Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/661

EARTHQUAKES 609 Lad tolerated a few Protestant heretics in their midst. be built capable of resisting any assumed acceleration. To avoid a recurrence of disaster certain of these were Experience has shown that yielding first shows itself at baptized by force. In the myths relating to underground the base of a pier, a wall, or a building, and it is therefore monsters and personages that are said to be the cause clear that the lower portion of such structures should be of earthquakes Ave see the direct effects which exhibitions of greater dimensions or stronger than that above. Piers of seismic and volcanic activity have produced upon the having these increased dimensions below, and tapering imagination. The beliefs, or more properly, perhaps, the upwards in a proper manner, so that every horizontal poetical fancies thus engendered have exhibited themselves section is sufficiently strong to resist the effects of the in various forms. Beneath Japan there is said to be a inertia of its superstructure, are employed to carry railways catfish, which in other countries is replaced by a mole, in Japan. In that country cast-iron piers are things of a hog, an elephant, or other living creature, which when it the past, whilst piers of masonry, together with their is restless shakes the globe. The Kamchadales picture foundations, no longer follow the rules of ordinary a subterranean deity called Tuil, who in Scandinavian engineering practice. mythology is represented by the evil genius Loki. To After flood, fire, earthquake, or when opportunity come still nearer home, we have only to think of the refer- presents itself, changes are introduced in the construction ence in the Decalogue forbidding the making of graven of ordinary buildings. In a so-called earthquake-proof images of that which is in the earth beneath, to see in house, although externally it is similar to other dwellings, early Biblical history evidence of a subterranean mytho- we find rafters running from the ridge pole to the floor logy. It cannot be positively stated how this arose, but sills, an exceedingly light roof, iron straps and sockets it seems probable that the same causes which led to the replacing mortices and tenons, and many other departures creation of Pluto, Vulcan, and Poseidon gave rise to prac- from ordinary rules. Masonry arches for bridges or tices condemned by Moses. The marked effect which arched openings in walls (unless protected by lintels), these imaginary deities and creatures have had upon the heavy gables, ornamental copings, cappings for chimneys, literature and the pictorial and other arts of many nations have by their repeated failure shown that they are hardly requires comment. The mental effects which undesirable features for construction in earthquake counaccompany and follow small earthquakes may be compared tries. As sites for buildings it is well to avoid soft to those we pay for when we ride upon a switchback. ground, on which the movement is always greater than on When an earthquake commences there is perhaps a little hard ground. Excessive movement also takes place along anxiety as to Avhat may happen next, but when it is over the face of unsupported openings, and for this reason the the imagination is excited as to its cause, and there is edges . of scarps, bluffs, cuttings, and river banks are a feeling of satisfaction that a new sensation has been localities to be avoided. In short, the rules and preexperienced. Small earth-shiverings have often thawed cautions which have to be recognized so as to avoid or the formalities of a dinner-party, and made strangers in mitigate the effects of earthquake movement are so hotels acquainted with each other. numerous that students of engineering and architecture in Perhaps the greatest practical benefits derived during Japan receive a special course of lectures on this subject. the last few years from seismological investigations re- When it is remembered that a large earthquake may Building to t° important changes and new principles entail a loss of life greater than that which takes place in withstand which have been introduced into the arts of many wars, and that for the reconstruction of ordinary ea en ineer anci buildings, factories, and public works an expenditure of S builder when ^ Jlk' in earthquake countries. The constructing new rules several million pounds sterling is required, the importance and formulae, rather than being theoretical deductions of these studies cannot be overrated. Severe earthquakes from hypotheses, are the outcome of observation and are fortunately unknown in the British Isles, but we have experiment. True measures of earthquake motion have simply to turn our eyes to earthquake-shaken colonies and been given to us by modern seismometers, with the result lands in close commercial touch with Great Britain to that seismic destructivity can be accurately expressed in realize the importance of mitigating such disasters as much mechanical units. From observation we now know the as possible, and any endeavour to obviate the wholesale degreatest acceleration and maximum velocity of an earth struction of life should appeal to the civilized communities particle likely to be encountered j and these are measures of the world. of the destructivity. The engineer is therefore dealing An unexpected application of seismometry has been to with known forces, and he has to bear in mind that these record the vibration of railway trains, bridges, and steamare chiefly applied in a horizontal direction. A formula ships. _ An instrument of suitable construction A Uca connecting the acceleration requisite to overturn bodies will give records of the more or less violent t/onsof of different dimensions has been given. The acceleration jolting and vibratory movements of a train, seismowhich will fracture or shatter a column firmly fixed at its and so localize irregularities due to changes metry. foundation to the moving earth may be expressed as in the character of ballast and sleepers, to variation in follows:— gauge, &c. An instrument placed on a locomotive throws _1ffFAB considerable light upon the effects due to the methods of a ~6 fw ’ balancing the wheels, and by alterations in this respect a where saving of fuel of from 1 to 5 lb of coal per mile per a — the acceleration per sec. per sec. locomotive has sometimes been effected. F = the force of cohesion, or force per unit surface, By mapping the centres from which earthquakes which when gradually applied produces frac- originate off the coast of Japan, we have not only deterture. mined districts where geological activity is pronounced, A = area of base fractured. but have placed before the cable engineer well - defined B = thickness of the column. localities which it is advisable to avoid • and in the records /= height of centre of gravity of column above the of unfelt earthquakes which originate far from land fractured base. similar information is being collected for the deeper parts w = the weight of the portion broken off. of the. oceans. Occasionally these records have almost With this formula and its derivatives we are enabled immediately made clear the cause of a cable failure. to state the height to which a wall, for example, may From lack of such information in 1888, when three cables S. HI —77