February. 1911. 



KNOWLEDGE. 



53 



areas. For instance, for the intensit\' seven, the 

 disturbed area ma\' be as lo\\' as one thousand square 

 miles and as high as sixty-three thousand six hundred 

 square miles ; for the intensity six. the area ranges 

 from seventy-four to three thousand one hundred 

 square miles ; and so on. Thus, actual intensitv 

 near the centre cannot be regarded as a measure of 

 an earthquake's strength. Nor, on the other hand, 

 as some seismologists maintain, can the extent of 

 the disturbed area be employed as such a measure. 

 for this area depends on several conditions, of which 

 one of the most important is the time of occurrence. 

 For instance, the Pembroke earthquake of 1892 was 

 felt over fort\-four thousand eight hundred and sixtv 

 square miles, and the distinctl\- weaker shock of 1S93 

 over sixty-three thousand six hundred square miles, 

 the reason being that the former movement occurred at 

 0.24 a.m. and the latter at 5.45 p.m. In like 

 manner the disturbed areas of the Derby earth- 

 quakes of 1903 and 1904 were twelve thousand and 

 twenty-fi\e thousand square miles, the former 

 occurring at 1.30 p.m. and the latter, which was 

 somewhat weaker, at .).21 on a Sunday afternoon. 

 In each case the disturbed area of the earlier shock 

 was bounded by a line of intensity four, and of the 

 latter by one of intensity three. On the whole, if area 

 is to be used at all as a measure of strength, it would 

 seem better to employ the area within a given 

 isoseismal line, or line of ecjual intensity, say that 

 corresponding to intensit\- four. F'or British earth- 

 quakes it is convenient to regard as straii!^ all those 

 in which this area exceeds five thousand square miles, 

 as moderate all those in which it lies between one 

 thousand and five thousand square miles, and as 

 slif;lif all those in which it is less than one thousand 

 square miles. Making use of this convention it 

 would appear, then, that during the twenty-one years 

 considered there ha\e been in this countr\' nine 

 strong, seven moderate, and two hundred and twenty 

 three slight earthquakes, and eleven earth-sounds. 



DlSTKIlU"TI()X. 



There is \-ery little apj)roach to unitormit\- in the 

 distribution of British earthquakes. Some parts of 

 the country are frequenth' visited, others onl\- rarelw 

 or not at all. Thus, of the total number, fifty 

 originated in England. twenty-se\en in Wales, and 

 one hundred and seventy-three in Scotland. In the 

 latter country certain limited districts are subject to 

 numerous shocks. Thus, in the low-l\-ing countr\- 

 between the Ochil Hills and the P'irth of Forth 

 eighty-three shocks were felt, several of them of 

 intensities six and seven ; in Cilen Garr\', in 

 Inverness-shire, fort\"-one shocks, all of them slight ; 

 while, in the small tract Iving along the line of the 

 Caledonian Canal between Inverness and Loch Ness. 

 thirty shocks originated, two of them disturbing areas 

 of seven thousand five hundred, and thirt\--thrce 

 thousand square miles. 



In England, the country between Hereford and 

 Ross has been visited by thirteen shocks, one of 

 them the strongest felt in this country during the last 



quarter of a century, in whichever wav strength be 

 measured. In Derbyshire, betw^een Ashbourne and 

 \\'irksworth, eight earthquakes havo occurred, two 

 of them strong. Other parts ot 'he country are nut 

 specially favoured, with the excep^' on perhaps of the 

 county of Corn-,' all, to which ten si.^'it shocks must 

 he credited. 



Tlie Welsh ea> iquakes are occasioi Ih' > !; con- 

 siderable strength. '.'I "y occur for the m::.?: pa,rt in 

 three districts, one b ing in the north- ; ;:t cf 

 Carnarvonshire, the othii^ in the south ot ■.hs 

 country, in Pembrokeshire and Glamorgan. Th 

 number among them four of the nine strong edth- 

 quakes, and all four, it mav be noticed, were felt 

 across the channel in the eastern and south-eastern 

 counties of Ireland. 



Only a small part of Great Britain has been 

 undisturbed by any sensible earthquake during the 

 t\\ent\'-one years, the only unshaken districts being 

 the extreme north-east of England and the southern 

 part of Scotland. The greater part of Ireland has 

 entireh' escaped from all terrestrial disturbance, and, 

 so far as I know, not a single earthquake has actually 

 originated during the inter\al considered \\ithiii the 

 area of this island. 



NATrRi-; OF THK Shock and SorxD. 



The first intimation that we generall}- receive of 

 the coming earthquake is a low rumbling noise. 

 \\'ithin a second or two, as the noise grows louder, 

 a tremor begins to be felt, the separate vibrations 

 being small and occurring at the rate of about five or 

 six a second. Within two or three seconds these 

 merge into the main part of the shock, consisting of 

 larger vibrations or jolts, with a period of perhaps 

 one-third of a second and a total range, even in the 

 strongest earthquakes, of probably only a fraction of 

 an inch. Close to the centre these vibrations have 

 been described as like the fierce beats of a railway 

 engine travelling rapidl\" ; but at a distance of fift\' 

 miles or more they become smoother and slower, 

 like the movements felt in a carriage with good 

 springs. During the whole of this time the rumbling 

 sound continues, becoming louder and more grating 

 with the principal vibrations, and occasionally inter- 

 spersed with deep explosive crashes. .\s a rule the 

 strong vibrations begin to die away after two or 

 three seconds, and are succeeded by a weaker tremor 

 and noise, until, finall\', after the lapse of six or 

 eight seconds, both die away, the sound continuing 

 for perhaps a second or two after the shock. The 

 total duration of the shock in a strong earthquake is 

 thus from six to eight or nine seconds. 



In slight earthquakes the phenomena are much 

 simpler. Sometimes only a tremor is felt, lasting 

 for at most two or three seconds, and accompanied 

 b\- the usual ruml)ling noise: but, as a rule, among 

 the tremors, and generally at the beginning, one 

 prominent vibration is felt, so that it seems as if a 

 heav\- weight had fallen with a thud upon the 

 ground, with the brief quiver following as such a 

 thud might be expected to cause in a building. 



