mm. The greatest value was that observed for the destructive shock of

Chapter 275,008 wordsPublic domain

Feb. 22, 1880, which was ·56 mm.

By means of a number of instruments distributed at various localities round Tokio, the chief of which were pendulums with friction pointers to render them ‘_dead beat_,’ and with magnifying apparatus to show the actual motion of the ground, the author arrived at results similar to those obtained by Dr. Wagener—namely, that the earth’s maximum horizontal motion at the time of a small earthquake was usually only the fraction of a millimetre, and it seldom exceeded three or four millimetres. When we get a motion of five or six millimetres, we usually find that brick and stone chimneys have been shattered.

The results obtained for vertical motion were also very small. In Tokio it is seldom that vertical motion can be detected, and when it is recorded it is seldom more than a millimetre.

These results, which were put forward some years ago, have since received confirmation by the use of a variety of instruments in the hands of different observers.

Mallet, in his account of the Neapolitan earthquake of 1857, approximated to the amplitude of an earth particle by observing the width, at the level of the centre of gravity, of fissures formed through and remaining in great masses of very inelastic masonry.

Taking stations situated on or very nearly on the same line passing through the seismic vertical (_epicentrum_), Mallet observed the amplitude increased as some function of the distance, as will be seen from the following table:—

+-------------------------+------+-------+-------+---------+-------+ | Station |Polla |La Sala|Certosa|Tramutola|Sarconi| +-------------------------+------+-------+-------+---------+-------+ | Distance from Seismic } | | | | | | | Vertical in } | 3·45 | 11·60 | 16·50 | 20·60 | 26·7 | | geographical miles } | | | | | | | Amplitude in inches | 2·5 | 3·5 | 4·0 | 4·5 | 4·75 | +-------------------------+------+-------+-------+---------+-------+

The possibility of a law such as this having an existence for places at a distance from the seismic vertical comparable with the vertical depth of the centrum will be shown farther on.

With regard to the maximum displacement of an earth particle. Mallet was of opinion that there was evidence to show that it had in some cases been over one foot. M. Abella, in an earthquake which occurred in the Philippines in 1881, made a rough observation of the motion of the earth to a distance of about _two metres_. This, as might be expected, was beyond the elastic limits of the material, and caused fissures to be formed, which were seen to open and shut.

_Intensity of Earthquakes._—In speaking of the strength of an earthquake, we usually employ terms like ‘weak,’ ‘strong,’ ‘violent,’ &c. Although these expressions, accompanied by illustration of the effects which an earth quake has produced, convey a general idea of the strength of a shock as felt at some particular locality, our ideas nevertheless wanting in definiteness; and if we endeavour to compare one shock with another, as a whole, our want of exactness is augmented. We have seen that Palmieri’s seismograph indicates intensity by a certain number of degrees, which, to a certain extent, is a measure of the violence of the motion as indicated at a particular locality. The degrees, as before stated, refer to the height to which in consequence of the shaking, a certain quantity of mercury was washed in a tube, which is a function of the depth of mercury in the tube, and also of the duration of the disturbance.

From this it seems possible that a very slow motion of small amplitude, continuing over a sufficient period of time, might, if it agreed with the period of the mercury, indicate an earthquake of many degrees of intensity, whilst residents in the neighbourhood might not have noticed the disturbance; and, on the other hand, a short but intense shock creating considerable destruction might have been recorded as of only a few degrees of intensity.

Although objections like these might be raised to such a method of recording intensity, in practice it would appear that such results are not pronounced, and the indications of the instrument usually give us approximate indications of relative intensity.

In writing about the Neapolitan earthquake of 1857, Mallet says that ‘area alone affords no test of seismic energy.’

The area over which a shock is felt will depend not only upon the initial force of the disturbance, but also upon the focal depth of a shock, the form and position of that focus, the duration of the disturbance, and the nature and arrangement of the materials which are shaken.

From observations in Japan, it is clearly shown that massive mountain ranges exert a considerable influence upon the extension of seismal disturbances. On one side of a large range of mountains large cities might be laid in ruins, whilst on the other side the disturbance creating this destruction might not be noticed.

_Velocity and Acceleration of an Earth Particle._—We now pass on to methods of determining the intensity of an earthquake which are less arbitrary than those which have just been discussed. These methods have already been discussed when speaking of artificial disturbances, where it was shown that the intensity of an earthquake as measured by its destructive effects greatly depended upon the suddenness with which the backward and forward motions of the ground were commenced or ended.

Amongst the earlier investigators of seismic phenomena who observed that there existed a connection between the distance to which bodies had been projected during an earthquake and the suddenness or initial velocity with which the ground had been moved beneath them, was Professor Wenthrop of Cambridge, Massachusetts, who noted that bricks from his chimneys had, by the New England earthquake of 1755, been thrown thirty feet. From this and the known height of the chimney, he calculates that the bricks had been projected with an initial velocity of twenty-one feet per second.[13]

The calculations made by Mallet respecting the maximum velocity of an earth particle at the time of the Neapolitan earthquake in 1857 depended upon the overthrow, projection, and fracture of bodies.

The principles which guided him in making the calculations will be understood from the following illustration.

If a column, A B C D, receive a shock or be suddenly moved in the direction of the arrow, the centre of gravity, G, of this column will revolve round the edge, and tend to describe the path G O. If it passes O, the column will fall. The work done in such a case as this is equal to lifting the column through the height _o_ _h_.

If G A = _a_, the angle G A _h_ = φ, and the weight of the body = W, then the above work equals

W_a_ (1 - cos φ).

This must equal the work acquired—that is to say, the kinetic energy of rotation of the body, or

W _w_^2 K^2 W_a_ (1 - cos φ) = ———————————. 2 _g_

Where _w_ is the angular velocity of the body at starting, K the radius of gyration round A, and _g_ the velocity acquired by a falling body in one second. Whence

_w_^2 K^2 = 2 _ga_ (1 - cos φ),

but _w_, the angular velocity, is equal to the statical couple applied, divided by the moment of inertia, or,

V_a_ cos φ _w_ = ——————————, K^2

squaring and substituting

K^2 1 - cos φ V^2 = 2_g_ × ——— × —————————, _a_ cos^2 φ

K^2 and since the length of the corresponding pendulum is _l_ = ———, _a_

1 - cos φ V^2 = 2_gl_ × —————————. cos^2 φ

To apply this to any given case we must find the value of _l_ or of

K^2 ———. _a_

Mallet finds these values for the cube, solid and hollow rectangular parallelopipeds, solid and hollow cylinders, &c. In these formulæ we have a direct connection between the dimensions and form of a body and the velocity with which the ground must move beneath it to cause its overthrow.

Not only is the case discussed for horizontal forces, but also for forces acting obliquely. Similar reasonings are applied to the productions of fractures in walls, but as there is uncertainty in our knowledge of the co-efficient of force necessary to produce fracture _through joints across_ beds of masonry, the deductions ought not to be applied as the measures of velocity. Where the fractures occur at the base or in horizontal planes, or in those of the continuous beds of the masonry, or through homogeneous bodies, the uncertainty is not so great, and for cases like these Mallet gives several illustrations. The distance to which bodies had been projected, as, for example, ornaments from the tops of pedestals, coping-stones from the edges of roofs, were also used as means of determining the angle at which the shock had emerged, or, if this be known, for determining the velocity.

Thus by a shock in the direction O C, a ball, A, on the top of a pedestal would describe a trajectory to the point C. Let the angle which O C makes with the horizon be _e_, the vertical height through which the ball has fallen be _b_, and the horizontal distance of projection be _a_; then

_a_^2 _b_ = _a_ tan _e_ + ————————————, 4H cos^2 _e_

H being the height due to the velocity of projection. Whence

___________________ 2H ± √4H(H + _b_) - _a_^2 Tan _e_ = ——————————————————————————. _a_

_a_^2 _g_ V^2 = ———————————————————————————————. 2 cos^2 _e_ (_b_ - _a_ tan _e_)

For the back motion or subnormal wave in the direction C O,

___________________ 2H ± √4H(H + _b_) - _a_^2 Tan _e_ = ——————————————————————————. _a_

_a_^2 _g_ V^2 = ———————————————————————————————. 2 cos^2 _e_ (_b_ + _a_ tan _e_)

A serious error which may enter into calculations of this description when practically applied has been pointed out when speaking of columns as seismometers. It was then shown that such bodies before being overthrown may often be caused to rock, and therefore that their final overthrow may not have any direct connection with the impulse of the succeeding shock.

Another point to which attention must be drawn respecting the above calculations is that if there was no friction or adherence between the projected body and its pedestal, in consequence of its inertia it would be left behind by the forward motion of the shock, and simply drop at the foot of its support. In the case of frictional adherence it would be carried forward by the velocity acquired before this adherence was broken, and thrown in a direction _opposite_ to that given in the figure—that is to say, in the direction of the shock.[14]

_The Absolute Intensity of the Force exerted by an Earthquake._—No doubt it has occurred to many who have experienced an earthquake that the power which gave birth to such a disturbance must have been enormously great. The estimates which we shall make of the absolute amount of energy represented by an earthquake cannot, on account of the nature of the factors with which we deal, be regarded as accurate. They may, however, be of assistance in forming estimates of quantities about which we have at present no conception. One method of obtaining the result we seek is that which was employed by Mallet in his calculations respecting the Neapolitan earthquake. Although disbelieving in the general increment of temperature as we descend in the earth at an average rate of 1° F. for every fifty or sixty feet of descent, for want of better means. Mallet assumes this law to be true, and, knowing from a variety of observations the depth of various parts of the cavity from which the disturbance sprang, he calculates the temperatures of this cavity in various parts as due to its depth beneath the surface. Next, it is assumed that steam was suddenly admitted into this cavity, which might exert the greatest possible pressure due to the maximum temperature. This was calculated as being about 684 atmospheres.

Next, he determined the column of limestone necessary to balance such a pressure, which is about 8,550 feet in height. As the least thickness of strata above this cavity was 16,700 feet, the pressure of 684 atmospheres was not sufficient to blow away its cover, but if suddenly admitted or generated in the cavity it might have produced the wave of impulse by the sudden compression of the walls of the cavity.

The pressure of 684 atmospheres is equivalent to about 4·58 tons on the square inch, and, as the total area of the walls of the cavity is calculated at twenty-seven square miles, the total accumulated pressure would be more than 640,528 millions of tons. Mallet, however, shows that it is probable that the temperature of the focal cavity was much greater than that due to the hypogeal increment, and that therefore the pressure may have been greater.

The capability of producing the earthquake impulse, however, depends on the _suddenness_ with which the steam is flashed off. According to the experiments of Boutigny and others, Mallet tells us that the most sudden production of steam would take place at a temperature of 500°-550° C., which is but a few degrees below that calculated for the mean focal depth.

Assuming the above calculated pressure to be true, and knowing the co-efficient of compression of the materials on which it acted, the volume of the wave at a given moment near the instant of starting—that is, at the focus—can be calculated, and from this the wave amplitude on reaching the surface may be deduced.

Proceeding backwards, if we have observed the wave amplitude, calculated the depth of the focus, and know the co-efficient of expansion, then the total compression may be calculated and the temperature due to the pressure producing this may be arrived at. In this way earthquakes may be used as a means of calculating subterranean temperature at depths that can never be attained experimentally.

A method of proceeding which is probably more definite than that adopted by Mallet would be the application of the method indicated when speaking of the intensity of artificial disturbances.

If for a given earthquake the origin of which is known we have determined by seismographs the mean acceleration of an earth particle at two or more stations at different distances from that origin, we are enabled to construct a curve of intensity the area between which and its asymptotes was shown to be a measure of the total intensity of the shock. Comparing this area with that of a unit disturbance produced, say, by the explosion of a pound of dynamite, one may approximately calculate in terms of this unit the initial intensity of the earthquake.

_Radiation of an Earthquake._—The tremors preceding the more violent movements of an earthquake may be due, as Mallet has suggested,[15] to the free surface waves reaching a distant point before the direct vibrations.

The fact that earth vibrations produced by striking a blow on or near the surface of the ground are wholly obliterated in reaching a cutting or valley, there being no underground waves of distortion to crop up on the opposite side of the valley, indicates that the disturbance is one that travels on the surface; the same fact is illustrated when we endeavour to transmit vibrations through the side of a hill into a tunnel.

In the tunnel, although the distance may be small, no sensible effects are produced, whilst the same disturbance may be recorded at a long distance from its origin on the surface of the ground outside the tunnel.

Lastly, we may refer to the experiences of miners underground.

Occasionally it has happened that miners when deep underground, as in the Marienberg in the Saxon Erzgebirge, have felt shocks which have not been noticed on the surface. These observations are rare, and it is possible that they may be explained by the caving in of subterranean excavations.

The usual experience is, that if a shock is felt underground it is also felt on the surface, as for example in the lead mines in Derbyshire at the time of the Lisbon disturbance (1755).

The most frequent observation, however, is that a shock may be felt on the surface while it is not remarked by the miners beneath the surface, as at Fahlun and Presburg in November, 1823.

At the Comstock Lode in Colorado about twelve years ago many earthquakes were felt. On one particular day twenty-four were counted. Superintendent Charles Foreman told the author when he visited Virginia City in 1882, that special observations were made to determine whether these shocks were felt as severely deep down in the mines as on the surface, where they were on the verge of being destructive. The universal testimony of many observers was that in most cases they were not felt at all underground, and when a shock was felt it was extremely feeble. At Takashima Colliery, in Japan, it is seldom that shocks are felt underground.

The explanation of these latter observations appears to be either that, in consequence of a smaller amplitude of motion in the solid rocks beneath the surface as compared with the extent of motion on the surface, the disturbances are passed by unnoticed, or else the disturbance is, at a distance from its origin, practically confined to the surface.

_Velocity of Propagation of an Earthquake._—Although many have written upon earthquakes and have endeavoured to give to us the velocity with which they were propagated, the subject is one about which we have as yet but little exact information.

The importance of this branch of investigation is undoubtedly great. By knowing the velocity with which an earthquake has travelled in various directions we are assisted in determining the locality of its origin; we may possibly make important deductions respecting the nature of the medium through which it has passed; perhaps also we may learn something regarding the intensity of the disturbance which created the earthquake. In the Report of the British Association for 1851 Mallet gives the table on next page, in which are placed together the approximate rates of transit of shocks of several earthquakes which he discusses. Some of these, it will be observed, are records of disturbances which must have passed through or across the bed of the ocean.

In Mallet’s British Association Report for 1858, he gives data compiled by Mr. David Milne[16] respecting the Lisbon earthquakes of 1755 and 1761, from which data the tables of velocities (p. 89) have been calculated, omitting those which Mr. Mallet has marked as uncertain.

The distances are marked in degrees of seventy English miles each, and the time is reduced to Lisbon time.

+---------------------+-------+--------------------------+----------+ | |Approx.| | | | |rate in| Formation constituting | | | Occasion and Place |feet | Range on surface as far |Authority | | | per | as known or conjectured | | | |second | | | +---------------------+-------+--------------------------+----------+ |Rev. John Mitchell’s |1,760 |Sea bottom, probably on | Mitchell | | guesses from the | | slates, secondary and | | | Lisbon earthquakes | | crystalline rocks | | |Von Humboldt’s |1,760 |From observations in | Humboldt | | guesses from South | to | various South American | | | America |2,464 | rocks in great part | | | | | volcanic | | | | | | | | _Lisbon Earthquake | | | | | of 1761._ | | | | |Lisbon to Corunna |1,994 |Transition, carboniferous | ‘Annual | | | | and granitoid |Register’ | |Lisbon to Cork |5,228 |Transition, carboniferous | „ | | | | crystalline slates and | | | | | granitoid, probably, | | | | | under sea bottom | | |Lisbon to Santa Cruz |3,261 |The same with many | „ | | | | alterations | | | | | | | | _Antilles._ | | | | |Pointe à Pitre to |6,586 |Probably volcanic rocks |Stier and | | Cayenne (doubtful) | | under sea bottom | Perrey’s | | | | | memoran- | | | | |dum, Dijon| | | | | | | _India._ | | | | |Cutch to Calcutta, |1,173 |Alluvial, secondary, | ‘Royal | | 1819 | | granitoid and later | Asiatic | | | | igneous rocks | Journal’ | |India, Nepauls, and | | | | | basin of the Ganges,| | | | | 1834:-- | | | | |Rungpur to Arrah |2,314 }|Deep alluvia, with | ‘Royal | |Monghyr to Gorackpur |3,520 }| occasional transition, | Asiatic | |Rungpur to Monghyr | 990 }| carboniferous, granitoid,| Journal’ | |Rungpur to Calcutta |1,210 }| and later igneous rocks | | |Ships ‘Rambler’ and |1,056 |Sea bottom resting on |‘Nautical | | ‘Millwood,’ at sea, | | unknown rock |Magazine’ | | 1851; between lat. | | | | | 16° 30′ N.L., 54° | | | | | 30′ W., and lat. 23°| | | | | 30′ N.L., 58° 0′ W. | | | | +---------------------+-------+--------------------------+----------+

THE LISBON EARTHQUAKE ON NOVEMBER 1, 1755.

+----------------------------+---------+--------+--------+ | | Moment |Distance|Velocity| | Localities |observed | from |in feet | | |of shock |presumed| per | | | | origin | second | +----------------------------+---------+--------+--------+ | | h. m. | ° ′ | | | Presumed focus of shock, | } 9 23 | -- | -- | | lat. 30°, long. 11° W. | } | | | | A ship at sea in lat. 38°, | } 9 24 | 0 30 | 3,091 | | long. 10° 47′ W. | } | | | | Colares | 9 30 | 1 30 | 1,325 | | Lisbon | 9 32 | 1 30 | 1,030 | | Oporto | 9 38 | 2 30 | 1,030 | | Ayamont | 9 50 | 4 0 | 916 | | Cadiz | 9 48 | 5 0 | 1,236 | | Tangier and Tetuan | 9 46 | 5 30 | 1,478 | | Madrid | 9 43 | 6 0 | 1,855 | | Funchal | 10 1 | 8 30 | 1,382 | | Portsmouth | 10 3 | 12 30 | 1,431 | | Havre | 10 23 | 13 0 | 1,339 | | Reading | 10 27 | 13 30 | 1,304 | | Yarmouth | 10 42 | 15 0 | 1,174 | | Amsterdam | 10 6 | 17 0 | 2,444 | | Loch Ness | 10 42 | 18 0 | 1,409 | +----------------------------+---------+--------+--------+

THE LISBON EARTHQUAKE OF MARCH 31, 1761.[17]

+----------------------------+---------+--------+--------+ | | Moment |Distance|Velocity| | Localities |observed | from |in feet | | |of shock |presumed| per | | | | origin | second | +----------------------------+---------+--------+--------+ | | h. m. | ° ′ | | | Presumed focus, lat. 43°, | } 11 51 | -- | -- | | long. 11° W. | } | | | | Ship at sea in lat. 43°, | } | | | | many leagues from coast | } 11 52 | 0 30 | 3,091 | | of Portugal | } | | | | Ship in lat. 44° 8′ and | } 11 54 | 1 45 | 3,607 | | about 80 leagues from | } | | | | coast | } | | | | Corunna | 11 51 | 2 30 | 2,576 | | Ship lat. 44° 8′ and 80 | } | | | | leagues NNW. of Cape | } 11 58 | 3 30 | 3,091 | | Finisterre | } | | | | Lisbon | noon | 4 30 | 3,091 | | Madeira | 12 6 | 10 0 | 4,122 | | Cork | 12 11 | 9 30 | 2,937 | +----------------------------+---------+--------+--------+

These tables, owing to the nature of the materials which Mallet had at his disposal, are but rude approximations to the truth. Two interesting facts are, however, observable: the first being that the velocities for the earthquake of 1761 are much higher than those obtained for the earthquake of 1755; and, secondly, that in both cases the velocities as determined from the observations of ships at sea closely approximate to each other, in all cases being nearly the same as that with which a sound wave would travel through water.

The great differences in transit velocity obtained for different earthquakes is a point worthy of attention.

Seebach’s velocity is a _true_ transit velocity, and its determination is dependent on the assumption that the shock radiated from the _centrum_ and not from the _epicentrum_, Seebach’s method is explained when speaking about the determination of origins.

Some interesting observations on the velocity with which the earthquake of October 7, 1874, was propagated, are given by M. S. di Rossi.[18]

One assumption is that the disturbance radiated from an origin to surrounding points of observation, whilst another is that the disturbance followed natural fractures, the direction of which is derived from the crest of certain mountain ranges. These velocities are as follows, Maradi being at or near the origin of the disturbance:—

+-----------------------------+-----------------------------------+ | Velocity in feet per second | Velocity in feet per second by | | with direct radiation | propagation along mountain chains | +-----------------------------+-----------------------------------+ | Modigliana 820 | By the Valley of Marenzo 1,080 | | Bologna 656 | „ „ Saveno 1,080 | | Forli 874 | „ „ Montone 1,080 | | Modena 518 | „ „ Panaro {1,080 | | | { 984 | | Firenze 273 | „ „ Sieve 540 | | Compiobbi 328 | „ „ „ 540 | +-----------------------------+-----------------------------------+

Another set of interesting results are those of P. Serpieri on the earthquake of March 12, 1873. The curious manner in which this shock radiated is described in the chapter on the Geographical Distribution of Earthquakes (see p. 231). Two large areas appear to have been almost simultaneously struck, so that, there being no time for elastic yielding, the velocities calculated between places situated on either of the areas are exceedingly great.[19]

From Ragusa to Venice the velocity was 2,734 feet per second „ Spoleto „ „ 4,101 „ „ „ Perugia to Orvieto „ 601 „ „ „ „ „ Ancona „ 1,640 „ „ „ „ „ Rome „ { 1,640 „ „ {or, 2,186

The following are examples of approximate earthquake velocities which have been determined in Japan.

_The Tokio Earthquake of October 25, 1881._—From records respecting this earthquake it appears to have been felt over the whole of Yezo and the northern and eastern coast of Nipon, a little farther south than Tokio. It was severest at Nemuro and Hakodate, and at the former place a little damage was done. From these facts, together with the indications of instruments recording direction of movement and a general inspection of the time records, it seems that the disturbance must have originated beneath the sea on the east coast of Yezo at a very long distance to the north-east of Tokio, from which place it passed in a practically direct line on to Yokohama.

As the disturbance was felt at Yokohama twenty-one seconds later than at Tokio, and the distances between these two places is about sixteen geographical miles, for this portion of its course the disturbance must have travelled at a rate of at least 4,300 _feet per second_. If we assume that the shock, after having reached Hakodate, travelled on at the same rate as it did between Tokio and Yokohama in order to reach Saporo, where the shaking was felt eighteen seconds after Hakodate, it must have had about thirteen geographical miles to travel after Hakodate was shaken before Saporo felt its effect.

Drawing from Hakodate a tangent to the eastern side of a circle of thirteen miles radius described round Saporo, the origin of the disturbance must be on the line bisecting this tangent at right angles. As it also lies on a line drawn through Tokio and Yokohama, it lies in a position about 41 N. lat. and 144° 15′ E. long., which is a position somewhat nearer to Nemuro than Hakodate, as we should anticipate. If this be taken as approximately indicating the origin, then the shock, after reaching Hakodate from the Hakodate _homoseist_, travelled about 218 miles to reach Tokio in 128 seconds, which gives a _velocity of 10,219 feet per second_.

The method here followed is equivalent to that of the hyperbola and one direction (see p. 204). The hyperbola is described on the assumption that the velocity deduced from the time taken to travel between Tokio and Yokohama is correct, and also that the earth waves travelled with approximately the same velocity in the vicinity of Saporo as near Tokio. The probability, however, is that they travelled more quickly. If this be so, then the origin is thrown somewhat to the south-east and the velocity between the Hakodate homoseist and Tokio reduced. Thus, if the velocity in the Saporo district be double that observed in the Tokio district, the origin is shifted about twenty-eight miles to the south-west, and the last-mentioned velocity is reduced to about 9,000 feet per second.

If we work by the method of circles, and assume the velocity to have been constant in all directions, then this velocity must have been about 6,000 feet per second. If we assume that the indications of direction obtained from seismographs and other sources give to us by this intersection a proper origin, the velocity in some directions may have been as much as 17,000 feet per second.

An origin thus determined, or even if determined by the method of circles, is in discord with the fact that places like Nemuro, in the north-east of Yezo, were nearer to the origin than any of the other places which have been mentioned.

The conclusion which we are therefore led to with regard to this shock, assuming of course that the time observations are tolerably correct, is that the velocity of propagation was variable, being greater when measured between points near to the origin than between points at a distance. The velocities estimated vary between 4,000 and 9,000 feet per second.

In the case of the earthquake which has just been discussed, we have an example of a disturbance which must have passed between Tokio and Yokohama in what was almost a straight line from the origin. As this direction ought to give the maximum time of transit if all earthquakes are propagated with the same velocity, the following table is given of the interval between the time of observation of several shocks at these two stations:—

FROM YOKOHAMA TO TOKIO. FROM TOKIO TO YOKOHAMA.

As these are observations which have been made with the assistance of a telegraphic signal daily employed to correct and rate the clocks from which the observations were obtained, they may be regarded as being tolerably, correct.

The disturbance of February 6, the two shocks of March 1, appear, like that of October 25, to have passed in almost a direct line from an origin in the N.N.E, through Tokio on to Yokohama. Their velocities of propagation as calculated from the above intervals are approximately 3,900, 1,900, and 1,400 feet per second. The shock of February 16 appears to have had its origin near to a point in Yedo Bay about eight miles east of Yokohama. Assuming this to be the case, the shock between the Yokohama homoseist and Tokio travelled at the rate of 2,454 feet per second, but between the Tokio homoseist and Chiba at the rate of 750 feet per second; that is to say, the velocity of propagation rapidly decreased as the disturbance spread outwards.

At Yokohama it was recorded at 5.31.54, at Tokio at 5.32.16, and at Chiba at 5.33.48. These times are given in Tokio mean time.

The shock of March 11, which was recorded at Tokio at 7.51.22 P.M. and at Yokohama at 7.51.33 P.M., appears, from the indications of instruments which were exceptionally definite in their records, to have originated in the N.E. corner of Yedo Bay, about nineteen miles S.S.W. from Chiba. This shock was rather severe, fracturing several chimneys. From the Tokio homoseist it appears to have travelled on to Yokohama at the rate of about 2,200 feet per second. Assuming these observations to be _approximately_ accurate, if we take them with the records of previous observers they lead us to the following conclusions:—

1. Different earthquakes, although they may travel across the same country, have very variable velocities, varying between several hundreds and several thousands of feet per second.

2. The same earthquake travels more quickly across districts near to its origin than it does across districts which are far removed.

3. The greater the intensity of the shock the greater is the velocity.