DOCUMENTARY HISTORY OF THE FLORIDA CANAL 169

 22. It may be remarked here that the Cape Cod Canal is crossed by only two
highway bridges, each with a vertical clearance of 135 feet, and one railway
lift bridge, which is habitually in an open position high above the water. It is
lowered on the approach of a train. This mode of operation is the reverse of
the customary arrangement whereby the bridges are usually closed and are
opened only upon the approach of a vessel. It may be observed that a train
or motor truck approaching a bridge can apply brakes and come to a full stop,
whereas a ship is moving in a fluid which is itself highly mobile. All that is
possible is for a ship to reverse its propellors and this causes a certain move-
ment of the water forward with the ship. Moreover, this ship is not on a fixed
track and must maintain steerage way to prevent sheering and going aground.
All of these factors relating to transportation by land and water must have
special consideration, particularly when waterborne commerce is heavy as in
congested harbors or waterways.
 TOLLS

 23. The Kiel Canal Is operated as a toll canal. In 1931 the revenues were
S6,137,000 marks and the average charge was 0.34 mark per net registered ton.
At the average rate of exchange, this toll was equal to 8.0843 cents per net
registered ton.
 24. At one time a corporation was formed by act of State legislature with
 a view to constructing and operating a Florida ship canal Application was
 made to the Reconstruction Finance Corporation for a loan and it was neces-
 sary to consider the canal as a self-liquidating project. Under these circum-
 stances, it could only be liquidated by assessing tolls and this idea, long since
 abandoned, is apparently the origin of the-comment that the project cannot be
 liquidated on a toll basis. There is, however, good reason for stating that on
 the very low basis of 8-cent toll per ton, the revenues would serve to maintain
 and operate the canal and amortize the principal in 80 years, or less-particu-
 larly if interest be computed at the low rates at which the Federal Government
 has been able to obtain funds. Moreover, the benefits to accrue to shipping
 would be such as to Justify a toll considerably in excess of 8 cents per ton.
 On petroleum products, the savings, conservatively figured, would exceed 14
 cents per long ton of cargo and would exceed 19 cents per ton of ordinary
 dry cargo as carried on freighters and combination freight and passenger ves-
 sels. However, since there are more than a dosen kinds of tons, it is im-
 portant to observe a proper distinction between a ton of cargo which could be
 2,000 or 2,240 pounds and a net registered ton which has 100 cubic feet of
 space. Tolls are usually based on the net registered ton but the Suez and
 Panama Canals have their own rules for measuring the tonnage of vessels.
 25. However, with reference to tolls, it should be noted that in 1796 Con-
 gress passed a resolution declaring navigable waters of the United States to
 be public highways forever free. This resolution was reenacted in somewhat
 different language in 1884 and is a fixed policy of the Federal Government.
 Many millions of dollars have been spent by the Government in creating arti-
 ficial channels in B'scayne Bay for the benefit of Miami, in improving natural
 channels in Tampa Bay for the benefit of Tampa and other ports and in im-
 proving St. Johns River Channel for the benefit of Jacksonville. The Govern-
 ment has spent many other millions for numerous other seaports and for the
 improvement of the Mississippi, Ohio, and other rivers, as well as for artificial
 inland waterways, such as the Atlantic intracoastal and the Gulf canals. It
 has never sought to levy tolls or port dues on vessels using these improvements.
 The expenditure of Federal funds has been predicated on general benefits to
 commerce, including such intangibles as "Increased safety and convenience in
 navigation." (See Reports on Miami Harbor, H. Doc. 15, 71st Cong., 2d sees.
 and on Tampa Bay, S. Doc. 22, 72d Cong., 1st sess. and numerous other Federal
 project documents.)
 26. The official reports on the Florida ship canal contain extended discussion
 of the tolls that might be collected and the extent to which they would Justify
 the project if tolls should be levied. The reports also contain extended discus-
 sions of the general intangible benefits which would justify the project in the
 interests of the commerce of the United States. The reports on the canal fully
 justify its construction as a Federal project in the same sense that they fully
 justify the extensive improvements now being made in Biscayne Bay, in Tampa
 Bay, and those projected for the harbor of Jacksonville and many other cities.
 27. Congress has power to levy tolls despite the policy heretofore established.
 It might decide to levy tolls on foreign vessels and not levy tolls on American-