severely from earthquakes. 

 But, on the other hand, there are many regions studded 

 with volcanoes, and yet fearfully liable to subterranean 

 shocks. Southern Italy, Sicily, the west coast of South 

 America, Iceland, the Sunda Islands, and Japan, all con- 

 spire to prove that volcanoes at least do not prove efficient 

 safety-valves for the excessive pressure. Very closely con- 

 nected with this theory is another view which pays par- 

 ticular attention to the fact that both volcanic and seismic 

 action haunt especially the sea-coasts. We have no authentic 

 records of either eruptions or earthquakes of any import- 

 ance in the vast plain which extends through northern 

 Germany, Russia, and northern Asia. The interior of 

 Africa and the great river-valleys and alluvial plains of 

 eastern South America seem free from both these scourges. 



Hence it is contended that the cause — or, at least, one of 

 the causes — of earthquakes must be the water of the sea 

 finding its way into the interior of the earth, and there 

 coming in contact with matter which may effect its decom- 

 position or its evaporation. Here we have at least two 

 possible cases. It is familiarly known that the alkaline 

 metals, such as potassium and sodium, have the power of 

 decomposing water, combining with its oxygen and setting 

 its hydrogen at liberty. If there exist, as was suggested 

 by Sir H. Davy, considerable underground deposits of any 

 of these metals the access of water would determine a con- 

 vulsion which, according to local circumstances, will take 

 the form of an earthquake or of a volcanic eruption. 



But another case is open. Suppose the water of the sea 



finds sudden access to intensely-heated strata, whether these 

 be liquid or solid. An immediate liberation of steam on an 

 immense scale would be the necessary result. Those who 

 are familiar with steam-explosions will best appreciate the 

 probable results if millions of gallons of water are thus 

 flashed into vapour. It is further known that if water 

 falls upon an intensely heated surface it does not at once 

 evaporate, but remains in the so-called spheroidal condition 

 until the temperature is sufficiently reduced, when it evapo- 

 rates with explosive violence. 



We are far from contending that ordinary earthquakes 

 are thus occasioned. But from a variety of circumstances 

 which cannot here be given in detail, the fearful and alto- 

 gether exceptional catastrophe of Krakatoa was probably 

 thus produced. That chemical reactions do play a part in 

 the phenomena of volcanoes and of earthquakes will appear 

 from the fact that hydrochloric, hydrosulphuric, and sul- 

 phurous acids, all in the gaseous state, have been given off 

 as well from chasms in the earth as from the craters of 

 volcanoes. But on this point much further investigation is 

 needed. 



Not a few scientific men have contended that earthquakes 

 are, in many cases at least, the outcome of electric action. 

 This view is advocated in Blaclnvood's Magazine for July, 

 1869; in the Quarterly Review for July, 1881 ; and in the 

 Journal of Science for December, 1883, and for Januarj-, 

 1884. The writer of the two last-mentioned articles, 

 Colonel Arthur Parnell, R.E., gives the particulars of 490 

 earthquakes, of which 437 were accompanied by thunder- 

 storms, magnetic disturbances, or other meteorological 

 phenomena generally considered to be causally connected 

 with electric action. He writes that : — " It is now well 

 established that in India, at all events, earthquakes are 

 almost always accompanied by furious storms of thunder." 

 On the other hand, a correspondent of the same journal, 

 who has experienced six earthquakes in India, states that 

 they were unattended by thunder, lightning, or rain, or, in 

 fact, by any atmospheric disturbance whatever. 



The late Cromwell Varley — certainly no mean authority 

 — was of opinion that some earthquakes at least are due to 

 subterranean electric discharges. But if we find that earth- 

 quakes are generally preceded, accompanied, and followed 

 by electric disturbances we have still to ask which of the 

 two classes of phenomena is cause, and which effect. We 

 may even first raise the qviestion whether the relation 

 between the two is anything more than a coincidence. 

 Certain it is, at least, that thunderstorms of the severest 

 character may be, and often are, unaccompanied by earth- 

 quakes. This may be observed in the Transvaal, and indeed 

 generally in the interior of Africa — a region at once singu- 

 larly liable to electric tempests, and as singularly exempt 

 from earthquakes. But before any satisfactory decision on 

 the electric character of earthquakes can be reached it will 

 be necessary to make, in some country where shocks are fre- 

 quent, such as Japan, prolonged series of electric and mag- 

 netic determinations. 



The alleged periodicity of earthquakes must not be left 

 unconsidered. Mallet considered that there was a maximum 

 about the middle of each century, and a second, but less 

 marked maximum, towards the end. He held also that earth- 

 quakes were most prevalent about the time of the winter 

 solstice. Now, if we consider that our centuries are purely 

 artificial periods, having, so far as we know, no reference to 

 a connection with any recurrent natural phenomena, we shall 

 find it hard to admit that their middles can be thus signalized. 

 Concerning the late earthquake at Charleston it is noticed in 

 the Bulletin of the Washington Philosophical Society that it 

 coincided with an unusually high tide ; that the moon was 

 near perigee, and there had been an eclipse of the sun only 



