Sentence:  The big factories which are relatively near the centers of our cities - the rubber factories in Akron , Chrysler 's Detroit plants , U. S. Steel 's Pittsburgh works - <WSD>often</WSD> began on these sites at a time when that was the edge of the city , yet close to transport ( river ) , storage ( piers ) and power ( river ) . 
Meaning of the word marked with <WSD> tags:  many times at short intervals