Setting aside these general considerations, let us return to the question of the disaster at San Francisco on that fatal morning of April 18th. The shock did not come unexpectedly. A month previous there had been a severe earthquake in the Island of Formosa, and many lives were lost there, while an enormous amount of damage was done. Only a few days before the event in San Francisco there was another earthquake in the same island. Still greater havoc was caused by it than by the earthquake in March, but fewer lives were lost, the reason being that the people were warned in time. Early in April the eruption of Mount Vesuvius reached its height and devastated the country around the volcano, covering an enormous territory with ashes, and caused the loss of hundreds of lives.
On Tuesday night, April 17th, word was received from Piatigorsk, Circassia, that there had been two severe earthquake shocks the previous day in Northern Caucasia. The same night a telegram from Madrid said that the newspapers there reported that the long-dormant volcano on Palma, the largest of the Canary Islands, was showing signs of eruption, columns of smoke issuing from the crater.
While scientists as a rule doubt that there was any connection between these volcanic phenomena and the earthquake at San Francisco, yet reports from the Mount Weather observation station in Virginia, a few miles from Washington, show that the eruptions of Vesuvius acted on the magnetic instruments by electro-magnetic waves in such a way as to disturb the electrical potentials at that place. Be this as it may, there is one remarkable circumstance in regard to all this activity. All the places mentioned—Formosa, Southern Italy, Caucasia, and the Canary Islands—lie within a belt bounded by lines a little north of the fortieth parallel and a little south of the thirtieth parallel. San Francisco is just south of the fortieth parallel, while Naples is just north of it. The latitude of Calabria, where the terrible earthquakes occurred in 1905, is the same as that of the territory affected by the recent earthquake in the United States. This may or may not have some bearing on the question.
Whatever be thought of all this, one thing is certain, the earthquake which laid San Francisco in ruins was felt the world over, wherever there were instruments in position to detect and record it. The seismograph in the government observatory at Washington showed that the first wave, on April 18th, came at 8.19—equivalent to 5.19 at San Francisco; that at 8.25 there was a stronger wave motion, and that from 8.32 to 8.35 the recording pen was carried off the paper. The vibrations did not entirely cease until 12.35 P. M., during this period there having been nearly half an inch of to and fro motion in the surface of the earth.