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such cases, the original seat of the disturbing forces must necessarily fall short of the centre of the earth, and also be unconnected with any such continuous fluid nucleus, as many suppose to exist at no very great distance from the surface. When from the action of any disturbing cause, the equilibrium of a continuous fluid mass was deranged, the resulting motions would be communicated in all directions radiating from the point of original disturbance, and if this was near the centre of the earth, the movements ought to affect its whole surface, so that shocks would be experienced nearly simultaneously over the whole world. But however extensive may be the connection of certain Earthquakes on record, we have nothing approaching to any such universality of effect as this, and the theory of local action (using this expression in a large sense) appears to agree best with the present state of our knowledge relative to the phenomena of Earthquakes and their causes.
On the night of the 5th of March, 1842, another very severe shock was experienced, which appears to have been more limited in its range than the preceding, and exhibited essentially distinct phenomena. The motion in this instance, instead of being like the rounded swell of a fluid or viscid mass, was sharp and sudden, like the effect of a concussion, than of an undulation, and seemed indeed to be a much magnified " jarr," similar in kind to that experienced by the hand when a hammer held by it, is struck forcibly on a hard unyielding body. One intelligent friend, who was in his study when the shock occurred, described the effect to be, as if he and his chair had received a sudden and severe blow from behind, by which they were impelled forward, while to me, it seemed as if my chair had been suddenly lifted from the ground, and dashed down again with great force.
The following interesting detail of the effects of the shock, as experienced at Berkeri, a station on the Doab Canal, about 20 miles south of Saharunpore, was communicated to me by Serjeant and Overseer J. Petrie, to whom I feel much indebted for the trouble he has taken in preparing it.
Letter from Serjeant John Petrie, dated Berkeri, 5th March, 1842.
Sib,—We had a very smart shock of an Earthquake here at 9 o'clock this evening; so much so indeed, that every' thing in this bungalow shook and rattled again. I had just laid down to rest with a book in my hand when it came on, and I started up and called out for assistance, thinking the house was coming down. Every one about the place felt it, and came running to me. I found that the south door of the inner room, which I had bolted before I went to bed, had been forced open by the bolt falling down. Indeed every thing in the house shook, and I was very much afraid of its falling, after having read the accounts from our Army near Peshawur. At that place, a number of houses have been destroyed, and many lives lost, from the last Earthquake.
Although this shock did not last so long as the one of the 19th of last month, in my opinion it was much more severe for the time.
The rate of propagation of this shock appears to have been great, since no perceptible difference was observed in the times of its arrival at the following places: Simlah and Mussoorie in the Himalayas, Deyrah in the Deyrah Dhoon, Saharunpore, and Berkeri. There is, therefore, every reason to think, that on this occasion the shock was propagated after the circular method, as I have denned it above, and the nature of the shock appears to indicate, that the seat of the disturbing force was either within the rocky crust of the earth, or at a very small distance indeed beneath it. Such a supposition is necessary to account for the peculiar "jarring" sensation characteristic of this shock. Its effects appear to have been most severe at Deyrah, where a large house is said have been split from top to bottom, but no particulars of this accident have reached me. I am somewhat disposed to think that the actual force of disturbance was situated somewhere in the valley of Deyrah, and propagated thence to the hills on one side, and to the plains on the other; a more extensive collection of facts would however be necessary to give probability to this impression, and these have not in this instance been collected. It may be stated, however, that all who had experienced both shocks in this neighbourhood, concurred in opinion that they came in different directions, and as the first was from West to East, it is not impossible the second may have been from North to South. The southern door of the inner room of the Berkeri Canal bungalow, which is stated by Serjeant Petrie to have been driven open by the shock, would on the above supposition receive the first impulse, and the effect produced upon it, tends in some measure, to confirm
the view I have taken of the direction in which the shock was propagated.
Hie occurrence of Earthquakes throughout these provinces, and indeed throughout India generally, is so frequent, and their connection with geological theories of such an interesting character, that it is highly desirable to facilitate, as much as possible, the collection of minute, well authenticated, and carefully detailed facts relative to these various phenomena. I will therefore conclude this note, by pointing out briefly those points on which information is peculiarly desirable, and the attention of observers is earnestly solicited to them.
1. The Time.—The startling discrepancies that occur in regard to time, in otherwise most satisfactory accounts of Earthquakes, indicate the great necessity for precaution in observing it, since it is undoubtedly the point on which the most interesting conclusion relative to such occurrences must be based. When, therefore, the period of a shock is marked by a watch, means ought to be taken, whenever possible, to verify the time shewn by this watch, by some simple celestial observation, or some data should be given by which the time could be ascertained independently within very trifling limits, as for example, by a specification of the exact length of the shadow of a vertical object of fixed and determinate length, on a horizontal level, at a precise moment, not too near noon; or if near the coast or at sea, the first appearances and last disappearances of the sun's upper and lower border, above and below the sea horizon, etc.* Without this minute identification of time, it is impossible to maintain the connection of shocks felt at far distant places; calculations of the rate of progress of the undulations or vibrations can only be approximative, and other interesting points are rendered inconclusive.
2. The Duration.—On this point also, the most striking discrepancies are to be observed, arising no doubt from each observer making his own sensation the measure of duration, and estimating the latter without reference to some determinate standard. When the mind is intently occupied either by feelings of alarm or intense interest, it is wholly unfitted for estimating duration correctly, and the watch ought only to be trusted. The general tendency is to make the duration
• Sir J. Herscbell's Meteorological Instruction, Prof. Papere, Roy. Engrs. vol. ii. of shock longer than it really is, and in most instances, considerable deductions might with safety be made from recorded observations on this point, to bring them near the truth. The duration of intervals between shocks should also be carefully noted.
3. Nature of the Shocks.—This is very frequently twofold: one kind throws the crust of the earth into a tremulous state. This was the nature of the shock of the 19th February. The second kind is of the nature of a concussion or blow, and does not always occur. Sometimes both of these are conjoined in one and the same shock, and the latter is felt generally in the middle of the former.
4. Nature of the motion on the Earth's surface.—Three different varieties of this have been observed. First, a horizontal motion by which bodies are, as it were, pushed horizontally forward. Second, a vertical motion by which they are lifted up and dashed down again. The conjunction of these two kinds of motion produces the third, which is of an undulatory character, partaking both of the horizontal and vertical movements. This kind is the most frequent of all, and produces those sensations of nausea, so commonly alluded to.
5. Rents in the ground and subsidencies are very common accompaniments of Earthquakes, and their appearance ought to be represented on paper, and their dimensions carefully measured. These are often accompanied by loud noises of various kinds.
6. Meteorological phenomena are highly important, and some curious and interesting relations have been observed, between these and the occurrences of Earthquakes. This is especially true as regards the state of the barometer and thermometer, and the electric condition of the atmosphere. Such points therefore merit peculiar attention.
7. Geological structure of affected District.—When the observer is qualified to furnish information relative to this, his remarks will be additionally important, as it has been observed, that in localities exhibiting certain geological features, Earthquakes always occur with much greater frequency than in others. Wherever powerful and extensive volcanic action has occurred, where faults and fissures communicating with the internal seats of disturbing forces are found, there Earthquakes occur with greater frequency and higher intensity, and they are frequently observed to pursue a direction, parallel to that of the principal faults or fissures.
8. The direction of the Shock.—I am not aware of any instrument haying yet been actually employed for ascertaining this point, but the following simple apparatus has been proposed for the purpose by Prof. Babbage, in his admirable little volume on the Economy of Manufactures and Machinery; and although it must be confessed, that several of the jchemes he has proposed in that work, remind us a little of the designs of the sages in Swift's College of Laputa, this is not one of them, but seems adapted to its proposed object.
"An earthquake," he remarks "is a phenomena of such frequent occurrence, and so interesting both from its fearful devastations, as well H from its connexion with geological theories, that it became important to possess an instrument which shall, if possible, indicate the direction of a shock, as well as its intensity. An observation made a few rears since at Odessa, after an Earthquake which happened during the night, suggests a simple instrument by which the direction of the shock mar be determined.
"A glass vase, partly filled with water stood on the table of a room in a house at Odessa; and from the coldness of the glass, the inner part of the vessel above the water was coated with dew. Several very perceptible shocks of an Earthquake happened between three and four o'clock in the morning; and when the observer got up, he remarked that the dew was brushed off at two opposite sides of the glass, by a wave which the Earthquake had caused in the water. The line joining the two highest points of this wave, was of course that in which the shock travelled. This circumstance which was accidentally noticed by an Engineer at Odessa,* suggests the plan of keeping, in countries subject to Earthquakes, glass vessels partly filled with treacle or some unctuous fluid, so that when any lateral motion is communicated to them from the earth, the adhesion of the liquid to the glass shall enable the observer, after some interval of time, to determine the direction of the shock.
"In order to obtain some measure of the vertical oscillation of the earth, a weight might be attached to a spiral spring, or a pendulum might be sustained in a horizontal position, and a sliding index be moved by either of them, so that the extreme deviations might be
• Memoires tie I'Acadeinie des Sciences lie Petersburg!), 6me series, tome i. p. 4.