SEISMOMETRY.
Nature of earthquake vibrations—Many instruments called
seismometers only seismoscopes—Eastern seismoscopes, columns,
projection seismometers—Vessels filled with liquid—Palmieri’s
mercury tubes—The ship seismoscope—The cacciatore—Pendulum
instruments of Kreil, Wagner, Ewing, and Gray—Bracket
seismographs—West’s parallel motion instrument—Gray’s conical
pendulums, rolling spheres, and cylinders—Verbeck’s ball and
plate seismograph—The principle of Perry and Ayrton—Vertical
motion instruments—Record receivers—Time-recording apparatus—The
Gray and Milne seismograph.
Before we discuss the nature of earthquake motion, the determination
of which has been the aim of modern seismological investigation, the
reader will naturally look for an account of the various instruments
which have been employed for recording such disturbances. A description
of the earthquake machines which have been used even in Japan would
form a bulky volume. All that we can do, therefore, is to describe
briefly the more prominent features of a few of the more important of
these instruments. In order that the relative merits of these may be
better understood, we may state generally that modern research has
shown a typical earthquake to consist of a series of small tremors
succeeded by a shock, or series of shocks, separated by more or less
irregular vibrations of the ground. The vibrations are often both
irregular in period and in amplitude, and they have a duration of from
a few seconds to several minutes. We will illustrate the records of
actual earthquakes in a future chapter, but in the meantime the idea
that an earthquake consists of a single shock must be dismissed from
the imagination.
To construct an instrument which at the time of an earthquake shall
move and leave a record of its motion, there is but little difficulty.
Contrivances of this order are called _seismoscopes_. If, however,
we wish to know the period, extent, and direction of each of the
vibrations which constitutes an earthquake, we have considerable
difficulty. Instruments which will in this way measure or write down
the earth’s motions are called _seismometers_ or _seismographs_.
Many of the elaborate instruments supplemented with electro-magnetic
and clockwork arrangements are, when we examine them, nothing more than
elaborate seismoscopes which have been erroneously termed seismographs.
The only approximations to true seismographs which have yet been
invented are without doubt those which during the past few years have
been used in Japan. It would be a somewhat arbitrary proceeding,
however, to classify the different instruments as seismoscopes,
seismometers, and seismographs, as the character of the record given
by certain instruments is sometimes only seismoscopic, whilst at other
times it is seismometric, depending on the nature of the disturbance.
Many instruments, for instance, would record with considerable accuracy
a single sudden movement, but would give no reliable information
regarding a continued shaking.
_Eastern Seismoscopes._—The earliest seismoscope of which we find any
historical record is one which owes its origin to a Chinese called
Chôko. It was invented in the year A.D. 136. A description is given
in the Chinese history called ‘Gokanjo,’ and the translation of this
description runs as follows:—
‘In the first year of Yōka, A.D. 136, a Chinese called Chôko invented
the seismometer shown in the accompanying drawing. This instrument
consists of a spherically formed copper vessel, the diameter of which
is eight feet. It is covered at its top, and in form resembles a
wine-bottle. Its outer part is ornamented by the figures of different
kinds of birds and animals, and old peculiar-looking letters. In the
inner part of this instrument a column is so suspended that it can
move in eight directions. Also, in the inside of the bottle, there is
an arrangement by which some record of an earthquake is made according
to the movement of the pillar. On the outside of the bottle there
are eight dragon heads, each of which holds a ball in its mouth.
Underneath these heads there are eight frogs so placed that they appear
to watch the dragon’s face, so that they are ready to receive the ball
if it should be dropped. All the arrangements which cause the pillar to
knock the ball out of the dragon’s mouth are well hidden in the bottle.’
[Illustration: FIG. 1.]
‘When an earthquake occurs, and the bottle is shaken, the dragon
instantly drops the ball, and the frog which receives it vibrates
vigorously; any one watching this instrument can easily observe
earthquakes.’
With this arrangement, although one dragon may drop a ball, it is
not necessary for the other seven dragons to drop their balls unless
the movement has been in all directions; thus we can easily tell the
direction of an earthquake.
‘Once upon a time a dragon dropped its ball without any earthquake
being observed, and the people therefore thought the instrument of
no use, but after two or three days a notice came saying that an
earthquake had taken place at Rōsei. Hearing of this, those who doubted
the use of this instrument began to believe in it again. After this
ingenious instrument had been invented by Chōko, the Chinese Government
wisely appointed a secretary to make observations on earthquakes.’
Not only is this instrument of interest on account of its antiquity,
but it is also of interest on account of the close resemblance it bears
to many of the instruments of modern times.
Another earthquake instrument also of Eastern origin is the magnetic
seismoscope of Japan.
On the night of the destructive earthquake of 1855, which devastated
a great portion of Tokio, the owner of a spectacle shop in Asakusa
observed that a magnet dropped some old iron nails and keys which had
been attached to it. From this occurrence the owner thought that the
magnet had, in consequence of its age, lost its powers. About two hours
afterwards, however, the great earthquake took place, after which the
magnet was observed to have regained its powers. This occurrence led
to the construction of the seismoscope, which is illustrated in a book
called the ‘Ansei-Kembun-Roku,’ or a description of the earthquake of
1855, and examples of the instrument are still to be seen in Tokio.
These instruments consist of a piece of magnetic iron ore, which holds
up a piece of iron like a nail. This nail is connected, by means of a
string, with a train of clockwork communicating with an alarm. If the
nail falls a catch is released and the clockwork set in motion, and
warning given by the ringing of a bell. It does not appear that this
instrument has ever acted with success.
_Columns._—One of the commonest forms of seismoscope, and one which
has been very widely used, consists of a round column of wood, metal,
or other suitable material, placed, with its axis vertical, on a level
plane, and surrounded by some soft material such as loose sand to
prevent it rolling should it be overturned. The fall of such a column
indicates that a shaking or shock has taken place. Attempts have been
made by using a number of columns of different sizes to make these
indications seismometric, but they seldom give reliable information
either as to intensity or direction of shock. The indications as to
intensity are vitiated by the fact that a long-continued gentle shaking
may overturn a column which would stand a very considerable sudden
shock, while the directions in which a number of columns fall seldom
agree owing to the rotational motion imparted to them by the shaking.
Besides, the direction of motion of the earthquake seldom remains in
the same azimuth throughout the whole disturbance.
An extremely delicate, and at the same time simple form of seismoscope
may be made by propping up strips of glass, pins, or other easily
overturned bodies against suitably placed supports. In this way bodies
may be arranged, which, although they can only fall in one direction,
nevertheless fall with far less motion than is necessary to overturn
any column which will stand without lateral support.
_Projection Seismometers._—Closely related to the seismoscopes and
seismometers which depend on the overturning of bodies. Mallet has
described two sets of apparatus whose indications depend on the
distance to which a body is projected. In one of these, which consisted
of two similar parts arranged at right angles, two metal balls rest
one on each side of a stop at the lower part of two inclined \/ like
troughs. In this position each of the balls completes an electric
circuit. By a shock the balls are projected or rolled up the troughs,
and the height to which they rise is recorded by a corresponding
interval in the break of the circuits. The vertical component of the
motion is measured by the compression of a spring which carries the
table on which this arrangement rests. In the second apparatus two
balls are successively projected, one by the forward swing, and the
other by the backward swing of the shock. Attached to them are loose
wires forming terminals of the circuits. They are caught in a bed
of wet sand in a metal trough forming the other end of the circuit.
The throw of the balls as measured in the sand, and the difference
of time between their successive projections as indicated by special
contrivances connected with the closing of the circuits, enables
the observer to calculate the direction of the wave of shock, its
velocity, and other elements connected with the disturbance. It will be
observed that the design of this apparatus assumes the earthquake to
consist of a distinct isolated shock.
Oldham, at the end of his account of the Cachar earthquake of 1869,
recommends the use of an instrument based on similar principles. In his
instrument four balls like bullets are placed in notches cut in the
corners of the upper end of a square stake driven into the ground.
_Vessels filled with liquid._—Another form of simple seismoscope is
made by partially filling a vessel with liquid. The height to which the
liquid is washed up the side of the vessel is taken as an indication of
the intensity of the shock, and the line joining the points on which
maximum motion is indicated, is taken as the direction of the shock. If
earthquakes all lasted for the same length of time, and consisted of
vibrations of the same period, such instruments might be of service.
These instruments have, however, been in use from an early date. In
1742 we find that bowls of water were used to measure the earthquakes
which in that year alarmed the inhabitants of Leghorn. About the same
time the Rev. S. Chandler, writing about the shock at Lisbon, tells us
that earthquakes may be measured by means of a spherical bowl about
three or four feet in diameter, the inside of which, after being dusted
over with Barber’s puff, is filled very gently with water. Mallet,
Babbage, and De la Bêche have recommended the same sort of contrivance,
but, notwithstanding, it has justly been criticised as ‘ridiculous and
utterly impracticable.’[7]
An important portion of Palmieri’s well-known instrument consists
of horizontal tubes turned up at the ends and partially filled with
mercury. To magnify the motion of the mercury, small floats of iron
rest on its surface. These are attached by means of threads to a pulley
provided with indices which move in front of a scale of degrees. We
thus read off the intensity of an earthquake as so many degrees, which
means so many millimetres of washing up and down of mercury in a tube.
The direction of movement is determined by the azimuth of the tube
which gives the maximum indication, several tubes being placed in
different azimuths.
This form of instrument appears to have been suggested by Mallet, who
gives an account of the same in 1846. Inasmuch as the rise and fall
of the mercury in such tubes depend on its depth and on the period
of the earthquake together with its duration, we see that although
the results obtained from a given instrument may give us means of
making approximate comparisons as to the relative intensity of various
earthquakes, it is very far from yielding any absolute measurement.
Another method which has been employed to magnify and register the
motions of liquid in a vessel has been to float upon its surface a
raft or ship from which a tall mast projected. By a slight motion of
the raft, the top of the mast vibrated through a considerable range.
This motion of the mast as to direction and extent was then recorded by
suitable contrivances attached to the top of the mast.
A very simple form of liquid seismoscope consists of a circular trough
of wood with notches cut round its side. This is filled with mercury
to the level of the notches. At the time of an earthquake the maximum
quantity of mercury runs over the notches in the direction of greatest
motion. This instrument, which has long been used in Italy, is known as
a Cacciatore, being named after its inventor. It is a prominent feature
in the collection of apparatus forming the well-known seismograph of
Palmieri.
_Pendulum instruments._—Mallet speaks of pendulum seismoscopes and
seismographs as ‘the oldest probably of seismometers long set up in
Italy and southern Europe.’ In 1841 we find these being used to record
the earthquake disturbances at Comrie in Scotland.
These instruments may be divided into two classes: first, those which
at the time of the shock are intended to swing, and thus record the
direction of movement; and second, those which are supposed to remain
at rest and thus provide ‘steady points.’
To obtain an absolutely ‘steady point’ at the time of an earthquake,
has been one of the chief aims of all recent seismological
investigations.
With a style or pointer projecting down from the steady point to
a surface which is being moved backward and forward by the earth,
such a surface has written upon it by its own motions a record of
the ground to which it is attached. Conversely, a point projecting
upwards from the moving earth might be caused to write a record on the
body providing the steady point, which in the class of instruments
now to be referred to is supposed to be the bob of a pendulum. It is
not difficult to get a pendulum which will swing at the time of a
moderately strong earthquake, but it is somewhat difficult to obtain
one which will not swing at such a time. During the past few years,
pendulums varying between forty feet in length and carrying bobs of
eighty pounds in weight, and one-eighth of an inch in length, and
carrying a gun-shot, have been experimented with under a great variety
of circumstances. Sometimes the supports of these pendulums have been
as rigid as it is possible to make a structure from brick and mortar,
and at other times they have intentionally been made loose and
flexible. The indices which wrote the motions of these pendulums have
been as various as the pendulums themselves. A small needle sliding
vertically through two small holes, and resting its lower end on a
surface of smoked glass, has on account of its small amount of friction
been perhaps one of the favourite forms of recording pointers.
The free pendulums which have been employed, and which were intended
to swing, have been used for two purposes: first, to determine the
direction of motion from the direction of swing, and second, to see if
an approximation to the period of the earth’s motion could be obtained
by discovering the pendulum amongst a series of different lengths which
was set in most violent motion, this probably being the one which had
its natural period of swing the most nearly approximating to the period
of the earthquake oscillations.
Inasmuch as all pendulums when swinging have a tendency to change the
plane of their oscillation, and also as we now know that the direction
of motion during an earthquake is not always constant, the results
usually obtained with these instruments respecting the direction of
the earth’s motion have been unsatisfactory. The results which were
obtained by series of pendulums of different lengths were, for various
reasons, also unsatisfactory.
Of pendulums intended to provide a steady point, from which the
relative motion of a point on the earth’s surface could be recorded,
there has been a great variety. One of the oldest forms consisted of
a pendulum with a style projecting downwards from the bob so as to
touch a bed of sand. Sometimes a concave surface was placed beneath the
pendulum, on which the record was traced by means of a pencil. Probably
the best form was that in which a needle, capable of sliding freely up
and down, marked the relative horizontal motion of the earth and the
pendulum bob on a smoked glass plate.
It generally happens that at the time of a moderately severe earthquake
the whole of these forms of apparatus are set in motion, due partly to
the motion of the point of support of the pendulum, and partly to the
friction of the writing point on the plate.
Among these pendulums may be mentioned those of Cavallieri, Faura,
Palmieri, Rossi, and numerous others. It is possible that the
originators of some of these pendulums may have intended that
they should record by swinging. If this is so, then so far as the
determination of the actual nature of earthquake motion is concerned,
they belong to a lower grade of apparatus than that in which they are
here included.
A great improvement in pendulum apparatus is due to Mr. Thomas Gray
of Glasgow, who suggested applying so much frictional resistance to
the free swing of a pendulum that for small displacements it became
‘dead beat.’ By carrying out this suggestion, pendulum instruments
were raised to the position of seismographs. The manner of applying
the friction will be understood from the following description of a
pendulum instrument which is also provided with an index which gives a
magnification of the motion of the earth.
B B B B is a box 113 cm. high and 30 cm. by 18 cm. square. Inside this
box a lead ring R, 17 cm. in diameter and 3 cm. thick, is suspended as
a pendulum from the screw S. This screw passes through a small brass
plate P P, which can be moved horizontally over a hole in the top of
the box. These motions in the point of suspension allow the pendulum to
be adjusted.
[Illustration: FIG. 2.]
Projecting over the top of the pendulum there is a wooden arm W
carrying two sliding pointers H H, resting on a glass plate placed on
the top of the pendulum. These pointers are for the purpose of giving
the frictional resistance before referred to. If this friction plate
is smoked, the friction pointers will write upon it records of _large_
earthquakes independently of the records given by the proper index,
which only gives satisfactory records in the case of shocks of ordinary
intensity. Crossing the inside of the pendulum R there is a brass bar
perforated with a small conical hole at M. A stiff wire passes through
M and forms the upper portion of the index I, the lower portion of
which is a thin piece of bamboo. Fixed upon the wire there is a small
brass ball which rests on the upper side of a second brass plate also
perforated with a conical hole, which plate is fixed on the bar O O
crossing the box.
If at the time of an earthquake the upper part of the index I remains
steady at M, then by the motion at O, the lower end of the index which
carries a sliding needle at G, will magnify the motion of the earth in
the ratios M O: O G. In this instrument O G is about 17 cm.
The needle G works upon a piece of smoked glass. In order to bring
the glass into contact with the needle without disturbance, the glass
is carried on a strip of wood K, hinged at the back of the box, and
propped up in front by a loose block of wood Y. When Y is removed
the glass drops down with K out of contact with the needle. The box
is carried on bars of wood C C, which are fixed to the ground by the
stakes A A.
The great advantage of a pendulum seismograph working on a stationary
plate is, that the record shows at once whether the direction of motion
has been constant, or whether it has been variable. The maximum extent
of motion in various directions is also easily obtained.
The disadvantage of the instrument is, that at the time of a large
earthquake, owing perhaps to a slight swing in the pendulum, the
records may be unduly magnified.
On such occasions, however, fairly good records may be obtained from
the friction pointers, provided that the plates on which they work have
been previously smoked. It might perhaps be well to use two of these
instruments, one having a comparatively high frictional resistance, and
hence ‘dead beat’ for large displacements.
Many attempts have been made to use a pendulum seismograph in
conjunction with a record-receiving surface, which at the time of the
earthquake should be kept in motion by clockwork. In this way it was
hoped to separate the various vibrations of the earthquake, and thus
avoid the greater or less confusion which occurs when the index of the
pendulum writes its backward and forward motion on a stationary plate.
Hitherto all attempts in this direction, in which a single multiplying
index was used, have been unsuccessful because of the moving plate
dragging the index in the direction of its motion for a short
distance, and then allowing it to fall back towards its normal position.
In connection with this subject we may mention the pendulum
seismographs of Kreil, Wagener, Ewing, and Gray.
In the bob of Kreil’s pendulum there was clockwork, which caused a disc
on the axis of the pendulum to continuously rotate. On this continually
revolving surface a style fixed to the earth traced an unbroken circle.
At the time of an earthquake, by the motion of the style, the circle
was to be broken and lines drawn. The number and length of these lines
were to indicate the length and intensity of the disturbance.
Gray’s pendulum consisted of a flat heavy disc carrying on its upper
surface a smoked glass plate. This, which formed the bob of the
pendulum, was supported by a pianoforte steel wire. When set ready to
receive an earthquake, the wire was twisted and the bob held by a catch
so arranged that at the time of the earthquake the catch was released,
and the bob of the pendulum allowed to turn slowly by the untwisting of
the supporting wire. Resting on the surface of this rotating disc were
two multiplying indices arranged to write the earth’s motions as two
components.
In the instruments of Wagener and Ewing, the clockwork and moving
surface do not form part of the pendulum, but rest independently on
a support rigidly attached to the earth. In Wagener’s instrument one
index only is used, while in Ewing’s two are used for writing the
record of the motion.
A difficulty which is apparent in all pendulum machines is that when
the bob of such a pendulum is deflected it tends to fall back to its
normal position. To make a pendulum perfect it therefore requires
some compensating arrangement, so that the pendulum, for small
displacements, shall be in neutral equilibrium, and the errors due to
swinging shall be avoided.
Several methods have been suggested for making the bob of an ordinary
pendulum astatic for small displacements. One method proposed by Gray
consists in fixing in the bob of a pendulum a circular trough of
liquid, the curvature of this trough having a proper form. Another
method which was suggested, was to attach a vertical spiral spring
to a point in the axis of the pendulum a little below the point of
suspension, and to a fixed point above it, so that when the pendulum is
deflected it would introduce a couple.
Professor Ewing has suggested an arrangement so that the bob of the
pendulum shall be partly suspended by a stretched spiral spring, and at
the same time shall be partly held up from below by a vertically placed
strut, the weight carried by the strut being to the weight carried by
the spring in the ratio of their respective lengths. As to how these
arrangements will act when carried into practice yet remains to be seen.
Another important class of instruments are _inverted pendulums_. These
are vertical springs made of metal or wood loaded at their upper end
with a heavy mass of metal. An arrangement of this sort, provided at
its upper end with a pencil to write on a concave surface, was employed
in 1841 to register the earthquakes at Comrie in Scotland. In Japan
they were largely employed in series, each member of a series having
a different period of vibration. The object of these arrangements
was to determine which of the pendulums, with a given earthquake,
recorded the greatest motion, it being assumed that the one which was
thrown into the most violent oscillation would be the one most nearly
approximating with the period of the earthquake. The result of these
experiments showed that it was usually those with a slow period of
vibration which were the most disturbed.
_Bracket Seismographs._—A group of instruments of recent origin which
have done good work, are the bracket seismographs. These instruments
appear to have been independently invented by several investigators:
the germ from which they originated probably being the well-known
horizontal pendulum of Professor Zöllner. In Japan they were first
employed by Professor W. S. Chaplin. Subsequently they were used by
Professor Ewing and Mr. Gray. They consist essentially of a heavy
weight supported at the extremity of a horizontal bracket which is free
to turn on a vertical axis at its other end. When the frame carrying
this axis is moved in any direction excepting parallel to the length
of the gate-like bracket, the weight causes the bracket to turn round
a line known as the instantaneous axis of the bracket corresponding to
this motion of the fixed axis. Any point in this line may therefore be
taken as a steady point for motions at right angles to the length of
the supporting bracket. Two of these instruments placed at right angles
to each other have to be employed in conjunction, and the motion of
the ground is written down as two rectangular components. In Professor
Ewing’s form of the instrument, light prolongations of the brackets
form indices which give magnified representations of the motion, and
the weights are pivoted round a vertical axis through their centre.
In the accompanying sketch B is a heavy weight pivoted at the end of
a small bracket C A K, which bracket is free to turn on a knife-edge,
K, above, and a pivot A, below, in the stand S. At the time of an
earthquake B remains steady, and the index P, forming a continuation
of the bracket, magnifies the motion of the stand, in the ratio of
A C : C N.
[Illustration: FIG. 3.]
In an instrument called a double-bracket seismograph, invented by Mr.
Gray, we have two brackets hinged to each other, and one of them to a
fixed frame. The planes of the two brackets are placed at right angles,
so as to give to a heavy mass supported at the end of the outer bracket
two degrees of horizontal freedom.
In all bracket machines, especially those which carry a pivoted weight,
it is doubtful whether the weight provides a truly steady point
relatively to the plate on which the record is written for motion
parallel to the direction of the arm.
[Illustration: FIG. 4.]
_Parallel motion Instrument._—A machine which writes its record as
two components, and which promises great stability, is one suggested
by Professor C. D. West. Like the bracket machines it consists of
two similar parts placed at right angles to each other, and is as
follows: A bar of iron A is suspended from both sides on pivots at C C
by a system of light arms hinging with each other at the black dots,
between the upper and lower parts of the rigid frame B C. The arms are
of such a length that for small displacements parallel to the length
of the bar, C C practically move in a straight line, and the bar is
in neutral equilibrium. A light prolongation of the bar _d_ works the
upper end of the light index _e_, passing as a universal joint through
the rigid support F. A second index _e′ _ from the bar at right angles
also passes through F. The multiplying ends of these indices are
coupled together to write a resultant motion on a smoked glass plate S.
_Conical Pendulums._—Another group of instruments which have also
yielded valuable records are the conical pendulum seismographs. The
idea of using the bob of a conical pendulum to give a steady point in
an earthquake machine was first suggested and carried into practice by
Mr. Gray. The seismograph as employed consists of a pair of conical
pendulums hung in planes at right angles to each other. The bob of each
of these pendulums is fixed a short distance from the end of a light
lever, which forms the writing index, the short end resting as a strut
against the side of a post fixed in the earth. The weight is carried by
a thin wire or thread, the upper end of which is attached to a point
vertically above the fixed end of the lever.
_Rolling Spheres and Cylinders._—After the conical pendulum
seismographs, which claim several important advantages over the bracket
machines, we come to a group of instruments known as rolling sphere
seismographs. Here, again, we have a class of instruments for the
various forms of which we are indebted to the ingenuity of Mr. Gray.
The general arrangement and principle of one of these instruments will
be readily understood from the accompanying figure. S is a segment
of a large sphere with a centre near C. Slightly below this centre a
heavy weight B, which may be a lead ring, is pivoted. At the time of
an earthquake C is steady, and the earth’s motions are magnified by
the pointer C A N in the proportion of C A : A N. The working of this
pointer or index is similar to that of the pointer in the pendulum.
[Illustration: FIG. 5.]
Closely connected with the rolling sphere seismographs, are Gray’s
rolling cylinder seismographs.
These are two cylinders resting on a surface plate with their axes at
right angles to each other. Near to the highest point in each of these
cylinders, this point remaining nearly steady when the surface plate
is moved backwards and forwards, there is attached the end of a light
index. These indices are again pivoted a short distance from their ends
on axes connected with the surface plate. In order that the two indices
may be brought parallel, one is cranked at the second pivot.
_Ball and Plate Seismograph._—Another form of seismograph, which is
closely related to the two forms of apparatus just described, is
Verbeck’s ball and plate seismograph. This consists of a surface plate
resting on three hard spheres, which in turn rests upon a second
surface plate. When the lower plate is moved, the upper one tends to
remain at rest, and thus may be used as a steady mass to move an index.
_The Principle of Perry and Ayrton._—An instrument which is of interest
from the scientific principle it involves is a seismograph suggested
by Professors Perry and Ayrton, who propose to support a heavy ball on
three springs, which shall be sufficiently stiff to have an exceedingly
quick period of vibration. By means of pencils attached to the ball by
levers, the motions of the ball are to be recorded on a moving band of
paper. The result would be a record compounded of the small vibrations
of the springs superimposed on the larger, slower, wave-like motions
of the earthquake, and, knowing the former of these, the latter might
be separated by analysis. Although our present knowledge of earthquake
motion indicates that the analysis of such a record would often present
us with insuperable difficulties, this instrument is worthy of notice
on account of the novelty of the principle it involves, which, the
authors truly remark, has in seismometry been a ‘neglected’ one.
_Instruments to record Vertical Motion._—The instruments which have
been devised to record vertical motion are almost as numerous as those
which have been devised to record horizontal motion. The earliest
form of instrument employed for this purpose was a spiral spring
stretched by weight, which, on account of its inertia, was supposed
at the time of a shock to remain steady. No satisfactory results have
ever been obtained from such instruments, chiefly on account of the
inconvenience in making a spring sufficiently long to allow of enough
elongation to give a long period of vibration. Similar remarks may
be applied to the horizontally placed elastic rods, one end of which
is fixed to a wall, whilst the opposite end is loaded with a weight.
Such contrivances, furnished with pencil on the weight to write a
record upon a vertical surface, were used in 1842 at Comrie, and we
see the same principle applied in a portion of Palmieri’s apparatus.
Contrivances like these neither give us the true amplitude of the
vertical motion, insomuch as they are readily set in a state of
oscillation; nor do they indicate the duration of a disturbance, for,
being once set in motion, they continue that motion in virtue of their
inertia long after the actual earthquake has ceased. They can only be
regarded as seismoscopes.
[Illustration: FIG. 6.]
The most satisfactory instrument which has yet been devised for
recording vertical motion is Gray’s horizontal lever spring seismograph.
This instrument will be better understood from the accompanying sketch.
A vertical spring S is fixed at its upper end by means of a nut _n_,
which rests on the top of the frame F, and serves to raise or lower
the spring through a short distance as a last adjustment for the
position of the cross-arm A. The arm A rests at one end on two sharp
points, _p_, one resting in a conical hole and the other in a V-slot;
it is supported at B by the spring S, and is weighted at C with a lead
ring R. Over a pin at the point C a stirrup of thread is placed which
supports a small trough, _t_. The trough _t_ is pivoted at _a_, has
attached to it the index _i_ (which is hinged by means of a strip of
tough paper at _h_, and rests through a fine pin on the glass plate
_g_), and is partly filled with mercury.
Another method of obtaining a steady point for vertical motion is that
of Dr. Wagener, who employs a buoy partly immersed in a vessel of
water. This was considerably improved upon by Mr. Gray, who suggested
the use of a buoy, which, with the exception of a long thin style, was
completely sunk.
Among the other forms of apparatus used to record vertical motion may
be mentioned vessels provided with india-rubber or other flexible
bottoms, and partially filled with water or some other liquid. As the
vessel is moved up and down, the bottom tends to remain behind and
provides a more or less steady point. Pivoted to this is a light index,
which is again pivoted to a rigid frame in connection with the earth.
Instruments of this description have yielded good records.
_Record Receivers._—A large number of earthquake machines having been
referred to, it now remains to consider the apparatus on which they
write their motions. The earlier forms of seismographs, as has already
been indicated, recorded their movements in a bed of sand; others
wrote their records by means of pencils on sheets of paper. Where we
have seismographs which magnify the motion of the earth, it will be
observed that methods like the above would involve great frictional
resistances, tending to cause motion in the assumed steady points of
the seismographs. One of the most perfect instruments would be obtained
by registering photographically the motions of the recording index by
the reflection of a ray of light. Such an instrument would, however,
be difficult to construct and difficult to manipulate. One of the best
practical forms of registering apparatus is one in which the record is
written on a surface of smoked glass. This can afterwards be covered
with a coat of photographer’s varnish, and subsequently photographed by
the ‘blue process’ so well known to engineers.
To obtain a record of all the vibrations of an earthquake it is
necessary that the surface on which the seismograph writes should at
the time of an earthquake be in motion. Of record-receiving machines
there are three types. First, there are those which move continuously.
The common form of these is a circular glass plate like an old form of
chronograph, driven continuously by clockwork. On this the pointers of
the seismograph rest and trace over and over again the same circles.
At the time of an earthquake they move back and forth across the
circles, which are theoretically fine lines, and leave a record of
the earthquake. Instead of a circular plate, a drum covered with
smoked paper may be used, which, after the earthquake, possesses the
advantage, after unrolling, of presenting the record in a straight
line, instead of a record written round the periphery of a circle, as
is the case with the circular glass plates. Such records are easily
preserved, but they are more difficult to photograph.
The second form of apparatus is one which is set in motion at the
time of a shock. This may be a contrivance like one of those just
described, or a straight smoked glass plate on a carriage. By means
of an electrical or a mechanical contrivance called a ‘starter,’ of
which many forms have been contrived, the earthquake is caused to
release a detent and thus set in motion the mechanism which moves the
record receiver.
The great advantage of continuously-moving machines is that the
beginning and end of the shock can usually be got with certainty, while
all the uncertainty as to the action of the ‘starter’ is avoided.
Self-starting machines have, of course, the advantage of simplicity and
cheapness, while there is no danger of the record getting obliterated
by the subsequent motion of the plate under the index.
_Time-recording Apparatus._—Of equal importance with the instruments
which record the motion of the ground, are those instruments which
record the time at which such motion took place. The great value of
time records, when determining the origin from which an earthquake
originates, will be shown farther on. The most important result which
is required in connection with time observations, is to determine
the interval of time taken by a disturbance in travelling from one
point to another. On account of the great velocity with which these
disturbances sometimes travel, it is necessary that these observations
should be made with considerable accuracy. The old methods of adapting
an apparatus to a clock which, when shaken, shall cause the clock to
stop, are of little value unless the stations at which the observations
are made are at considerable distances apart. This will be appreciated
when we remember that the disturbance may possibly travel at the rate
of a mile per second, that its duration at any station may often extend
over a minute, and that one set of apparatus at one station may stop,
perhaps, at the commencement of the disturbance, and the other near
the end. A satisfactory time-taking apparatus will therefore require,
not only the means for stopping a clock, but also a contrivance which,
at the same instant that the clock is stopped, shall make a mark on a
record which is being drawn by a seismograph. In this way we find out
at which portion of the shock the time was taken.
[Illustration: FIG. 7.]
Palmieri stops a clock in his seismograph by closing an electric
circuit. Mallet proposes to stop a clock by the falling of a column
which is attached by a string to the pendulum of the clock. So long as
the column is standing the string is loose and the pendulum is free
to move; but when the column falls, the string is tightened and the
pendulum is arrested. The difficulty which arises is to obtain a column
that will fall with a slight disturbance. The best form of contrivance
for causing a column to fall, and one which may also be used in drawing
out a catch to relieve the machinery of a record receiver, is shown in
the accompanying sketch.
S is the segment of a sphere about 4·5 cm. radius, with a centre
slightly above C. L is a disc of lead about 7 cm. in diameter resting
upon the segment. Above this there is a light pointer, P, about 30
cm. long. On the top of the pointer a small cylinder of iron, W, is
balanced, and connected by a string with the catch to be relieved. When
the table on which W P S rests is shaken, rotation takes place near
to C, the motion of the base S is magnified at the upper end of the
pointer, and the weight overturned. This catch may be used to relieve
a toothed bar axled at one end, and held up above a pin projecting
from the face of the pendulum bob. When this falls it catches the
projecting pin and holds the pendulum.
Another way of relieving the toothed bar is to hold up the opposite
end to that at which it is axled by resting it on the extremity of a
horizontal wire fixed to the bob of a conical pendulum—for example, one
of the indices of a conical pendulum seismograph. The whole of this
apparatus, which may be constructed at the cost of a few pence, can be
made small enough to go inside an ordinary clock case.
The difficulty which arises with all these clock-stopping arrangements
is that it is difficult for observers situated at distant stations
to re-start their clocks so that their difference in time shall
be accurately known. Even if each observer is provided with a
well-regulated chronometer, with which he can make comparisons, the
rating of these instruments is for all ordinary persons an extremely
troublesome operation.
In order to avoid this difficulty the author has of late years used a
method of obtaining the time without stopping the clock. To do this a
clock with a central seconds hand is taken, and the hour and minute
hands are prolonged and bent out slightly at their extremities at right
angles to the face, the hour hand being slightly the longest. Each
hand is then tipped with a piece of soft material like cork, which is
smeared with a glycerine ink. A light flat ring, with divisions in it
corresponding to those on the face of the clock, is so arranged that
at the time of a shock it can be quickly advanced to touch the inked
pads on the hands of the clock and then withdrawn. This is accomplished
by suitable machinery, which is relieved either by an electro-magnet
or some other contrivance which will withdraw a catch. In this way an
impression in the form of three dots is received on the disc, and the
time known without either stopping or sensibly retarding the clock.
For ordinary observers, if a time-taker is not used in conjunction
with a record receiver, as good results as those obtained by ordinary
clock-stopping apparatus are obtainable by glancing at an ordinary
watch. Subsequently the watch by which the observation was made should
be compared with some good time-keeper, and the local time at which the
shock took place is then approximately known.
From what has now been said it will be seen that for a complete
seismograph we require three distinct sets of apparatus—an apparatus to
record horizontal motion, an apparatus to record vertical motion, and
an apparatus to record time. The horizontal and vertical motions must
be written on the same receiver, and if possible side by side, whilst
the instant at which the time record is made a mark must be made on
the edge of the diagram which is being drawn by the seismograph. Such
a seismograph has been constructed and is now erected in Japan. It is
illustrated in the accompanying diagram.
_The Gray and Milne Seismograph._—In this apparatus two mutually
rectangular components of the horizontal motion of the earth are
recorded on a sheet of smoked paper wound round a drum, D, kept
continuously in motion by clockwork, W, by means of two conical
pendulum seismographs, C. The vertical motion is recorded on the same
sheet of paper by means of a compensated-spring seismograph, S L M B.
The time of occurrence of an earthquake is determined by causing the
circuit of two electro-magnets to be closed by the shaking. One of
these magnets relieves a mechanism, forming part of a time-keeper,
which causes the dial of the timepiece to come suddenly forwards on
the hands and then move back to its original position. The hands are
provided with ink-pads, which mark their positions on the dial, thus
indicating the hour, minute, and second when the circuit was closed.
The second electro-magnet causes a pointer to make a mark on the paper
receiving the record of the motion. This mark indicates the part of the
earthquake at which the circuit was closed.
[Illustration: FIG. 8.]
The duration of the earthquake is estimated from the length of the
record on the smoked paper and the rate of motion of the drum. The
nature and period of the different movements are obtained from the
curves drawn on the paper.
Mr. Gray has since greatly modified this apparatus, notably by the
introduction of a band of paper sufficiently long to take a record for
twenty-four hours without repetition. The record is written in ink by
means of fine siphons. In this way the instrument, which is extremely
sensitive to change of level, can be made to show not only earthquakes,
but the pulsations of long period which have recently occupied so much
attention.