EARTHQUAKES.

Northland Age, Volume 2, Issue 46, 19 June 1906, Page 2

EARTHQUAKES.

■ —* — THE IMMENSE PRESSURE IN THE EARTH'S INTERIOR. 4 * No matter where an earthquake occurs, it sends its own news all over the world almost immediately. Wherever there is an observatory equipped with the little instrument called the seismograph, the message j of disaster, perhaps in the uttermost I parts of the earth, is received and recorded. Only such details as the number of lives lost and cities destroyed remain to be reported by* other means.

Professor John Milne may be rightly described as the leader of the new seismology. The new science of earthquake study began its work with the invention of the seismograph, which is an instrument used for analysing earthquake motion and recording it in conjunction with a time .scale. By means of these instruments it became possible to inaugurate an entirely new series of investigations of the nature of earthquakes, and ultimately to create a new branch of science.

The earliest form of seismoscope was, of course, one which signalled a tremor in the ground ; but in order to gain a more instructive idea of the movement we need an instrument which will trace the whole motion automatically from the beginning of the quake to the end, showing at every instant the direction, the amplitude, and the frequency or periods of the vibrations. Such an instrument is called a “seismograph. There is an intermediate class of instruments which give continuous records of the movements of the ground, and also show the directions and amplitudes, but do not unravel the various ■ complicated motions. These are called seismometers. A seismograph is a far more complex instrument. Its first problem was and is, to devise a “steady point”— that is to say, a mass which shall always remain at rest, while everything around it and even the support that holds it, is in a constant state of vibratory motion. AN INCONCEIVABLE STRAIN. To attain it is far from easy ; but if it can be attained, then we can imagine a plate attached to it with a prepared surface capable of receiving and recording the movements of a pencil or point. And we can imagine a tracing pencil attached to supports which vibrate with the earth, and the other end resting l'ghtly on the smoked (or surfaced) plate. Or we can imagine the thing reversed, and the tracing point attached to our unmoving, immovable mass, and tho smoked plate attached to the supports that vibrate with the earth. In either case the movements of the earth will cause a traced line to be drawn on the plate, showing the motion of the earth, and the support, relative to the “steady point.” In this way, though this is but a rough sketch of the theory of a very ingenious instrument,' with many complexities and improvements for magnifying the extent of the motion, or rectifying the direction of the moving point, earthquakes are measured ; and this is the principle of the beautiful “seismograph,” which ‘Professor Milne and others have perfected.

Professor Milne has mapped out the earthquake centres of origin of the world into eighteen great districts ; and he regards the recent great earthquakes of Dharmsula in India as having had its origin in the nearest of these great districts, the Himalayan seismic ellipse. All this is made possible by the earthly rigidity due to the enormous pressure existing throughout its interior. It is a law of physics that in any, concentric spherical surface* the resistance of the enclosed nucleus must be just equal to the pressure of the surrounding shells resting upon it, and thus the* strain upon tho matter of the globe increases towards the centre.

This pressure is sustained by the increasing density and rising temperature of the matter in the earth’s interior, which is thus under an inconceivable strain, far surpassing the strength of any known substance. As the matter is above the critical temperature of every element, it is essentially a gas reduced by pressure to a hardness greater than that of steel, and with an elasticity and rigidity infinitely near to perfection. The result is that the explosive strain upon the matter of our globe from within, which is everywhere just equal to the pressure sustained by the enclosed nucleus, renders the interior matter more rigid than any known substance ; and even the outer layers, which are but slightly compressed, yield so little under the action of external forces that the globe as a whole is more rigid than steel. It is this extreme rigidity which causes vibrations of any shock, external or within the earth, to.be transmitted throughout the whole bulk.—“ Science Siftings.”