Nowadays the Admiralty have a large and well-fitted tank at Portsmouth, the United States Navy have one at Washington, private shipbuilders have the use of a national tank at Bushey, near London, while several of the large firms have tanks of their own.
The national tank at Bushey, by the way, was given to the nation by Mr. Yarrow, a famous shipbuilder, in memory of Mr. Froude, it being called the "William Froude Tank" in recognition of the great work done by him.
Now these tanks may be described as rather elongated swimming-baths. Such a structure is generally a little narrower than the average bath, but it is longer and much deeper.
At one end there are miniature docks in which the models float when not in use, while at the other there is a sloping beach upon which the waves caused by the models expend their energy harmlessly.
Along each side there runs a rail upon which are supported the ends of a travelling bridge. Driven by electric motors, this bridge can run to and fro from end to end of the tank, and its purpose is to drag the models through the water.
Carried upon the bridge is a platform which bears a number of instruments, chief among which is a self-recording dynamometer.
Now a dynamometer is an instrument for measuring the force of a "pull," and when we call it self-recording we mean that it automatically takes a record of a series of pulls or of a varying pull. In this case there projects below the bridge a lever, to
the end of which the model under test is attached. As the bridge rushes along it pulls the model through the water by means of this lever, and the force which is expended in doing so is recorded in the form of a wavy line upon a sheet of ruled paper.
If the model slips through the water very easily there is little pull upon the lever and the line drawn by the pen of the instrument remains low down upon the chart. If, however, much power is needed and the pull is a strong one the pen moves and the line rises towards the top of the paper. Any change, whether increase or decrease, is thus shown by the rise or fall of the ink line.
One model can be thus tried at various speeds and its behaviour noted under different conditions. Other matters can be investigated too, such as whether or not the bow rises in the water or falls when the boat is in motion, also how much such rise or fall may amount to.