If a common barometer be moved a few inches up and down by the hand, the column of mercury will be found to oscillate up and down in the tube in directions opposite to the motions of the instrument, the range of the mercury depending upon the velocity and range of motion of the whole instrument. A barometer fixed to the earth, therefore, if we could unceasingly watch it, would give the means of measuring the vertical element of the shock-wave ; and If wo could lay it down horizontally, it would do the same for the amplitude, or horizontal element. This we cannot do ; but the same principle may be put into use by having a few pounds of mercury and some glass tubes bent into the form of the letter L, sealed close at one end, and open at the other ; together with some common barometer tubes, having the open end turned up like an inverted syphon. The instruments to be con structed of these are of the natore of fluid pendulums. They are superior to common solid where the dimensions of the shocks are small ; but where these are great and very violent, heavy solid suspended pendulums will be found more applicable. The length of the seconds pendulum for the latitude of Greenwich will always be desirable.
Mr. Mallet thus describes a solid pendulum, of easy construction, which will answer several important purposes of seismometry. " Fix a heavy ball, such as a four-pound shot, at ono end of an elastic stick, whose direction passes through the centre of gravity of the ball : a stout rattan will do. Fix the stick vertically in a socket in a heavy block of wood or atone, and adjust the length above the block as near as may be to that of the seconds pendulum for Greenwich. Prepare a hoop of wood, or other convenient material, of about 8 inches diameter ; bore four smooth holes through the hoop in the plane of its circle, and at points 90' distant from each other : adjust through each of these a smooth round rod of wood (an uncut pencil will do well), and make them, by greasing, &c., slide freely, but with slight friction, through the holes. Secure the hoop horizontally at the level of the centre of the ball by struts from the block, and the ball being in the middle of the hoop, slide in the four ;rods through the hoop until just in contact with the ball. It is now obvious that a shock, causing the ball to oscillate in any direction, will move one or more of the rods through the holes in the hoop, and that they will remain to mark the amount of oscillation.
" A similar apparatus, with the pendulum-rod secured horizontally (wedged into the face of a stout low wall, for example), will give the vertical element of the wave. Two of these should be arranged, one north and south, the other east and west. One objection to this and all apparatus upon the same principle is, that as the centre of elastic effort of the pendulum-rod never can be insured perfectly in the plane passing through the centre of gravity of the ball, for every possible plane of vibration, so an impulse in a single plane produces a conical vibration of the pendulum, and hence the ball deranges the position, more or less, of the index rode which are out of the true direction of shock. Moving the apparatus by hand, and a little practice in observa tion of its action, will, however, soon enable a pretty accurate elusion as to the true lino of shock to be deduced from it." The observer must record minutely the dimensions and other con ditions of such apparatus, to enable calculations as to the wave of scientific value to be made from his observations of the range of either fluid or solid pendulums. " A common bowl partly filled with a
viscid fluid, such as molasses, which, on being thrown by oscillation up the aide of the bowl, shall leave a trace of the outline of its surface, has been often proposed as a seismometer. This method has many objections; it can only give a rude approximation to the direction of the horizontal element; but as it is easily need, should never be neglected as a check on other instruments. A common cylindrical wooden tub, with the sides rubbed with dry chalk and then carefully half filled with water or dye stuff, would probably be the best modification." Such instruments and contrivances as those now described, suffice to give the direction of transit of the earth-wave, and its dimensions. Its rate of progress or transit over the shaken country remains to be observed ; and wherever it may be possible to connect three or more such instruments, at moderately distant stations, say from fifteen to thirty miles chart, by galvanic telegraph wires, so as to register at one point the moment of time at which each instrument was affected, the best and most complete ascertainment of transit rate may bo expected. On the entire subject of extemporaneous seismometry, we must again refer the Intending observer to Mr. Mallet's article in the Admiralty Manual' before referred to.
Since the publication of the article EARTHQUAKES, Mr. Mallet has applied the method of investigating their phenomena, announced in his reports published in those of the British Association, to the earth quake of December 16,1857, noticed in that article, the greatest that has occurred in Italy since that of 1783; having personally examined its effects. He has incorporated the results in an elaborate report read to the Royal Society on the 24th of May, 1860, and of which an abstract is given in vol. z. of the Society's Proceedings,' p. 486-494, the report itself being reserved for the Philosophical Transactions: From that abstract the following particulars are derived, being the result of the first exact investigation of the phenomena of a great earthquake. It was felt over nearly the whole of the Italian peninsula, south of Terracina in the States of the Church, near the Neapolitan frontier, about 56 miles south-east from Rome, and of Gar gano, on the Adriatic. Its area of greatest destruction, or meizoseismal area, within which nearly all the towns were wholly demolished, was an oval, whose major axis was in a direction north-west and south east nearly, and about twenty-five geographical miles iu length by ten in width. The first isoseismal area beyond this, within which build ings were everywhere more or less prostrated and people killed, is within an oval of about sixty geographical miles by thirty-five ; the second isoseismal is also an oval within which buildings were every where fissured, but few prostrated, and few or no lives lost. The third isoseismal embraces a greatly enlarged area, within which the earthquake was everywhere perceived by the unassisted senses, but did not produce injury. A fourth isoseismal was partially traced, within which the shock was capable of being perceived by instrumental means, and which probably reached beyond Rome to the northward.