The lack of knowledge of some of those who in years past criticized the Gatun Dam was illustrated by an amusing incident that occurred at a senatorial hearing on the Isthmus. Philander C. Knox, afterwards Secretary of State, was then a Senator and a member of the committee which went to the Isthmus. Another Senator in the party had grave doubts about the stability of Gatun Dam, and asked Colonel Goethals to explain how a dam could hold in check such an immense body of water. Colonel Goethals, in his usual lucid way, explained that it was because of that well-known principle of physics that the outward pressure of water is determined by its depth and not by its volume--that a column of water 10 feet high and a foot thick would have just as much outward pressure as a lake 200 square miles in extent and 10 feet deep. Still unconvinced, the Senator pressed his examination further. At this juncture Senator Knox, who is a past master at the art of answering a question with a question, interposed, and asked his colleague: "Senator, if your theory holds good, how is it that the dikes of Holland hold in check the Atlantic Ocean?" CHAPTER IV THE LOCKS Ships that pass Panama way will climb up and down a titanic marine stairway, three steps up into Gatun Lake and three steps down again. These steps are the 12 huge locks in which will center the operating features of the Isthmian waterway. The building of these locks represents the greatest use of concrete ever undertaken. The amount used would be sufficient to build of concrete a row of six-room houses, reaching from New York to Norfolk, via Philadelphia, Baltimore, Washington and Richmond--houses enough to provide homes for a population as large as that of Indianapolis. The total length of the locks and their accessories, including the guide walls, approximates 2 miles. The length of the six locks through which a ship passes on its voyage from one ocean to the other is a little less than 7,000 feet.