Boys' Second Book of Inventions

by Ray Stannard Baker

The accompanying diagram gives a very clear idea of the arrangement of the apparatus. The "boom" is the pendulum. It is customary to think of a pendulum as hanging down like that of a clock, but this is a horizontal pendulum. Professor Milne has built a very solid masonry column, reaching deep into the earth, and so firmly placed that nothing but a tremor of the hard earth itself will disturb it. Upon this is perched a firm metal stand, from the top of which the boom or pendulum, about thirty inches long, is swung by means of a "tie" or stay. The end of the boom rests against a fine, sharp pivot of steel (as shown in the little diagram to the right), so that it will swing back and forth without the least friction. The sensitive end of the pendulum, where all the quakings and quiverings are shown most distinctly, rests exactly over a narrow roll of photographic film, which is constantly turned by clockwork, and above this, on an outside stand, there is a little lamp which is kept burning night and day, year in and year out. The light from this lamp is reflected downward by means of a mirror through a little slit in the metal case which covers the entire apparatus. Of course this light affects the sensitive film, and takes a continuous photograph of the end of the boom. If the boom remains perfectly still, the picture will be merely a straight line, as shown at the extreme right and left ends of the earthquake picture on this page. But if an earthquake wave comes along and sets the boom to quivering, the picture becomes at once blurred and full of little loops and indentations, slight at first, but becoming more violent as the greater waves arrive, and then gradually subsiding. In the picture of the Borneo earthquake of September 20, 1897, taken by Professor Milne in his English laboratory, it will be seen that the quakings were so severe at the height of the disturbance that nothing is left in the photograph but a blur. On the edge of the picture can be seen the markings of the hours, 7.30, 8.30, and 9.30. Usually this time is marked automatically on the film by means of the long hand of a watch which crosses the slit beneath the mirror (as shown in the lower diagram with figure 3). The Borneo earthquake waves lasted in England, as will be seen, two hours fifty-six minutes and fifteen seconds, with about forty minutes of what are known as preliminary tremors. Professor Milne removes the film from his seismograph once a week—a strip about twenty-six feet long—develops it, and studies the photographs for earthquake signs.

Seismogram of a Borneo Earthquake that Occurred September 20, 1897.

Besides this very sensitive photographic seismograph Professor Milne has a simpler machine, not covered up and without lamp or mirror. In this instrument a fine silver needle at the end of the boom makes a steady mark on a band of smoked paper, which is kept turning under it by means of clockwork. A glance at this smoked-paper record will tell instantly at any time of day or night whether the earth is behaving itself. If the white line on the dark paper shows disturbances, Professor Milne at once examines his more sensitive photographic record for the details.

It is difficult to realise how very sensitive these earthquake pendulums really are. They will indicate the very minutest changes in the earth's level—as slight as one inch in ten miles. A pair of these pendulums placed on two buildings at opposite sides of a city street would show that the buildings literally lean toward each other during the heavy traffic period of the day, dragged over from their level by the load of vehicles and people pressing down upon the pavement between them. The earth is so elastic that a comparatively small impetus will set it vibrating. Why, even two hills tip together when there is a heavy load of moisture in a valley between them. And then when the moisture evaporates in a hot sun they tip away from each other. These pendulums show that.

Nor are these the most extraordinary things which the pendulums will do. G. K. Gilbert, of the United States Geological Survey, argues that the whole region of the great lakes is being slowly tipped to the southwest, so that some day Chicago will sink and the water outlet of the great fresh-water seas will be up the Chicago River toward the Mississippi, instead of down the St. Lawrence. Of course this movement is as slow as time itself—thousands of years must elapse before it is hardly appreciable; and yet Professor Milne's instruments will show the changing balance—a marvel that is almost beyond belief. Strangely enough, sensitive as this special instrument is to distant disturbances, it does not swerve nor quiver for near-by shocks. Thus, the blasting of powder, the heavy rumbling of wagons, the firing of artillery has little or no effect in producing a movement of the boom. The vibrations are too short; it requires the long, heavy swells of the earth to make a record.

Professor Milne tells some odd stories of his early experiences with the earthquake measurer. At one time his films showed evidences of the most horrible earthquakes, and he was afraid for the moment that all Japan had been shaken to pieces and possibly engulfed by the sea. But investigation showed that a little grey spider had been up to pranks in the box. The spider wasn't particularly interested in earthquakes, but he took the greatest pleasure in the swinging of the boom, and soon began to join in the game himself. He would catch the end of the boom with his feelers and tug it over to one side as far as ever he could. Then he would anchor himself there and hold on like grim death until the boom slipped away. Then he would run after it, and tug it over to the other side, and hold it there until his strength failed again. And so he would keep on for an hour or two until quite exhausted, enjoying the fun immensely, and never dreaming that he was manufacturing wonderful seismograms to upset the scientific world, since they seemed to indicate shocking earthquake disasters in all directions.

Mr. Cleveland Moffett, to whom I am indebted for much of the information contained in this chapter, tells how the reporters for the London papers rush off to see Professor Milne every time there is news of a great earthquake, and how he usually corrects their information. In June, 1896, for instance, the little observatory was fairly besieged with these searchers for news.

"This earthquake happened on the 17th," said they, "and the whole eastern coast of Japan was overwhelmed with tidal waves, and 30,000 lives were lost."

"That last is probable," answered Professor Milne, "but the earthquake happened on the 15th, not the 17th;" and then he gave them the exact hour and minute when the shocks began and ended.