308 ANNUAL OF SCIENTIFIC DISCOVERY. 



African iron forwarded to this country, which have given rise to the sup- 

 position, owing their apparently natural structure to a peculiar method of 

 smelting the ore adopted by the natives. 



EARTHQUAKE PHENOMENA. 



The following is an abstract of a lecture on the above subject recently 

 delivered before the Royal Institution, London, by Professor Ansted: 



The whole number of recorded earthquakes upon which any dependence 

 can be placed amounts, at present, to somewhere near 7000. (This calcula- 

 tion is made only up to 18-30, and there have been many since). Of these 

 7000 we know actually the dates of a large proportion, that is, the time 

 of year when they took place. The whole number recorded up to the year 

 1500 is 787 only. During the three following centuries, that is, from the 

 beginning of the sixteenth to the end of the eighteenth, there were 2804, 

 four times as many as in all previous time. The whole number recorded 

 since 1800 down to 1850 is 3-340. We may conclude that 3240 in half a cen- 

 tury is the nearest approximation as to numbers that we can at present 

 obtain. From this, then, we may calculate our average, and we find that 

 certainly more than one earthquake takes place every week on some or 

 other of the visited parts of the earth. Of these, however, not more than 

 one in every forty on an average are of great importance. An important 

 earthquake occurs, therefore, once in every eight months; and we under- 

 stand by this a disturbance of considerable magnitude, capable of doing 

 much mischief if it occurs near human habitations. Such, at any rate, has 

 been the average of the first half of the present century, even without 

 making allowance for those numerous cases which must have happened, 

 although we have heard nothing about them. 



If, however, we take the statistics as far as Europe only is concerned, we 

 shall find that during the last ten years, or rather during a period of ten 

 years in which calculation was made very carefully (from 1833 to 1812 inclu- 

 sive), thirty-three earthquakes occurred on an average in every year. In 

 other words, in these ten years there were 320 distinct earthquakes in Europe 

 only; and, therefore, in Europe an earthquake occurs every twelve days. 

 It is evident that the records for other parts of the world will be much less 

 perfect than those of Europe, and that the number generally may be far 

 greater than is above stated. This result is startling, and I confess I was not 

 prepared for it when the evidence came before me. 



Earthquakes must be regarded as the undulations produced by the exercise 

 of some great expaasive forces acting beneath the earth's surface on tolerably 

 well-defined zones, whenever such expansive force rapidly breaks asunder, 

 instead of quietly and slowly heaving up the vast weight of overlying earth 

 and sea that intervenes between its point of action and the upper surface. 

 The convulsive or paroxysmal movement thus occasioned by the fracture of 

 rocks at a great depth generally causes an earth-wave to be propagated to a 

 distance which is proportioned to the amount of force exercised and the 

 depth at which it acts, and not unfrequently opens out a communication to 

 the surface, and terminates in a volcanic eruption. After one great wave is 

 thus formed and sent on, it is often repeated before it quite dies away. In 

 any case, the result at the surface consists of the propagation of a wave 

 through rocks of various degrees of elasticity. The cracking of the earih 

 at the surface may have nothing whatever to do with the fracture below; and 



