KNOWLEDGE. 



February. 1911. 



The streets -vce quiet, observers are 



iiig down, ari ^, 'hen they awake, as 



:::12 a.m.. the\- are in a 



^j are alert and read\- to 



,ent. The increase in 



.ill. seems to be due to 



the sixteen earthquakes 



.UHir. seven occurred on a 



-1 ethers possibh' in the restful 



' an early tea. Now. as shocks 



.talh' show a tendencN' to cjreatest 



observation 

 generall)- i' 



persons ;>ften do between 

 nervor.s condition, their i 

 detect the slightest '::■ 

 frequencv from 4 

 similar condition 

 .ecorded duriuL' 

 Sunday afternoii 

 interval devoted 

 recorded instru- 



bc^ local noon at an\' place, it follows 

 : • shocks, which are onl\' perceptible 

 conditions alluded to abo\'e. must be 



frequenc\- a 

 that the" ': 

 under i^e 

 much more numerous 

 than our catalogues would 

 lead us to suspect. We 

 si I all probably not be 

 over-estimating their 

 number if we consider 

 that t\\cnt\- earthquakes 

 occur in this countr\- on 



Pkriodk'ity. 



^^'hether there be anv 

 real \ariation of frequenc\' 



3^1 



throughout the da\- must 

 for the present remain 

 uncertain in the case of British eartlniuakes. Eor 

 this purpose the only records of any value are those 

 which are registered by properly erected instruments, 

 isolated completely from all artificial disturbances. 

 Personal obser\-ations are. however, sufficient to 

 determine whether any annual variation exists in the 

 frequency of earthquakes, for there is no reason for 

 supposing the conditions of observation to be sensibh- 

 better at one time of the year than another. In 

 Figure 2. the continuous line represents the numbers 

 of earthquakes recorded during the different months, 

 account, of course, being taken of their varying 

 lengths. The curve, it will be seen, is irregular, the 

 monthly number of shocks being greatest in 

 September and December. The actual frequenc\- of 

 earthquakes may, however, be due to different 

 causes, which ma\- be themselves subject to variations 

 of different periods. — just as, when a chord is plaved 

 on any instrument, the moxement of a particle of 

 air in its neighbourhood is compounded of several 

 movements, each with a different period correspond- 

 ing to those of the notes that are struck. To 

 .separate out the different periods which produce the 

 actual variation in frequency, some method of 

 harmonic analysis must be emploved. and, as great 

 accuracy is not essential, the method of overlappin;. 



Figure 2. A curve representing the number of shocks 

 recorded in this country during different months of the vear. 



the end of January : the mean of those for December 

 to Ma\' corresponds to the end of Februarv. and so 

 on. In this wa\" the broken line in Figure 2 is 

 obtained. The effect of taking six-monthl\- means is 

 to smooth the curve hv eliminating or reducing 

 periods of six months and less. The broken line 

 therefore, represents with sufficient accurac\' the 

 annual variation in tre(iuenc\- of British earthquakes. 

 The maximum of the annual period, it will be 

 noticed, falls in October, that is. in the middle of two 

 months when the actual frequencv is less than in the 

 two months of September and December on either 

 side. .\ similar method ma\' be used for determining 

 whether a si\-montbl\- period exists, but the resulting 



\'ariation is not pro- 

 notmced enough for us 

 to feel convinced of the 

 realitx' of a period of this 

 length. All that we can 

 regard as proved is that 

 there is a marked annual 

 periodicitv, and that its 

 m a X i m 11 m occurs in 

 October and its minimum 

 in .\pril. 



Intensity. 



The intensitx' of an 

 earthquake is generally 

 denoted by its greatest 

 intensit\- within the central region of the dis- 

 turbed area. l'"or this purpose an arbitrar}- scale 

 is used, known as the Kossi-Forel scale. In 

 this there are ten degrees, the two lowest and 

 the two highest being inapplicable to the earth- 

 quakes here considered, 

 are as follows, onh 

 each heading : the shock being strong enough : 

 Intensity 



3 for the direction or duration to be sensible ; 



4 to make doors, w indows. and so on, rattle ; 

 .T to cause the observer's seat to be percep- 

 tibly raised or mo\-ed : 



6 to make chandeliers, pictures, and so on, 



The remaining degrees 

 one test being given under 



7 to overthrow ornaments, vases, and so on ; 



8 to throw dow n chimnevs or crack the walls 



of some houses. 

 Of the two hundred and hfty earthquakes, three 

 were of intensit\- 8. nine of intensit}' 7, se\'en 

 of intensitv 6, twenty-nine of intensity 5, sixt_\--four 

 of intensitv 4, one hundred and twenty-seven of 

 intensit\- 3 or about 3, while the remaining eleven 

 were mereh' sounds without any tremor being felt. 

 .\s a rule, of course, the area disturbed hv an 



means will be be found sufficient for the purpose, earthquake increases w ith its intensity, the average 



The method consists in finding consecutive six- disturbed area of an earthquake of intensity three 



monthly means of the monthly numbers of being one hundred and twent\--six square miles, and 



earthquakes. Thus, the mean of the numbers for of one of intensit\- eight about sixty-six thousand 



the months from November to .Vpril inclusi\-e, square miles. Earthquakes of the same degree of 



corresponds to the middle of that interxal, that is, to intensitv are, however, felt over w idely differing 



