EARTH TREMORS.
Artificially produced tremors—Observations of Kater, Denman,
Airy, Palmer, Paul—Natural tremors—Observations of Zöllner,
M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With
seismoscopes, microphones, pendulums—Work in Italy—Bertelli,
Count Malvasia, M. S. di Rossi—Instruments employed in
Italy—Tromometers, microseismographs, microphones—Results
obtained in Italy—in Japan—Cause of microseismic motion.
During the past few years considerable attention has been drawn towards
the study of small vibratory motions of the ground, which to the
unaided senses are usually passed by without recognition. These motions
are called _earth tremors_. Their discovery appears to have been due
to accident, and not to the results of inductive reasoning. No sooner
had philosophers contrived astronomical and other instruments for the
purpose of making refined measurements and observations than they at
once discovered that they had an enemy to contend against in the form
of microscopic earthquakes.
_Artificially produced tremors._—Artificial disturbances of this
description exist in all our towns, and near a railway line they are
perceptible with every passing train. Those who have used microscopes
of high power will readily appreciate how small a disturbance of
the ground is visible in the apparent movement of the object under
examination.
Captain Kater found that he could not perform his pendulum experiments
in London on account of the vibrations produced by the rolling of
carriages. Captain Denman, who made some observations on artificially
produced tremors, found that a goods train produced an effect 1,100
feet distant in marshy ground over sandstone. Vertically, however,
above a tunnel through the sandstone, the effects only extended 100
feet.
A remarkable example of the trouble which artificially produced earth
vibrations have occasioned those who make astronomical observations
occurred some twenty years ago at the Greenwich Observatory. When
determining the collimation error of the transit circle by means of the
reflexion of a star in a tray of mercury, it was found that on certain
nights the surface of the mercury was in such a state of trembling
that the observers were unable to complete their observations until
long after midnight. After obtaining a series of dates on which these
disturbances occurred, it was found that they coincided with public and
bank holidays, on which days crowds of the poorer classes of London
flocked to Greenwich Park, and there amused themselves with running and
rolling down the hill on which the observatory is situated. On these
occasions it was found that the disturbances in the mercury were such
that observations could not be made until two or three hours after the
crowds had been turned out of the neighbouring park.[137]
To obviate this difficulty Sir George Airy suspended his dish of
mercury in a system of india-rubber bands, and in this way succeeded in
eating the intruders up.
Lieutenant-Colonel H. S. Palmer, R.E., when engaged with the transit of
Venus expedition in New Zealand, in 1874, was troubled with vibrations
produced from a neighbouring railway. To escape the enemy he intrenched
his instruments by placing them in pits. With pits 3½ feet deep he
found himself sufficiently protected. The distance from the line was
about 400 yards, and the soil through which the disturbances were
propagated was a coarse pebbly gravel.[138]
Before the United States Naval Observatory was established at
Washington, Professor H. M. Paul was deputed to make a tremor survey
to discover stable ground. The results of these experiments were
exceedingly interesting. By watching the reflected image of a star in
a dish of mercury a passing train would be noticed at the distance of
a mile. Its approach could be detected by the trembling of the image
before its coming could be heard. At one point of observation the
disturbance appeared to be cut off by a ravine. The strata was gravel
and clay.[139]
These few examples of artificially produced tremors, to which many
more might be added, have been given because they teach us something
respecting their nature. Hitherto earth tremors have only been regarded
as intruders, which it was necessary to escape from or destroy. From
what has been said they appear to be a superficial disturbance which
is propagated to an enormous distance. This distance appears to
depend upon the propagating medium, upon the intensity of the initial
disturbance, and upon its duration. In the observation of these
artificial disturbances, which are accessible to every one, and which
hitherto have been so neglected, we have undoubtedly a fruitful source
of study.
_Natural tremors._—Next let us turn to those microscopical
disturbances of our soil which are due to natural causes. Thus far they
seem to have been recorded wherever instruments suitable for their
detection have been erected, and it is not improbable that they are
common to the surface of the whole globe.
Some of the more definite observations which have been made upon earth
tremors were those made in connection with experiments on the deviation
of the vertical due to the attractive influence of the moon and sun.
Professor Zöllner, who invented the horizontal pendulum which he used
in the attempt to measure the change in level due to lunar and solar
attraction, found his instruments so sensitive that the readings were
always changing.
The most interesting observations which were made upon small
disturbances of the soil were those of M. d’Abbadie, who carried on his
experiments at Abbadia, in Subernoa, near Hendaye, 400 mètres distant
from the Atlantic, and 62 mètres above sea level. The soil was a loamy
rock. Here M. d’Abbadie constructed a concrete cone 8 mètres in height,
which was pierced down the centre by a vertical hole or well, which was
continued two mètres below the cone into the solid rock. At the bottom
of this hole or well a pool of mercury was formed which reflected
the image of cross wires placed at the top of the hole. These cross
wires and their reflection were observed by means of a microscope.
The observations consisted in noting the displacement and azimuth of
the reflected image relatively to the real image of the wires. After
allowing this structure five years to settle, M. d’Abbadie commenced
his observations. To find the surface of the mercury tranquil was
a rare occurrence. Sometimes the mercury appeared to be in violent
motion, although both the air and neighbouring sea were perfectly calm.
At times the reflected image would disappear as if the mercury had
been disturbed by a microscopic earthquake.
The relative positions of the images were in part governed by the state
of the tide. Altogether the movements were so strange that M. d’Abbadie
did not venture any speculations as to their cause, but he remarks
that the cause of the changes he observed were sometimes neither
astronomical nor thermometrical. These observations, the principal
object of which was to determine changes in level rather than earth
vibrations, were carried on between the years 1868 and 1872.[140]
_Observations at Cambridge._—Another instructive set of observations
were those which were made in the years 1880–1882, by George and Horace
Darwin, in the Cavendish Laboratory, at Cambridge. The main object
in these experiments was to determine the disturbing influence of
gravity produced by lunar attraction. The result which was obtained,
however, showed that the soil at Cambridge was in such an incessant
state of vibration that whatever pull the moon may have exerted upon
the instrument which was employed was masked by the magnitude of the
effects produced by the earth tremors, and the experiments had, in
consequence, to be abandoned.
The principle of this instrument was similar to one devised by Sir
William Thomson, and put up by him in his laboratory at Glasgow.
As erected by the brothers Darwin, at Cambridge, it was briefly as
follows: A pendulum, which was a massive cylinder of pure copper,
was hung by a copper wire, about four feet long, inside a hollow
cylindrical tube rising from a stone support. A small mirror was then
hung by two silk fibres, one of which was fastened to the bob and the
other to the stone basement. A ray of light sent from a lamp on to
the mirror was reflected to a scale seven feet distant, and by this
magnification any motion of the bob relatively to the stone support
was magnified 50,000 times. In several ways the apparatus was insulated
from all accidental disturbances. The spot of light was observed from
another room by means of a telescope. This instrument was so delicate
that even at the distance of sixteen feet the shifting of your weight
from one foot to the other caused the spot of light to run along the
scale. So sensitive was the instrument that, notwithstanding its being
cut off from the surrounding soil by a trench filled with water and
the whole instrument being immersed in water to damp out the small
vibrations, it would seem that the ground was in a constant state of
tremor; in fact, so persistent and irregular were these movements that
it seemed impossible to separate them from the perturbations due to the
attraction of the moon.[141]
As a result of observations like these, the world had gradually
forced upon it the fact that the ground on which we live is probably
everywhere in what is practically an incessant state of vibration.
This led those who were interested in the study of earth movements to
establish special apparatus for the purpose of recording these motions
with the hope of eventually discovering the laws by which they were
governed.
_Experiments in Japan._—The simpler forms of apparatus which have been
used in Japan may be described as delicate forms of seismoscopes,
which, in addition to recording earth tremors, also record the
occurrence of small earthquakes.
A simple contrivance which may be used for the purpose of recording
small earthquakes can be made with a small compass needle.
If a light, small sensitive compass needle be placed on a table, it
will be found that a small piece of iron like a nail may be pushed so
near to it that the needle assumes a position of extremely unstable
equilibrium. If the table now receives the slightest tap or shake this
condition is overcome, and the needle flies to the iron and there
remains. By making the support of the needle and the iron the poles of
an electric circuit it is possible to register the time at which motion
took place with considerable accuracy.
With crude apparatus like this a large number of small earth
disturbances have been recorded.
Another form of apparatus, employed in Japan, has been a delicately
constructed _circuit closer_. The motions of this instrument were
recorded by causing an electro-magnet to deflect a pencil which was
tracing a circle on a revolving dial. The revolving dial was a disc of
wood covered with paper fixed to the hour-hand axle of a common clock.
A third form of apparatus used in Japan consisted of a small piece of
sheet lead about the size of a threepenny piece suspended by a short
loop from a rigid support. Projecting from the lead a fine wire, about
two inches in length, passed freely through a hole in a metallic plate.
By the slightest motion of the support the small pendulum of lead was
set into a state of tremor and caused its pointer to come in contact
with one or other side of the hole in the metal plate and thus to close
an electric circuit.
A more refined kind of apparatus which has been employed in Japan was
similar to that used by the Darwins at Cambridge. This was so arranged
that any deflection of the mirror was permanent until the instrument
was reset, and in this way the maximum disturbance which had taken
place between each observation was recorded.
In addition to these and other contrivances, experiments were made with
microphones.
The microphones used were small doubly pointed pencils of carbon
about three centimetres long, saturated with mercury, and supported
vertically in pivot holes bored in other pieces of carbon, which were
the terminals of an electric circuit. These microphones were screwed
down on the top of stakes driven deeply into the ground. They were
covered with a glass shade thickly greased at its base. The stakes were
in the ground at the bottom of a small pit—about two feet square and
two feet deep—which was lined with a box. The box was covered with a
lid, and earth to the depth of nine inches or one foot. One of these
pits was in the middle of a lawn in the front of my house, and the
other was at the foot of a hill at the back of the house. The wires
from the microphone passed through the side of the box into a bamboo
tube and thence up to my dining-room and bed-room. In one of the
circuits there were three Daniell’s cells, a telephone, and a small
galvanometer. I used the galvanometer because I found that when there
was sufficient motion in the microphone to produce a sound in the
telephone a motion in the needle of a galvanometer was produced. If in
any case motion took place in the magnetic needle during my absence, it
was held deflected by a small piece of iron with which it was brought
into contact by the movement.
The sensitiveness of the arrangement may be judged of from the fact
that if a person walked on the grass within six feet of the microphone,
each step caused a creak in the telephone, and the needle of the
galvanometer was caused to swing and come in contact with the iron.
Dogs running on the grass had no effect. A small stone one or two
inches in diameter thrown from the house so that it fell near to the
microphone pit caused a sharp creak in the telephone and a movement in
the needle.
The nature of the records I received from this contrivance may be
judged of from the following extract from my papers.
h. m.
1879. Nov. 12th 7 0 P.M. contact of needle
7 2 „ difficult to set the needle
7 3 „ needle swings and telephone creaks
7 4 „ „ „ „
7 5 „ „ „ „
7 6 „ „ „ „
7 10 „ 3 more swings
7 11 „ again „
Here I went out, took away the covering, and examined the microphone.
Nothing wrong was to be observed. All that I saw was one small ant. I
do not think that this could have caused the disturbance, because it
could not get near the instrument.
On the succeeding nights I experienced similar disturbances, and it
seemed as if they might possibly have been the prelude to several small
shocks which occurred about this time (November 15, 16, and 17). On
November 17, at 8 A.M., the needle was found in contact, and again at
5 P.M., and at 6 P.M. the shock of a small earthquake was felt _which
caused a rattling sound in the telephone for about one minute after the
motion had appeared to cease_. The needle swung considerably, but did
not come in contact.
The great objection to these observations is that it is possible
that the movements and sounds which I have recorded might, with the
exception of one case when the shaking was actually felt, possibly have
been produced by causes other than that of the movement of the ground.
To determine this I subsequently put up two distinct sets of apparatus
to determine whether the motions of each were synchronous. So far as
I went this appeared only to be sometimes the case:—but this is a
question difficult to determine, unless a recorder of time be added to
the apparatus.
The greatest objection to observations of this sort is that the
sensibility of the instrument is not constant. After a current has been
running for several days it is no longer sensible to slight shocks, it
appears as if its resistance had been increased. To overcome this it is
necessary to resharpen the carbon points and bore out the pivot holes
every three or four days. Farther, the battery varies. This might to
some extent be overcome by using a battery with large plates. These
two causes tend to reduce the sensitiveness of the galvanometer-like
recorder—the deflection of the needle gradually becoming less and less,
and therefore day by day needing a greater swing to bring it into
contact with the iron. For reasons such as these this instrument, to be
used successfully, appears to require considerable attention.
Another form of microphone employed by the author consisted of an
aluminium wire standing vertically on a metallic plate, its upper end
passing loosely through a hole in an aluminium wire standard.
The upper end of the vertical wire was loaded with lead. This
contrivance possesses all the sensitiveness of an ordinary microphone,
whilst, if it receives a sudden impulse, there is a sudden break in the
current, and the vertical wire is thrown from one side to the other of
the hole in the standard.
After many months of tiresome observation with instruments of this
description, and after eliminating all motions which might have been
produced by accidental causes, the general result obtained showed that
in Tokio there were movements of the soil to be detected every day,
and sometimes many times per day, which to ordinary persons were passed
by unnoticed.
_Work in Italy._—The most satisfactory observations which have been
made upon microseismic disturbances are those which have been made
during the last ten years in Italy. The father of systematical
microseismical research appears to have been Father Timoteo Bertelli,
of Florence.
In 1870 Father Bertelli suspended a pendulum in a cellar, and observed
it with a microscope. As the result of his observations it was
announced that he had perceived the earthquakes which shook Romagna,
although to the ordinary observer in Florence these shakings had not
been perceptible.
In 1873 Bertelli, by means of microscopes fixed in several azimuths,
made 5,500 observations on free pendulums. He also made observations on
reflections from the surface of mercury.[142]
One result of these observations was to show that microseismic motions
increased with a fall of the barometer. Similar observations were made
at Bologna by M. le Conte Malvasia, and also by M. S. di Rossi, near
Rome. On January 14, 15, and February 25 these three observers at their
respective stations simultaneously observed great disturbances.
Similar investigations were made at Nice by M. le Baron Prost.
Although doubt was cast upon Bertelli’s observations they appear to
have been the origin of a series of microseismical observations, a
distinguished leader in which is Professor Rossi, who, in 1874, found
that large earthquakes were almost always preceded or accompanied with
microseismical storms. In 1878 Professor Rossi worked upon these small
disturbances with the assistance of the microphone and telephone, and
his first results were published by Professor Palmieri.
Many of Professor Rossi’s observations were made in the grotto of Roca
de Papa, 700 mètres high and eighteen mètres under the soil. Here over
6,000 observations were made by means of microscopes, on pendulums of
different lengths, suspended in tubes cut in the solid rock.
_Instruments employed in Italy._—It is impossible to describe in detail
the various forms of apparatus which have been used by the Italian
investigators. A description of one or two of the more important
instruments may not, however, be out of place, inasmuch as they will
assist the reader to understand the manner in which the various results
respecting the laws governing microseismic movements have been arrived
at.
The most important of these instruments is the _Normal Tromometer_ of
Bertelli and Rossi.
This consists of a pendulum 1½ metres long, carrying, by means of
a very fine wire, a weight of 100 grammes. To the base of the bob
a vertical stile is attached, and the whole is enclosed in a tube
terminated, at its base, by a glass prism of such a form that when
looked through horizontally the motion of the stile can be seen in
all azimuths. In front of this prism a microscope is placed. Inside
the microscope there is a micromatic scale, so arranged that it can
be turned to coincide with the apparent direction of oscillation of
the point of the stile. In this way not only can the amplitude of the
motion of the stile be measured, but also its azimuth. The extent of
vertical motion is measured by the up and down motion of the stile due
to the elasticity of the supporting wire. This instrument is shown in
the accompanying drawing.
[Illustration: FIG. 37.—Normal Tromometer.
B, bob of pendulum; P, prism; M, microscope; S, rim of scale.]
Another apparatus is the _Microseismograph_ of Professor Rossi. Here we
have an arrangement which gives automatic records of slight motions.
It consists of four pendulums, each about three feet long, suspended
so that they form the corners of a square platform. In the centre
of this platform a fifth, but rather longer, pendulum is suspended.
The four pendulums are each connected just above their bobs to the
central pendulum with loose silk threads. Fixed to the centre of each
of these threads, and held vertically by a light spring, is a needle,
so adjusted that each thread is depressed to form an obtuse angle of
about 155°. These needles form the terminals of an electric circuit,
the other termination of which is a small cup of mercury placed just
below the lower end of the needle. By a horizontal swing of one of the
pendulums this arrangement causes the needle to move vertically, but
with a slightly multiplied amplitude. By this motion the needle comes
in contact with the mercury, and an electro-magnet with a lever and
pencil is caused to make a mark on a band of paper moved by clockwork.
The five pendulums being of different lengths allows the apparatus to
respond ‘to seismic waves of different velocities.’[143]
Lastly, we have Professor Rossi’s _Microphone_. This consists of a
metallic swing arranged like the beam of a balance. By means of a
movable weight at one end of the beam this is so adjusted that it
falls down until it comes in contact with a metallic stop. This can be
so adjusted that a slight tap will cause the beam to slightly jump from
the stop. The beam and the stop form two poles of an electric circuit,
in which there is a telephone. The slightest motion in a _vertical_
direction causes a fluctuation in the current passing between the stop
and the beam, and a consequent noise is heard in the telephone.
With instruments analogous to these, observations have been made by
various observers in all portions of Italy, extending over a period
of ten years. Every precaution appears to have been taken to avoid
accidental disturbances, and the experiments have been repeated in a
variety of forms.
_Results obtained in Italy._—The results which from time to time have
been announced are of the greatest interest to those who study the
physics of the earth’s crust, and they appear to be leading to the
establishment of laws of scientific value.
It would seem that the soil of Italy is in incessant movement, there
being periods of excessive activity usually lasting about ten days.
Such periods are called seismic storms. These storms are separated by
periods of relative calm. These storms have their greater regularity
in winter, and sharp maximums are to be observed in spring and autumn.
In the midst of such a period or at its end there is usually an
earthquake. Usually these storms are closely related to barometric
depressions. To distinguish these movements from those which occur
under high pressure, the latter are called baro seismic movements, and
the former _vulcano seismic_ movements. The relation of these storms
to barometric fluctuation has been observed to have been very marked
during the time of a volcanic eruption.
At the commencement of a storm the motions are usually small, and one
storm, lasting two or three days, may be joined to another storm.
In such a case the action may be a local one. It has been observed
that a barometrical depression tended to bring a storm to a maximum,
whilst an increase of pressure would cause it to disappear. Sometimes
these actions are purely local, but at other times they may affect a
considerable tract of land.
If a number of pendulums of different length are observed at the same
place, there is a general similarity in their movements, but it is also
evident that the free period of the pendulum more or less disturbs
the character of the record. The greatest amplitude of motion in a
set of pendulums is not reached simultaneously by all the pendulums,
and at every disturbance the movement of one will predominate. From
this Rossi argues that the character of the microseismical motions is
not constant. Bertelli observed that the direction of oscillation of
the pendulums is different at different places, but each place will
have its particular direction dependent upon the direction of valleys
and chains of mountains in the neighbourhood. Rossi shows that the
directions of movement are perpendicular to the direction of lines of
faults, the lips of these fractures rising and falling, and producing
two sets of waves, one set parallel to the line of fracture, and the
other perpendicular to such a direction. These movements, according
to Bertelli, have no connection with the wind, rain, change of
temperature, and atmospheric electricity.
[Illustration: FIG. 38.]
The disturbances, as recorded at different towns, are not always
strictly synchronous, but succeed each other at short intervals. If,
however, we take monthly curves of the disturbances as recorded at
different towns in Italy, we see that these are similar in character.
The maximum of disturbance occurs about the winter solstice, and the
minimum about the summer solstice, and in this respect they exhibit
a perfect accordance with the curves drawn by Mallet to show the
periodicity of earthquakes. The accompanying curves taken from one
of Bertelli’s original memoirs not only show this general result but
also show the close accord there is between the results obtained at
different towns during successive months.
At Florence, before a period of earthquakes there is an increase in the
amplitude and frequency of vertical movements. These vertical movements
do not appear to coincide with the barometrical disturbances, but they
appear to be connected with the seismic disturbances.
They are usually accompanied with noises in the telephone, but as the
microphone is so constructed as to be more sensitive to vertical motion
than to horizontal motion, this is to be expected. This vertical motion
would appear to be a local action, inasmuch as the accompanying motions
of an earthquake which originates at a distance are horizontal.
Storms of microseismical motions appear to travel from point to point.
Sometimes a local earthquake is not noticed in the tromometer,
whilst one which occurred at a distance, although it may be small,
is distinctly observed. To explain this, Bertelli suggests the
existence of nodes. Similar conclusions were arrived at by Rossi when
experimenting on different portions of the sides of Vesuvius. Galli
noticed an augmentation in microseismic activity when the sun and moon
are near the meridian. Grablovitz found from Bertelli’s observations
a maximum two or three days before the syzygies, and a minimum
three days after these periods. He also found that the principal
large disturbances occurred in the middle of periods separating
the quadrature from the syzygies, the apogee from perigee, and the
lunistigi period from the nodes, whilst the smallest disturbances
happened in the middle of periods opposed to these.
P. C. Melzi says that the curves of microseismical motions,
earthquakes, lunar and solar motions, show a concordance with each
other.
With the microphone Rossi hears sounds which he describes as roarings,
explosions, occurring isolated or in volleys, metallic and bell-like
sounds, ticking, &c., which, he says, revealed natural telluric
phenomena. Sometimes these have been intolerably loud. At Vesuvius the
vertical shocks corresponded with a sound like volleys of musketry,
whilst the undulatory shocks gave the roaring. Some of these sounds
could be imitated artificially by rubbing together the conducting wires
in the same manner in which the rocks must rub against each other in
an earthquake. Other sounds were imitated by placing the microphone
on a vessel of boiling water, or by putting it on a marble slab and
scratching and tapping the under side of it.
These, then, are some of the more important results which have been
arrived at by the study of microseismic motions. One point which seems
worthy of attention is that they appear to be more law-abiding than
their more violent relations, the earthquakes, and as phenomena in
which natural laws are to be traced they are certainly deserving our
attention. As to whether they will ever become the means of forewarning
ourselves against earthquakes is yet problematical. Their systematic
study, however, will enable us to trace the progress of a microseismic
storm from point to point, and it is not impossible that we may yet
be enabled to foretell where the storm may reach its climax as an
earthquake. These, I believe, are the views of Professor di Rossi, who
is at the present time engaged in the establishment of a system of
microseismic observations throughout Italy.
Before the earthquake of San Remo (Dec. 6, 1874) Rossi’s tromometer
was in a state of agitation, and similar disturbances were observed at
Livorno, Florence, and Bologna.
Since February 1883 I have observed a tromometer in Japan, and such
results as have been obtained accord with results obtained in Italy.
The increase in microseismical activity with a fall of the barometer
is very marked. Other peculiarities in the behaviour of the instrument
will be referred to under ‘Earth Pulsations.’
_Cause of microseismic movements._—As to the cause of tromometric
movements, we have a field for speculation. Possibly they may be
due to slight vibratory motions produced in the soil by the bending
and crackling of rocks produced by their rise upon the relief of
atmospheric pressure. If this were so we should expect similar
movements to be produced at the time of an increase of pressure. Rossi
suggests that they may be the result of an increased escape of vapour
from the molten materials beneath the crust of the earth, consequent
upon a relief of pressure. The similarity of some of the sounds which
are heard with the microphone to those produced by boiling water are
suggestive of this, and Rossi quotes instances when underground noises
like those which we should expect to hear from a boiling fluid have
been heard before earthquakes without the aid of microphones. One
instance was that of Viduari, a prisoner in Lima, who, two days before
the shock of 1824, repeatedly predicted the same in consequence of the
noises he heard.
A possible cause of disturbances of this order may be small but
sudden fluctuations in barometric pressure, which are visible during
a storm. During a small typhoon on September 15, 1881, when in the
Kurile Islands, I observed that the needle of an aneroid worked back
and forth with a period of from one to three seconds. This continued
for several hours. With every gust of wind the needle suddenly rose
and then immediately fell. At times it trembled. These movements were
observed in the open air. The extent of these sudden variations
was approximately from ·03 to ·05 inches. Beckoning an increase of
barometrical pressure of one hundredth of an inch as equivalent to a
load of twenty million pounds on the square mile, during this storm
there must have been the equivalent of loads of from 60 to 100 million
pounds to the square mile continually placed on and removed from a
considerable tract of the earth’s surface. If the period of application
of these stresses approximately coincide with the natural vibrational
period of the area affected, it would surely seem, especially when
we reflect upon the effect of an ordinary carriage, that tremors of
considerable magnitude ought to be produced.
An inspection of the following few observations taken from my note-book
for the same typhoon will suggest that even the large and slower
variations are capable of producing tremulous motions.
Time Barometer
h. m. reading
12 5 P.M. 29·02
12 10 „ 29·05
12 12 „ 29·07
12 13 „ 29·05
12 25 „ 29·10
12 50 „ 29·00
1 10 „ 29·00
1 20 „ 29·07