 By the early 1950s, Stone & Webster had built 27 separate hydroelectric plants constituting 5 percent of U.S. capacity, steam power plants aggregating six million kilowatts in capacity, and some 6,000 miles of power transmission lines.During this time, the company also obtained a variety of chemical process contracts in the United States, Canada, Japan and other countries to meet the worldwide demand for plastics.As the 1960s drew on, however, Stone & Webster's petrochemical and plastics activity began to slow as U.S. refinery capacity caught up with customer demand and declined accordingly.