« AnteriorContinuar »
THE NEW ZEALAND
JOURNAL OF SCIENCE.
No. I., Vol. 1.-JANUARY, 1882.
HE publication of a Magazine in the interests of Science,
be looked upon by many as premature, and it may be considered that the enterprise entered upon with this number is destined to that failure which attends so many efforts put forth in advance of the time. But no one interested in the spread of scientific knowledge, and, in particular, no one working at any branch of scientific study, will deny that great advantages may result from such a publication, if well conducted, ard that it may prove an efficient means of advancing scientific work and culture in our midst. The want of some closer and more frequent means of communication between our scientific men than is furnished by our excellent annual volume of Institute Transactions, has often been expressed, but no one seems to have considered the time sufficiently ripe for attempting to fill the existing gap. Former attempts, too, at periodical literature, both here and in the Australian Colonies, have not been of a sufficiently encouraging nature to tempt others into the field. Indeed it was not till last year, when the “Southern Science Record” was started in Mel. bourne, that anything like a successful attempt was made to supply the want. That journal, however, is almost exclusively Australian, so that so far as New Zealand science matters are concerned, we are still in statu quo ante.
There are many ways in which such a journal as this may prove both of special and general use. In the first place, workers in all parts of the Colony will be enabled to know what others are doing, and will thus be able to avoid clashing with one another, and also materially to help one another by interchange of ideas and hy suggestions. Anyone reading an original contribution for publication (in due course) in the “ Transactions of the New Zealand Institute,” will also secure priority of publication for names, descriptions, &c., by sending an abstract to our columns, Articles in English or Foreign periodicals which deal with New Zealand scientific matters, or which are of general interest and value, will be reproduced either whole or in a condensed form. This in itself will prove an important branch of our work, as it is only in a few of the more important centres of population that current scientific literature is accessible to those who take an interest in it. Again, there are many persons who devote the few spare hours of a busy lite to scientific pursuits, and who in the course of their researches accumulate a considerable amount of valuable information, but they shrink from publishing what they consider their fragmentary knowledge in our recognised channel for scientific work. But they need no longer hesitate to communicate the knowledge thus acquired, however fragmentary it may be. Indeed, it is hoped that such amateur workers—and there is a considerable number of them in the Colony–will freely avail themselves of the facilities now offered to them, and will make the “Notes” of this journal one of its most interesting and valuable features. Our columns will be open to all who desire to communi. cate or obtain information on scientific subjects.
It is hoped that the Secretaries of all scientific societies in the Colony will aid by forwarding notices of their meetings, together with abstracts of original papers read at them, to the Editor. And if all those interested in the cause of science will give their assistance to the publication now commenced, there need be little fear for its immediate and lasting success.
Finally, it may be well to add that the animating spirit which has led to the publication of The New ZEALAND JOURNAL OF Science is solely the desire of spreading knowledge, and of encouraging the search for truth. It is hoped that it will be always carried on in a similarly genuine spirit, and so we adopt for our motto, and as our standard for guidance, the old but true words of Lucretius
Judicio perpende : et si tibi vera videntur
WHAT IS AN EARTHQUAKE?
BY PROFESSOR F. W. HUTTON.
What are earthquakes? Where do they come from? How are they caused? These are questions which may interest many people in New Zealand, although we cannot, at present, see that any useful result is likely to follow from an investigation of earthquake phenomena. However, on this point no one can dogmatize. In no case have the practical results which followed from a scientific discovery been foreseen, and, perhaps, in no case has an important scientific discovery been made by a person who was looking only for practical results. The ancient philosophers were ridiculed for poring over the various figures presented when a cone was cut in different directions, and yet the practical application of astronomy to navigation was one of the results of their studies. The patient investigation of minute forms of life, which followed immediately on the invention of the microscope, was undertaken without any idea of its being useful; and yet nearly all our sanitary arrangements are based on the discovery that disease and living germs are intimately connected. The invention of the steam-engine seems at first sight to be an exception; but it is not so, for it was only a practical application of the law of the expansion of gases. Watt discovered nothing new : but it occurred to him how a very useful combination of things already known might be made. It is a mistake to call the invention of the steam-engine a scientific discovery ; a history of science might, and should, pass it by unnoticed, but it forms a very important era in the history of industrial art. certainly cannot expect to be able to prevent earthquakes, but neither can we prevent storms; yet every civilised Government spends money in the investigation of meteorological phenomena, with the hope that it may be possible to foretell the weather. And would it not be as useful to be able to foretell earthquakes? And is it impossible to do this ? He would indeed be but a halfhearted philosopher who could harbour such a thought for an instant.
But, to return to the question at the head of the paper, “What is an earthquake ?" When a heavily-laden waggon jumbles along a rough street, the room of the house we sit in shakes. A tremor has been communicated from the wheels of the waggon to the walls of the house. An earthquake has happened ; none the less a true earthquake because it has originated, as we say,
artificially. It will be worth our while to examine with some care what takes place in the ground while the waggon is passing over it. In the first place, the fact of the walls of the house shaking shows that the ground on which the foundations rested must have moved, and this movement must have been the result of a horizontal pressure passing through the particles of earth from the centre of the road to the house. But the direction of the blows of the wheel on the ground, as it passed over the rough stones, must have been vertical ; consequently, the vertical concussion of the wheel on the ground must have started a series of movements in the earth particles which radiated outward in all directions from the point of concussion. In the second place, the wheels of the waggon would, probably, leave a mark behind them, unless the road were very hard (as, for instance, if it were paved). That is to say, some of the particles moved by the concussion of the wheel would not have returned into their original position when the pressure was removed. But the foundations of the house would be found to retain exactly the position they had before the concussion; consequently, the earth-particles under the house must have moved in a horizontal direction, and, by virtue of the elasticity of the mass, returned once more into their original position. From these considerations we see that the wheels of the waggon must have compressed the particles below them. Some escaped the compression, by being moved on one side, where they remained ; others, unable to do this, compressed all the particles surrounding them; and these in their turn compressed the next row, and so on. The sudden impact of the wheel would make the compression much more severe than if it were due to the mere weight of the waggon, and the moment the impact was over, the elasticity of the compressed particles would enable them to recover their former position. Thus, a wave of movement would spread in all directions from the wheel ; each particle being moved directly outward from the wheel by compression, and returning into its place again by elasticity. This is a true earthquake wave, which is defined by Mr. Mallet as "a wave of elastic compression traversing the substance of the earth.”
Now, it is necessary to notice carefully that there are two kinds of movement in an earthquake wave. First, there is the movement of each particle as it travels forward and back again into its place. The rate of this movement is called the velocity of shock. Secondly, there is the movement communicated by one particle to another, or, in other words, the speed with which the wave moves outward. If all the particles were perfectly rigid and absolutely in contact, all would move together ; but this is never the case, and a certain amount of time is lost in transmitting the movement from one particle to another, which will vary according to the elasticity of the particles and their distance apart (the wave travelling much faster in a solid elastic rock than it would in loose sand or clay). The rate of movement of the wave outward is called the velocity of transit. The dis
tance each particle moves from its original position is called the amplitude of the wave. A familiar illustration will perhaps make these terms clearer. If a number of billiard balls are placed in a line so that they touch one another, and another ball is made to roll gently up and strike the first in the line, a wave of elastic compression will be transmitted through the line, and the last ball will move off. The time between the first ball being struck and the movement of the last one will be, when reduced to feet per second, the velocity of transit of the wave; while the velocity of the moving ball, also reduced to feet per second, will be the velocity of shock. It is evident that the velocity of transit must always greatly exceed the velocity of shock.
Let us now examine the effects of earthquake shocks on buildings. It is plain that when the velocity of transit is very great, the whole of the foundations of a building will practically move at the same time, and if the walls are firmly bound together, but little harm will be done. This is always the case when an earthquake wave traverses large masses of compact rock; but when the wave passes into beds of loose soil or shingle, the velocity of transit is very much reduced, and the damage done to buildings is proportionately increased. But the velocity of transit only determines whether the whole building moves together or whether different parts of it move at different times, it has nothing to do with the actual movement itself. This depends upon-(1) the velocity of shock, (2) the direction of the shock, and (3) the amplitude of the wave. Of course, the greater the velocity of shock, the greater is the destructive power of an earthquake, and it can only be guarded against by making buildings low and with light roofs. But the damage done by an earthquake depends very much on the direction of the shock, whether it be vertical or nearly horizontal-i.e., its direction in altitude; and from what point of the compass it comes—i.e., its direction in azimuth. On the one hand, the nearer the shock approaches to the horizontal, the greater will be its overturning power; while on the other hand, the nearer it approaches the horizontal, the greater will be the distance of the centre of impulse of the earthquake wave, and consequently the less will be the velocity of shock. Theoretically it can be shown that the destructive effect of an earthquake wave will be greatest when its angle of emergence from the earth is about 50 deg. The direction of the wave in azimuth is a very important consideration, for a wall built in the direction of the wave would stand, while one at right angles to it would be overthrown. The best position for a house would be one in which a corner faced the direction of the wave.
It is evident that the amplitude of the wave is another very important point ; for a small movement, however rapid, might not be so destructive as a larger movement with a less velocity. Now, in compact rocks, where the whole mass is bound together, the amplitude is small, but it is much increased when the ground is loose and incoherent; consequently, earthquakes are almost always more severely felt on alluvial plains than on solid rock,