Page:McClure's Magazine volume 10.djvu/415

Rh than a telescope would be at the theatre. The seismographs used all over Japan record earthquakes that can be felt; the Milne horizontal pendulums record earthquake waves that cannot be felt. After years given to the practical side of seismology, Professor Milne is now studying its theoretical side, although, as has been seen, much practical good is resulting from his investigations.

My first view of the instruments was at night. Professor Milne walked beside me, carrying a lantern, and his Japanese assistant, Shinobo Hirota, who is nicknamed "Snow" on the Isle of Wight, went ahead to open the doors of the strong-walled little houses where the pendulums were guarded. There are two of these pendulums, both constructed on the same principle, but the one more sensitive than the other. "Snow" showed us the sensitive one first; and when I saw it, I saw only a little lamp burning on a red box with steps to it. The box covered the pendulum. The whole place suggested some silent altar with undying flame. I could hear a clock ticking inside the box.

"What is the lamp for?" I asked.

"To photograph the end of the boom," said the Professor. "It lets a point of light down through that slit. When the earth moves, the boom swings."

"Oh," said I. "And what is the clock for?"

"The clock works the machinery. I'll explain it in the morning, and show you how 'Snow' develops the seismograms."

"Snow" looked pleased, and led the way to the other little house. Here we found a pendulum that was not covered up. It rested on a heavy column of masonry, and one end of it pressed a tiny silver needle against a vertical band of smoked paper that moved slowly between two rollers. There was a clock ticking here also, but no little lamp.

"This," said the Professor, "is an every-day pendulum, to let us know if anything is happening. If there is, then we look at the other pendulum for fuller details. The other one is not so easy to get at. Just glance along that paper band and you can see if there has been an earthquake anywhere in the last twenty-four hours. No, there has been nothing; the line is straight; see—that long white line—the needle makes it as the band turns."

"Suppose there had been an earthquake?"

"I'll show you what would have happened. Come around here; that's right. Now press against the column, not hard, just with your hand. There it goes. See?"

It was like pressing against a chimney, but the boom of the pendulum responded instantly, and the needle swerved out on the paper and then back again, marking a narrow loop.

"You tipped the column and altered its level just as an earthquake wave from Japan or Borneo would have done. That is the whole purpose of these instruments, to indicate slight changes of level. They are sensitive to a difference in level of one inch in ten miles. That's not a very steep grade, is it?"

And then he went on to tell how a pair of these pendulums, placed on two buildings at opposite sides of a city thoroughfare, 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.

"All these tons of weight make the earth's surface contract between the two rows of buildings, and that tips them together just as you tipped this column. You see the earth is so elastic that a comparatively small impetus will set it vibrating. Why, even two