The Causes and Phenomena of Earthquakes. 67 



From the beginning of 16th to end of 18th 2804 



From 1799 to 1850 3240 



(Or, in 1850 years, 6773) ... ... ... ... 6831 



Of these, 216 were great earthquakes. The deduction from this 

 is that, at present, we may calculate on one great earthquake 

 destructive of life and property in some part of the world every 

 eight months. (Mallet, Eep. B.A., 1858, p. 56.) and that in the 

 Southern hemisphere the observed earthquakes may be now esti- 

 mated at one in every two years. Sir Charlas Lyell calculates 

 that the average number of volcanic eruptions is 2000 per cen- 

 tury, or 20 per annum, since the ratio for earthquakes, according 

 to Mallet's table, from 1701 to 1850, is 35.310 per annum. 



On this is based one proof of the conclusion, that earthquakes 

 and eruptions have a connection. But this is not that of cause 

 and effect. Just now were mentioned great earthquakes. These 

 are arranged in the first class, as destroying multitudes of people 

 and devastating wide areas. 2. Mean earthquakes produce 

 damage of an inferior kind. 3. Minor earthquakes do little 

 harm and occur frequently. The shock of 18th June bordered 

 on the second class, and missed a very little of doing great 

 damage. 



The radii of these three kinds of shocks are assumed to be 

 540, 180, and 60 geopraphical miles. 



I have only hitherto selected such facts as throw a general 

 light on the great features of earthquake disturbances ; but in 

 order to apply, before I conclude, such as belong to the physical 

 history of this part of the world, it will be necessary to enumerate 

 a few of the deductions which have been derived from the exami- 

 nation of the 6000 or 7000 earthquakes already mentioned. 



"We are to understand, then, that earthquakes occur in all 

 parts of the earth and ocean, at all seasons and at all hours of day 

 and night. No part of the earth's crust has always, or at any 

 epoch, been more subject than another to shocks ; but there is a 

 periodicity in earthquakes, and those countries are most liable to 

 be shaken which are in proximity with the present lines of 

 volcanos. 



Tracts not now near volcanos nor centres of extinct action, are 

 subject to frequent shocks, whilst those tracts that are, are not 

 particularly distinguished. Thus, indeed, there are areas in which 

 igneous action action was formerly general, and even now is 

 partially not extinct, in which earthquakes are seldom or ever 

 felt. Such, according to Newbold (Proc. G-eol, Soc, Q.J., iv., 

 pp. 336 and 339, 1848), is the case in Egypt, and along the Red 

 Sea ; and, according to Mr. St. John, in the Nubian desert, 

 about seventy miles from Syene, whereby there is clear evidence 



