made in our century, which has first rendered a real seismology, a
scientific knowledge of the seismic conditions of the earth, possible
through the immense development of technics, by which a system of
instrumental observation of earthquakes was established. Only through
this could the acquisitions of recent times be utilized. While
formerly observations were macroscopic and touched only earthquakes
that could be directly felt, they now cover essentially microscopic
tremors of the earth's crust, of less than a thousandth of a
millimetre, that are wholly imperceptible to human senses; and we can
read them, enlarged at our pleasure, on our photographically
registering seismometers. We already had instruments which correctly
indicated the time of the beginning and possibly the direction of a
shock; but we needed and have invented new instruments--various sorts
of horizontal and vertical pendulums--for the observation and
representation of the whole course of the movement. The vertical
indicating instruments are much used in Italy, and the horizontal ones
almost exclusively in England, Japan, and Germany. The horizontal
pendulum was invented in Germany in 1832 by Hengler, adapted to
scientific use by Professor Zöllner, of Leipsic, and afterward applied
in that form by English, German, and other observers. The most
complete shape and the one best adapted to extremely delicate seismic
observations was given to it by the late German astronomer and
geographer Dr. Ernst von Rebeur Paschnitz, of Merseburg. Having
undergone a few small changes, fixed in a threefold combination it
serves as our most sensitive and accurate seismometer. Its movements
and its very exact time markings are photographically represented. The
pendulum box is only forty centimetres in diameter. In consequence of
its convenience and cheapness, its self-action and its serviceability,
it is becoming adopted more and more generally as an international
instrument.
Microseismic investigation and its wide extension over the earth have
raised seismology another step during the last twenty years, so that
it may be said that really exact seismic research began with it.
Modern seismology has confirmed many of the older results, such as the
localization of earthquakes on the shores of the Pacific, the
Mediterranean and in the mountain chains of the earth, and also the
importance of homoseists and the epicenter. It has, on the other hand,
greatly modified the former estimates of the velocity of propagation
of the shocks. It has cast much doubt on speculations as to the
seasons in which earthquakes are more or less frequent; and it has
demonstrated the inadequacy of former methods of determining the
central focus. It has furthermore brought us much that is new. First
is the momentous fact that the earth's crust is never at rest; that it
undergoes a multitude of very diversified movements besides those of
the earthquake. Thus a periodical swelling, a flood wave, is produced
by the attraction of the moon; and other heavings are induced by the
daily and annual course of the sun's heat. But such movements and
other similar ones do not come within the scope of this article.
Real earthquakes, or movements that originate in the depths of the
earth, also appear in very different forms. First are the directly
perceptible shocks, from the powerful ones that create great
disturbances to the merely local ones often hardly remarked. Of the
immediate workings of these shocks, microscopic instruments have
taught us nothing essentially new. But very many macroscopic
movements, often continuing for several hours, but which are not felt,
have been revealed, that have been shown in many instances to be
distant effects of other strong earthquakes; effects which are
sometimes propagated over the whole surface of the earth. There is,
furthermore, another series of movements, only partly explained as
yet, of a peculiar sort: first, small, quickly passing disturbances,
which appear in the photographic reproductions of the curves as larger
or smaller knots, and which are regarded with great probability as
distant effects of minor seismic movements most likely imperceptible
anywhere. They can not be local earthquakes, for they give entirely
different curves. There also appear, with considerable regularity, at
certain seasons of the year, very slow movements of the ground, called
pulsations; and finally the multitude of vibrations called tremors,
which assume various forms. Sometimes they come as forerunners,
accompaniments, or followers in close association with those great
disturbances that originate in distant earthquakes; sometimes as
shocks of minute intensity in separate groups, which it has not yet
been possible to account for; and in other cases they are traced to
the shaking of the ground by the wind. It is hardly necessary to
observe that the seismic apparatus should be most carefully guarded
against disturbance by the movements of trade, wagons, etc., so that
the problem shall not be complicated by them.
The theory of the nature of earthquake shocks, their transmission and
their velocity, has been set in a new light by the labors of Augustus
Smith, of Stuttgart. From some calculations of their velocity made by
G. von Nebeur, it is found that the earthquake of April 17, 1889, in
Tokio, Japan, was perceived in Potsdam, Prussia, nine thousand
kilometres distant, in thirteen minutes; that of October 27, 1894, in
Santiago, Chili, in Rome, eleven thousand five hundred kilometres
distant, in seventeen minutes, and in Charkow, Russia, two thousand
kilometres from Rome, between one and two minutes later. It reached
Tokio at the same time, after a transit of seventeen thousand four
hundred kilometres.
Still another task of modern seismology is the investigation of
earthquakes at sea, or seismic movements of the bottom of the ocean,
and the manner in which they are propagated through the water, of
which a very fine cartographic representation has been published by
Dr. C. Rudolph, of Strasburg.
The question of the origin of earthquakes stands in constant
connection with this external development of seismology. It is
significant and remarkable that the answers to it, though they may be
given differently from different scientific points of view, are always
consistent in one fact, that earthquakes are a phenomenon of the whole
earth. Some of the investigators seek to explain them, aside from
those that occur in volcanic regions, as a part of the great changes
in the earth's crust which have taken place during the last geological
epoch, and are still, perhaps, taking place; others find their seat
and cause in the unstable condition of the interior of the earth,
beneath its solid and red-hot envelope. The former explanation, the
older and heretofore the prevalent one, is called the tectonic theory,
because it is based, leaving out volcanic earthquakes, on the
structure of the earth's crust; the second, which is gaining ground,
and requires no separate explanation for volcanic earthquakes, may be
called, reviving an expression used by L. Fr. Naumann, of Leipsic, the
Plutonic theory, because it goes down into the unexplored depths of
the earth. If seismic manifestations depend upon the action of the
whole earth, a single explanatory principle, as is always the case
with great natural phenomena, is not sufficient, and tectonic as well
as Plutonic earthquakes must be recognized, and the reverse.
The tectonic theory is of geological origin, and properly supplanted
the older Plutonic theory of Humboldt, which was only an unverified
supposition. As a whole it was first worked out by Otto Volger in
1858, after various similar hypotheses had been set forth by other
investigators. He was confirmed by the independent researches of
Rudolf Hoernes, Edouard Suess, and most of the German, French, and
English seismologists.
Their theory supposes that there are large hollow spaces in the crust
of the earth, into which immense falls of material take place, and
that these are the cause of a part of the earthquakes; that the crust
of the earth is often and variously disturbed in consequence of the
constant contraction dependent upon the cooling of the globe. It is
broken up into separate masses which in their turn are dislocated
horizontally or vertically; is lifted up and folded into immense
mountain ranges, the arches of which, breaking, may again suffer
dislocation. Thus continuous action in movement of masses and foldings
is constantly going on in the earth. Edouard Suess, the distinguished
Austrian geologist, has indeed constituted a special earthquake type
to correspond with this type of mountain formation. Since, in
consequence of this condition, tension is present everywhere in the
crust of the earth, it may come to pass that it shall be relieved by a
distant earthquake, and another earthquake, which may be called a
relay or transmission earthquake, be produced thereby. Hence we have,
besides the volcanic, the landfall, the tectonic (in the strict
sense), and the transmission earthquakes. The sources of earthquake
force lie, then, according to this theory, in the incompleteness of
the earth's crust, the effects of gravity, and the earth's loss of
heat.
And is the supposition not very probable? Do we not see similar
processes going on over the whole earth, in the shape of earthquakes,
landslides, fissures, subsidences of land, and the like? And as the
Alps were lifted up, and the plain of the Rhine was depressed between
the Vosges and the Black Forest, may not mightier dislocations,
breaches, and destruction occur? Why may not the processes which took
place in the earlier epochs of the earth's history and were so
powerful in the more recent Tertiary be still going on? All this seems
so plausible that, with a few exceptions, the theory has been almost
universally agreed in.
I briefly mention here Falb's theory, which, accepting the earlier
views, ascribes earthquakes to periodical swellings of the fiery fluid
interior of the earth, only because of the effect it has had on the
public in connection with some wholly unscientific predictions. More
worthy of consideration is the theory of Daubrée, the late
distinguished master of French and especially Alsatian geology, who
did not attribute the similar phenomena of volcanic and nonvolcanic
earthquakes to different causes, but maintained that all earthquakes
were produced by superheated steam issuing from surface waters. But
this theory needs no refutation. There are, however, some serious
objections to the tectonic theory of earthquakes, plausible as it may
seem. In order to weigh them as we ought, we must as briefly as
possible construct a picture of the constitution of the earth's
interior.
The average distance from the earth's surface to its center is
sixty-three hundred and seventy kilometres. The temperature of the
earth increases with the depth, at the rate, on a moderate estimate,
of about one degree centigrade for every forty metres. Hence, at a
depth of one thousand kilometres we would have a temperature of
25,000° C.; even if we call it only 15,000°, we should expect to find
there only gases, and those in a simple state, for with that heat all
the compound gases would be dissociated. The zone of fluidity for all
rocks lies at a depth of about one hundred kilometres, where the
temperature is 2,500° C. While the crust of the earth is between 2.5
and three times as heavy as distilled water at 4° C., its specific
gravity rises toward the center of the earth to more than eleven, or
about fourfold. Iron has a specific gravity of 7.8, or about threefold
that of the crust of the earth; but the specific gravity of the earth
at the greatest depth is considerably higher than this. Hence must
arise an enormous pressure, steadily increasing toward the center,
where, according to the English geophysicist, the Rev. Osmond Fisher,
it reaches about three million atmospheres to the English square inch.
It results from these conditions that with the enormous pressure and
heat, and specific gravity, the interior of the earth consists of
dissociated gases compressed to great rigidity, which exert an immense
counter-pressure--for their tendency is always to expand. They pass
out continuously into a zone of fluid matter, and this again is held
by the pressure of the interior gases in a like compact condition.
Thus a very high pressure still prevails in the lower parts of the
solid crust of the earth, which is so high that even the most solid
rocks there are in a latent plastic condition--that is, they behave
toward different forces like plastic clay, and like it can be deformed
without breaking. Rents, slides, caves, and clefts are out of the
question there; things of that kind can exist only in the upper
strata.
This fact constitutes a very strong objection to the tectonic theory
of earthquakes, and thus the very depths of the earth speak against
it. We have already mentioned that K. von Seebach estimated the depth
of the earthquake focus from the movements of the waves, and found it
not very great. But his estimates, as Prof. August Schmidt has shown,
rest upon physically incorrect premises; according to Schmidt's more
correct calculation, the center of the Charleston earthquake of 1886
lay at a depth of one hundred and twenty kilometres, where there can
be no question of tectonic movements, because general fluidity is
reached at one hundred kilometres. Further, the earthquake at Lisbon,
if the tectonic theory is valid, might, taking the character of the
region into consideration, have been occasioned by a slide. But how
large must the plunging mass, how deep the plunge or slide have been
to produce such shocks as destroyed Lisbon and shook Europe to beyond
Bohemia! Where can we find room in the closely compressed interior of
the earth for such irruptions? Even if such a sudden sinking had left
no trace in the interior, it should have left its marks on the
surface. Mr. John Milne counts up not less than 8,331 considerable
earthquake shocks in Japan between 1885 and 1892; Julius Schmidt,
former director of the observatory in Athens, enumerated three hundred
severe and dangerous and fifty thousand light shocks for Phocis alone
between 1870 and 1873, of which not a trace of land changes or
depressions can be perceived, aside from superficial avalanches (on
Parnassus, for example) and subsidence of meadows and other spongy
soil, like the famous depression of the Molo at Lisbon.
All this speaks so emphatically against the tectonic origin of
earthquakes that it can not be considered as a general cause. Even the
mighty disturbances and shocks of the times when such ranges as the
Alps and Himalayas were lifted up can prove nothing for the present
time; for the conditions, the mechanical work and acting forces, of
the earth were quite different, and the latter much greater and more
acute than in our time, as the number and magnitude of the volcanoes
of those ages show, before which ours are almost as nothing. We have
no adequate comprehension of the way that mechanical work was done. A
depression like that of the plain of the Rhine could certainly not
have taken place without severe earthquakes; but we do not know how
they may have come to pass, for we have nothing analogous to them. The
upper strata of the earth's crust are broken up, fissured, and
cavernous; hence purely local minor earthquakes may undoubtedly be
produced by cavings-in, landslides, and settlings of small extent. But
this explanation, in view of the nature of the crust, is not possible
for strong earthquakes, even in the upper layers, which send their
waves far over the land; their origin must be, almost of necessity, in
the greater deeps beneath the crust, far down where the immense gas
globe of the interior is constantly forcing its way into the fluid
band, and this into the solid stone; in those zones of changing
conditions a mighty movement must be incessantly prevailing. The
pressure upon the gases of the interior diminishes here, and the
excessive temperature as well. This can not take place without
changes. Temperature and pressure now fall, now rise again, but
continue very high through it all. The dissociated gases unite and
separate again, and most violent explosions are infallibly produced
thereby. Water exists in the interior in immense masses, and that not
solely in consequence of percolation from the surface. Vapor at very
high pressure separates into its elements--hydrogen and oxygen--the
reunion of which ensues with violent explosions, similar to our gas
explosions, which must be very numerous in the interior of the earth,
and accompanied with great development of force. The principal effect
of such explosions is, of course, against the cooler and more weakly
resisting sides, and therefore not toward the interior but toward the
crust and the weakest parts of it, toward the rupture lines of the
zones of disturbance, the synclinals. Such attacks, striking the
earth's crust from within, occasion most earthquakes, especially
violent, destructive, deep-seated outbursts like those of Lisbon and
Charleston. The relation of the seismic and the volcanic phenomena is
clearly to be seen.
One series of seismic phenomena remains to be explained--the lighter
undulations, the tremors, and the remarkable irregularity of the
movements of the ground. The indications of the vertical pendulum
apparatus which represent these movements form an inextricable tangle
of lines running over and crossing one another. The late Japanese
professor of seismology, Sekiya, prepared an enlarged model of the
tracings of the seismic movements of a point of the earth's surface,
which has been much copied. It represents an extremely confusing
vibration of the lines.
Now we have to confront a very important fact which adds much to the
difficulty of seismic research. We never feel and observe the
earthquake shocks themselves, never directly in their simplicity or
multiplicity, but only the wave movements that are sent out from them
in the elastic crust of the earth. These, however multifold their
origin, proceed in an immense spherical wave which moves in more or
less numerous repetitions through the earth's interior. It is this
shaking of the earth by the spherical waves that our instruments
represent as earthquakes. We can not include as the earth's crust the
surface of the earth on which we live, and which consists of loose
materials disintegrated by weathering, breaking, and numerous causes,
but the solid crust, often lying at a considerable distance beneath
us, which bears these materials, and from which the spherical waves
emerge. As the waves of the sea, beating upon the coast, are turned,
split up, divided, thrown up, etc., in their surging, so surge, too,
the seismic waves upon the disintegrated surface of shingle, pebbles,
broken rocks, sand, and earth, in clefts and gorges. We thus never
observe the original spherical waves, but only their fragmentary
derivative forms, their resolution into numerous single waves which
come to us diverted into the most various directions. It is thus most
plainly shown that Mallet's effort to determine the center and origin
of the earthquake from the direction of the shock was futile. We can
only draw scientific conclusions respecting the time of beginning, the
duration, and force of the movement. It is thus evident that many of
the tremors (not all, by any means) originate in this division; that a
fixed point of the earth's surface must describe a very complicated
path in so intricate a wave movement; that the division is less marked
on firm ground than on loose; that the former, in consequence of the
more evenly protracted movement, is less dangerous than the latter;
and that multiplied waves interfere, overlay, weaken, or strengthen
one another just as water waves do. Thus are explained the earthquake
bridges or spots which always remain unmoved through repeated
earthquakes, either because they are firmer, or because the progress
of the waves is arrested at them by interference.
The sounds, too, which so frequently accompany earthquakes are
likewise simply results of this division of the waves and their escape
into the air, for we perceive wave motions in the air as sound. The
admirable delicacy of our sense of hearing is here manifested, for
seismic movements are not rarely perceptible, or heard, as air waves,
which we can not perceive as movements of the ground. Earthquake
thunder is caused, like storm thunder, by shocks to the air, of which
we hear the nearest and latest first, and the farthest and earliest
last. The different tone shades of the earthquake sound depend upon
their various sources, as from small, sharp fragments, clinking,
rattling, and humming; from sand and earth, dull rumbling; from trees,
whistling, etc. The echo in ravines not rarely operates to add
strength to them. Earthquake sounds that seem to come out of the air
from above are caused by earthquake waves reaching us by way of trees,
houses, etc.; the different directions and degrees of force which they
seem to indicate in different houses or in different rooms of the same
house are explainable by the different elasticity conditions of the
houses and rooms. But not the most insignificant conclusion can be
drawn from these sounds concerning the nature and causes of
earthquakes. It is important to emphasize this fact, for errors have
often originated in conclusions drawn from such things.--_Translated
for the Popular Science Monthly from the Deutsche Rundschau._
* * * * *
Examples of a race of curiously protectively colored mice
which inhabit the sandy island, the North Bull, in the Bay
of Dublin, were exhibited by Dr. H. Lyster Jameson in the
Zoölogical Section of the British Association. A
considerable percentage of them were distinctly lighter hued
than the ancestral type of house mouse, though every
possible gradation occurred between the typical house mouse
and the palest examples. The speaker regarded the marked
predominance of sand-colored specimens as due to the action
of natural selection. The hawks and owls which frequent the
island, and are the only enemies the mice have to compete
against, most easily capture the darkest examples, or those
that contrast most strongly with the color of the sand. Thus
a protectively colored race is becoming established. The
island came into existence only about a hundred years ago.
Consequently it is possible to fix a time limit within which
the sandy-colored race has been evolved. Its evolution also,
as Professor Poulton observed in his comment on Dr.
Jameson's paper, gives additional evidence to that afforded
by the shore crabs described by Professor Weldon in his
presidential address to the section, that the transmutation
of species is not necessarily so slow as to be
indiscernible.
A SHORT HISTORY OF SCIENTIFIC INSTRUCTION[33]
BY J. NORMAN LOCKYER, K. C. B., F. R. S.
The two addresses by my colleagues, Professors Judd and
Roberts-Austen, have drawn attention to the general history of our
college and the details of one part of our organization. I propose to
deal with another part, the consideration of which is of very great
importance at the present time, for we are in one of those educational
movements which spring up from time to time and mold the progress of
civilization. The question of a teaching university in the largest
city in the world, secondary education, and so-called technical
education are now occupying men's minds.
At the beginning it is imperative that I should call your attention to
the fact that the stern necessities of the human race have been the
origin of all branches of science and learning; that all so-called
educational movements have been based upon the actual requirements of
the time. There has never been an educational movement for learning's
sake; but of course there have always been studies and students apart
from any of those general movements to which I am calling attention;
still we have to come down to the times of Louis Quatorze before the
study of the useless, the _même inutile_, was recognized as a matter
of national concern.
It is perhaps the more necessary to insist upon stern necessity as
being the origin of learning, because it is so difficult for us now to
put ourselves in the place of those early representatives of our race
that had to face the problems of life among conditionings of which
they were profoundly ignorant: when night meant death; when there was
no certainty that the sun would rise on the morrow; when the growth of
a plant from seed was unrecognized; when a yearly return of seasons
might as well be a miracle as a proof of a settled order of phenomena;
when, finally, neither cause nor effect had been traced in the
operations of Nature.