Page:McClure's Magazine volume 10.djvu/418

26 from this and other reasons, that the transmission is straight through the earth. Do you understand?"

"You mean that these waves come to us along the chord of the arc instead of along the arc itself," I ventured, recalling my geometry.

"Exactly, and now I come to the most important thing: we find that all these waves from distant earthquakes reach Shide in practically the same number of minutes, no matter where the earthquakes occur. They come from Japan in sixteen minutes, from South America in sixteen minutes, from Java in sixteen minutes, and so on as far as our data extend. When all the stations are working, we shall be able to verify this conclusion; but it certainly looks already as if the period of wave transmission through the earth was uniform."

"I don't see, Professor, if all these different earthquake waves get here in the same time, how you can tell one from the other, or know that this one started in South America and that one in South Africa, and so on?"

"I may say in a general way," he replied, "that we know them by their signatures, just as you know the handwriting of your friends; that is, an earthquake wave which has traveled 3,000 miles makes a different record in the instruments from one that has traveled 5,000 miles, and that again a different record from one that has traveled 7,000 miles, and so on. Each one writes its name in its own way, as you have seen on the bands. It's a fine thing, isn't it, to have the earth's crust harnessed up so that it is forced to mark down for us on paper a diagram of its own movements!"

"Are these differences in the wave signatures due to differences in the distance traveled?"

"Exactly. See here, I can make it plain to you in a moment."

He took pencil and paper again, and dashed off an earthquake wave like this:

"There you have the signature of an earthquake wave which has traveled only a short distance, say 3,000 kilometers; but here is the signature of the very same wave after traveling, say, 9,000 kilometers.

"You see the difference at a glance; the second seismogram (that is what we call these records) is very much more stretched out than the first, and a seismogram taken at 12,000 kilometers from the start would be more stretched out still. This is because the waves of transmission grow longer and longer, and slower and slower, the further they spread from the source of disturbance. In both figures, the point A, where the straight line begins to waver, marks the beginning of the earthquake; the rippling line AB shows the preliminary tremors which always precede the heavy shocks, marked C; and D shows the dying away of the earthquake in tremors similar to AB.

"Now it is chiefly in the preliminary tremors (we call them the P.T.'s) that the various earthquakes reveal their identity. The slower waves come, the longer it takes to record them, and the more stretched out they become in the seismograms. And by carefully noting these differences, especially those in time, we get our information. Suppose we have an earthquake in Japan. If you were there in person you would feel the preliminary tremors very fast, five or ten in a second, and their whole duration before the heavy shocks would not exceed ten or twenty seconds. But these preliminary tremors, transmitted to the Isle of Wight, would keep the pendulums swinging from thirty to thirty-two minutes before the heavy shocks, and each vibration would occupy five seconds.

"There would be similar differences in the duration of the heavy vibrations; in Japan they would come at the rate of about one a second, here at the rate of about one in twenty or forty seconds. It is the time, then, occupied by the preliminary tremors that tells us the distance of the earthquake. Earthquakes in Borneo, for instance, give P. T.'s occupying about forty-one minutes, in Japan about half an hour, in the earthquake region east of Newfoundland about eight minutes, in the disturbed region of the West Indies about nineteen or twenty minutes, and so on."

"Then, really, the information you get from the seismogram is simply that an earthquake has occurred somewhere at a certain distance from the instrument?"

"Yes; but that is quite sufficient to locate the earthquake with absolute precision,