ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. AZ 
Of Seismic Energy in relation to Time. 
Plates I. II. II]. IV. V. and VI. carry down the stream of time the whole 
series of observations from 2000 years before the Christian era to the year 
1850. 
In all these chrono-seismic curves the ordinate is that of epoch, and must 
not be confounded with one expressing in anywise the duration of each 
shock or separate seismic effort. The abscissa is that of seismic intensity, 
which has been assumed proportional to the number of coincident seismic 
efforts, without taking any account in the curve of the variable intensity of 
different efforts. This is a source of uncertainty that would not have been 
avoided, but rather the tendency to error increased, by any conventional 
law of enlargement of the abscissa that could have been devised to suit the 
vague proportion of greater or less in earthquake narrations; but the means 
are given to the reader of applying such corrective as the information admits, 
by placing along the line of time down to the year 1750 the letter G above 
each epoch at which an earthquake of undoubtedly great and destructive 
intensity has been recorded, and the letter S above all! those that were so 
circumstanced as to have been followed by the influx of “ great sea waves.” 
This notation might have been carried on further, but that after the year 
1750, when observations rapidly multiply, the number of earthquakes re- 
corded as being “ great” are so numerous, that to distinguish their epochs 
thus would have involved the extension of the ordinate to a new and incon- 
veniently enlarged scale. For the first three centuries of historic time 
(according to our commonly accepted chronology) it will be seen that there 
are no earthquake records, and that, while between a.c, 1700 and a.c, 
1400 there are a few scattered facts, there is again from a.c, 1400 to a.c. 
900, nearly a period of five hundred years of perfect blank, followed again 
(with a few exceptions) by another blank from a.c, 800 to a.c. 600, Even 
in the succeeding century, but two earthquakes are recorded; so that, in 
fact, the record of any value for scientific analysis may be said to commence 
at the five hundredth year before the Christian era. 
It is only in the first century prior to our era that the curve shows that 
observations may be at length deemed even continuous, every previous cen- 
tury being interrupted by lengthened lacune. 
From the commencement of the Christian era downwards to the present 
day, the abscissz continually increase in closeness and magnitude, and at the 
first casual glance suggest the idea that earthquake energy has increased 
over the whole earth during the course of ages in a fearful manner, We 
shall see, however, reason to correct any such conclusion. 
Although periods of thirty and forty years occur in the second and third 
centuries of our era without the record of a single earthquake, it did not 
seem advisable to affirm as certain the want of all observation, by the sub- 
stitution here of lacune for the continuity of the curve. 
_ The end of the third century first gives evidence of numerical increase; 
and the increase thence is steadily progressive up to the year 1850. 
It is not, however, until the seventeenth century that the increased number 
of earthquakes becomes strikingly remarkable, increasing still more in the 
eighteenth, and presenting a far greater number in the first half of the 
nineteenth than in both the preceding centuries taken together. 
Yet this vast and rapid expansion, in the three last centuries especially, 
affords no proof whatever that there has been a corresponding, or even any 
inerease in the frequency of earthquake phenomena. Our chrono-seismic 
curve is, in fact, not only a record of earthquakes, but a record of the ad- 
