437 



SEISMOLOGY. 



SELENE. 



433 



circumstances in which fixed instruments cannot be appealed to. The 

 elements necessary to he recorded, Mr. Mallet states, in his article ' On 

 Observation of Earthquake Phenomena,' in the third edition of the 

 ' Admiralty Manual of Scientific Inquiry,' are such as will enable us to 

 calculate, in order to obtain finally the actual elements of the wave 

 itself, as stated above 1 , the direction in azimuth of the wave's motion 

 upon the earth's surface, and also its direction of emergence at the 

 points of observation. 2, Its velocity of transit upon the surface. 

 3, Its dimensions and form that is, its amplitude and altitude. 



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 we 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 nature of fluid pendulums. They are 

 superior to common solid, pendulums, 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 seismomctry. " Fix 

 a heavy ball, such as a four-pound shot, at one 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 stone, 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, Ac., 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 .'roils 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 rods 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 con 

 elusion as to the true line 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 nude from his observations of the range of either 

 Huiil or solid pendulums. " A common bowl partly filled with a 

 viscid fluid, such as molasses, which, on being thrown by oscillation 

 up the side 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 used, 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 instrument*, at moderately distant stations, nay from fifteen to 

 thirty miles apart, 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 be expected. 

 <>n 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 at 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. x. 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 in 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. 



The seismic vertical, Mr. Mallet determined from the independent 

 and concurrent evidence of above seventy separate wave-paths, was 

 close to the village of Caggiano, near the eastern extremity of the 

 valley of the Solaris. The depth of the focal point below the sea-level, 

 that is, themin focal depth was found to be about 5j geographical miles. 

 From all the information, observed and calculated, it was deduced that 

 the fofal-carity, within which the seismic energy originated, was a 

 curved lamellar cavity, or fissure, of about three geographical miles in 

 depth by nine in length, with an inclined vertical section, and a mean 

 focal depth (or depth of ita central point of surface) of of miles below 

 the level of the sea, as just stated. The probable horizontal form of 

 the cavity, when laid down upon a map, co-ordinates with the existing 

 lines of dislocation of the country in a remarkable manner. Mr. 

 Mallet, it may here be remarked, assumes the force that acted within 

 this cavity to have been due to steam of high tension, either suddenly 

 developed or suddenly admitted into a fissure rapidly enlarged by 

 rending. 



The most trustworthy of the observations of time correspond with 

 considerable exactness, and give a velocity of transit of the wave of 

 shock upon the surface, of between 700 and 800 feet per second. 

 Strictly speaking this is the velocity at which the ware-form was 

 propagated from point to point. The velocity of the wave itself, or of 

 the tcavc-partiele, was in round numbers between 13 and 14 feet per 

 second in the direction of the wave-path. Mr. Mallet points out a 

 remarkable relation between this velocity and that recorded for the 

 earthquake of Kiobamba, as the greatest whose effects have been ob- 

 served. The height due to the velocity of this wave is to the altitude 

 of Vesuvius as that due to the velocity recorded of the Riobamba wave 

 is to the mean height of the volcanic shafts of the Andes, and more 

 especially to the height of the volcanic vents nearest to Riobamba. 

 [VOLCAHOS.] The wave of shock, of course, decays in relation to 

 superficial distance from the seismic vertical. The amplitude of the 

 wave slowly and slightly increases, and its velocity decreases. In the 

 case of the Italian earthquake under investigation, the lowest velocity 

 at nearly 30 miles from the seismic vertical was still about 11 J feet per 

 second. The total modifying effects of this earthquake on the earth's 

 surface were insignificant. No great sea-wave accompanied it ; nor 

 was such possible, the focal point being inland. Mr. Mallet examined 

 with care more than 150 miles of sea-coast, as well as river-courses, for 

 evidence of any permanent elevation nf land having taken place con- 

 currently with the earthquake, but found none. This is an important 

 point, because geologists and travellers have often ascribed elevations 

 of the land to earthquakes, or to a succession of earthquake-shocks 

 directed upon the same spot ; but " earthquakes," Mr. M. observes, 

 " cannot produce elevations, although the latter have been known to 

 have taken place about the same time as earthquakes, and in the same 

 region." . . . . " The functions of elevation and depression," of an 

 earthquake, " are limited solely to the sudden rise and as immediate 

 fall, of that limited portion of the surface through which the great 

 wave is actually passing momentarily." 



The works and memoirs cited in this and in the article EARTH- 

 QUAKE, will make the student acquainted with the entire available 

 literature on the subject of earthquakes. 



SELECT VESTRY. [VESTRY.] 



SKLENALDINE. [TJUALDINK.] 



SKLKNK (2eA^nj), the Moon, was worshipped as a goddess by the 

 ancient Greeks. She is generally represented as the sister of Helios, or 

 the Sun ; sometimes as his daughter, and occasionally as his wife, and 

 the mother of the four Seasons. She is also said to have had children 

 by Zeus and several others. The story of Endymion, whom she 

 caused to fall into a perpetual sleep that she might unseen gaze upon 

 his beauty and enjoy his embraces, is well known from the use made . 

 of it by the poets. By him she is said to have been the mother of 

 fifty daughters. She was believed to drive her chariot through the 



