But as early as 1803, and ingenious Maryland farmer, Thomas Moore, had been on the right track. He owned a farm about twenty miles outside the city of Washington, for which the village of Georgetown was the market center. When he used an icebox of his own design to transport his butter to market, he found that customers would pass up the rapidly melting stuff in the tubs of his competitors to pay a premium price for his butter, still fresh and hard in neat, one-pound bricks. One advantage of his icebox, Moore explained, was that farmers would no longer have to travel to market at night in order to keep their produce cool.

　　47. What is the topic of the passage?

　　48. Where was ice used after the Civil War?

　　49. What was essential to a science of refrigeration according to the passage?

　　50. It can be inferred from the passage that the theoretical foundation of ice box should be that ________.

　　51. Without an ice box, farmers had to go to the market at night because ________.