Sentence:  The most infamous of all was launched by the explosion of the island of Krakatoa in 1883 ; it raced across the Pacific at 300 miles an <WSD>hour</WSD> , devastated the coasts of Java and Sumatra with waves 100 to 130 feet high , and pounded the shore as far away as San Francisco . 
Meaning of the word marked with <WSD> tags:  a period of time equal to 1/24th of a day