ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 49 
uniformity in the occurrence of earthquakes as distributed over long epochs 
of time. Setting aside (as contradicted by all other sources of analogy and 
information) the supposition that this, or any other phenomenon of occa- 
sional disturbance, has an increasing development upon our planet, we have 
two remaining alternatives ;—either that seismic energy is getting gradually 
spent and is dying out—this, the evidence before us appears sufficiently to 
contradict; or that, upon the whole, during our short and most imperfect 
acquaintance with it, it has remained pretty uniform throughout historic 
time, taking one long period with another. Yet, could we extend our view 
beyond the short limit of man’s history to the vast past duration of that of 
our globe itself, it might be found that seismic energy is really a slowly 
decreasing force. 
A conclusion thus appearing at the first glance even contradictory to the 
presented results from which it is drawn, may bear a certain boldness of 
aspect, for which I hope to find that the observations preceding, as to the 
true character of all earthquake records, and of the sort and amount of stress 
that may be laid upon them, will be held a justification. 
But while such uniformity or insensibly slow decadence may be the fact 
through time taken as a whole, there is also evidence of irregular and par- 
oxysmal energy in reference to shorter periods; that is to say, not only (as 
all know) do earthquakes occur at some times,-and not at others, in any 
given spot; but, taking the whole area of observation together (in which 
there is no moment, perhaps, or but a very brief one, wherein there is not an 
earthquake somewhere, or more than one), it will be found that there are 
epochs when they occur in greater numbers or intensity, either in the same 
or in several places within a limited time,—d.e. periods of paroxysmal 
energy. 
If we omit from our view all the curves of earlier periods and less ample 
observation, and limit our consideration to those of the last three centuries and 
a half, 7.e. from A.p. 1500 to 1850, this paroxysmal character becomes 
evident at a glance, and increasingly so in the last century and a half (the 
epoch of all human history the most replete with discovery), wherein the 
number of recorded observations is so great, that it was necessary for clear- 
ness to double the scale, of the ordinate of the diagram (Plate VI.) in rela- 
tion to the preceding ones. On examining these curves, they seem to 
justify the following deductions :— 
1. While the smallest or minimum paroxysmal interval may be a year 
or two, the average interval is from five to ten years of comparative 
repose. 
_ 2. The shorter intervals are in connexion with periods of fewer earthquakes 
—not always with those of least intensity, but usually so. 
3. The alternations of paroxysm and of repose appear to follow xo 
absolute law deducible from these curves. 
_ 4. Two marked periods of extreme paroxysm are observable in each 
century—one greater than the other—that of greatest number and 
intensity occurring about the middle of each century, the other 
towards the end of each. 
This is one of the most remarkable facts that these curves seem to point 
to: from about the fiftieth to the sixtieth year of each century, both the 
number and intensity of earthquakes will be observed suddenly to shoot up ; 
again, during the last quarter of the three complete centuries another but 
_ less powerful paroxysm is apparent. The paroxysmal power at these two 
| aed each century far exceeds any other paroxysms within their limits. 
. E 
