Seaplane tank testing is an offshoot of ship tank testing, the technique of which was founded by William Froude nearly 70 years ago and is now highly developed. Apart from the fact that a scale model is towed through still water, however, there is little in common between the two systems of test, though one link remains in spite of dissentient voices; Froude's laws are used as the basis of correlation between model and full scale. In other respects, the conditions of operation of a ship and a seaplane on the water are so different that when the R.A.E. tank was built, the whole existing technique was reviewed.