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ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 5] 
successive historic groups, of time extending over the whole record of the 
catalogue :— 
TABLE XXIX. 
Historic Group. Ratio per Month. | Ratio per Year. 
ANGO:t0 LOOO 8.0. ccnsssserceeess 0°00033 0:004 
1001 3.c. to Christian era ... 0:0045 0°054 
Bel TOAD. LOUOM cris cheeos 0°0185 0°222 
A.D. 1001 to A.D. 1850 4... 0°545 7°740 
Te AL tO AnD LSIOscneccs0s 1:450 17°370 
Ae TZOL tO AnD, LODO. .ccasns- 2°610 35°310 
These numbers are absolute as well as proportional; nothing can more 
distinctly show the relation between the expanding areas of our curves of 
time and the increase of observation. 
Sir Charles Lyell, at p. 428 (‘ Principles of Geology,’ 7th edit.), caleu- 
lates, upon approximate data, the average number of actual eruptions of 
volcanic matter at 2000 per century, or 20 per annum,—a result which har- 
monizes sufficiently with the preceding, and gives support to the commonly 
received view of the connected nature of volcanic and seismic phenomena. 
This connexion receives further confirmation from the facts recorded by 
Perrey (‘ Mem. on Chili,’ p. 201), as to the long duration there, of many 
earthquakes of a character much more violent and decisive than the tremors 
long continued, at Comrie, East Haddam, &c. He mentions earthquakes 
in 1647, 1730, 1751, 1819, 1822, and 1833, each of which lasted, with little 
intermission, for several months, and which, from other sources of in- 
‘formation, seem to have been in some instances contemporaneous with pro- 
longed activity of the neighbouring volcanic regions. 
Of Seismic Energy in relation to Season. 
I now proceed to such discussions as the data will admit, of the relations 
between seismic development and the time of year. In Plate VIII. are given 
the curves of mensual seismic energy obtained from the entire period of the 
catalogue, thirty-two centuries, 
The northern and southern hemispheres of observations have been 
separated for the following reasons. ‘The total number and value of the 
Observations in each, present great disparity between them respectively. 
We are enabled graphically to present 5879 observational results for the 
northern, and but 223 for the southern hemispheres; and, for convenience, 
the vertical or seismic abscissa of the former is on a scale which bears to 
that of the latter the ratio of 100: 1; the ordinate of time, which extends 
to the cycle of an entire year, and is divided and marked for the twelve 
Months in order, is the same for both figures. As the months, in fact, in- 
volve or contain the seasons of the year, and indeed all other divisions of 
our solar revolution, and as the latter are unlike for opposite hemispheres, 
and are hereafter to be compared, such subdivision is necessary. 
_ Examining figs. 1 and 2, Plate VIII., we find in the northern hemisphere 
the annual paroxysmal minimum in July, in the southern it appears to be 
in March. The duration of this minimum in the northern extends, with no 
very considerable fluctuation, over nearly two months, and suddenly rises 
E2 
