Page:McClure's Magazine volume 10.djvu/417

Rh As for the instruments, I saw that they are simple enough in principle, though most admirable in perfection of adjustment and delicacy of working. Beautiful devices they are, to do for our sense of level, if I may so express it, what the microscope does for our eyesight. A horizontal pendulum, or boom, poised against a knife edge at the base of a mast, that is the essential feature. A wire stay from the masthead supports the far end of the boom, and a weight hung from it keeps everything taut.

Then two backscrews allow either leg of the supporting tripod to be raised or lowered by the thickness of a spider's web, and even so small a change of level as that disturbs the end of the boom. And that makes the point of light move on the band of paper, and that movement is photographed, so that the record shows a slight loop. As nothing is allowed to disturb the boom, once the pendulum is adjusted, it follows that if the record band shows loops and curves instead of a straight line, it is because the earth's surface has moved underneath the supporting column and changed its level. As a matter of fact, the earth's surface moves very frequently with tremors like a creature of life, and with long heavings caused by distant seismic disturbances. And for each of these movements the pendulums give an individual record with characteristic waverings and loops on the band, and queer ups and downs that mean nothing to the inexperienced eye, but everything to the seismologist. When "Snow" brings in news of something on the band, there is excitement in that quiet house at Shide as among waiting tiger hunters at a crashing in the jungle.

In each of these records the time is marked in hours along the edge of the band, this being done automatically by hourly passage of the long hand of the watch over the slit in the red box, that shuts off the light for an instant and makes a line on the photographic film.

When a man finds himself in the midst of such an unfamiliar subject as earthquake shocks that cannot be felt, he naturally asks questions, and I asked a great many during my stay at Shide. For instance:

"Does the ground really move, Professor, when these waves come from the other side of the earth?"

"Undoubtedly; it rises and falls just as the ocean does. You see, the earth's crust is very elastic; it is constantly quivering and pulsatory, I might almost say breathing."

"How much does the ground rise and fall with one of these waves?"

"Oh, about three inches."

"What! the solid earth comes up three inches right under us and then goes down three inches?"

"Certainly, it does that very frequently."

"But why don't we see it or feel it?"

"Because it moves so slowly and evenly; fifteen seconds, perhaps, for the lift, and as many more for the descent. And then the waves are so long—several miles between two crests—that everything about us rises and falls together; half of all London heaves up and settles down with a single breathing.

"And how long does it take these waves to travel around the earth, say from Japan?"

"They don't travel around the earth—they travel through the earth; that is one of the most important discoveries we have made. If they were transmitted in the earth's crust around the circumference we should get two records for every earthquake—one coming the shortest way round, the other coming the longest way; for, of course, these wave-movements would be propagated in both directions. Waves through the air, for instance, from volcanic explosions, always come to us both ways around the earth, the one being recorded after the other. Do you see that?"

"Yes."

"Well, we never get two records of earthquakes, we only get one; so we conclude