Tesla's hammer

Great excerpt from Nicholas Tesla biography "Wizard, The life and Times of Nikolas Tesla" by Marc J. Seifer:

"...He decided to experiment.
With Georff Scherff present, Tesla placed one of his mechanical oscillators where his laboratory was located and adjusted the frequency to the point where the beam begun to hum. "While he was attending to something else for a few moments, it attained such crescendo of rhythm that it started to shake the building, then it began shaking the earth near about [and other buildings with support beams in resonant frequencies]... The Fire Department respond to an alarm frantically turned in; four tons if machinery flew across the basement and the only thing which saved the building from utter collapse was the quick action of Dr. Tesla in seizing a hammer and destroying his machine."
"The device could be a Frankenstein's monster," Tesla confided many years later. "If not watched, no substance can withstand the steadily applied rhythm when its resonance point is reached. Skyscrapers could easily be destroyed with the steady building up of resonance from the timed strokes of a five pound hammer."
In another rendition of the story, told at another time, Tesla claimed that he had taken his alarm clock-sized oscillator to a building site "in the Wall Street district." Finding one under construction, about ten stories high of steel framework..." he clamped the vibrator to one of the beams and fussed with the adjustment until he got it.
"In a few minutes I could feel the beam trembling," Tesla told a reporter Gradually the trembling increased in intensity and extended throughout the whole great mass of steel. Finally, the structure began to creak and weave, and the steel-workers cam to the ground panic-stricken, believing there had been an earthquake. Rumors spread that the building was about to fall, and the police reserves were called out. Before anything serious happened, I took off the vibrator, put in my pocket and went away. And, with the same vibrator, I could drop the Brooklyn Bridge into the East River in less than an hour"
Tesla told the report that he could split the earth in the same way, putting an end to mankind.
"The vibrations of the earth,", he said, "have a periodicity of approximately one hour and forty-nine minutes. That is to say, if I strike the earth this instant, a wave of contraction goes through it that will come back in one hour and forty-nine minute in the form of an expansion. As a matter of fact, the earth, like everything else, is in a constant state of vibration. It is constantly contracting and expanding.
"Now suppose that at the precise moment when it begins to contract, I explode a ton of dynamite, That accelerates the contraction, and in one hour and forty-nine minutes, there comes an equally accelerated wave of expansion. When the wave of expansion ebbs, suppose I explode another ton... and suppose this performance be repeated time after time. Is there any doubt as to what would happen? There is no doubt in my mind. The earth would split in two. For the first time in man's history, he has the knowledge with which he may interfere with cosmic processes."