sentences:  Eckart contributed to geophysics by linking theoretical hydrodynamic exercises to actual physical properties of water.In the following decades he did research on thermal layering in the ocean and atmospheres on which he wrote a book, the transmission of sound in the sea, turbulence, air-sea interactions, the generation and structure of surface, and internal ocean waves.After WW II, Eckart collected his work and the work of others on underwater detection and published it a classified volume entitled "Principles and Applications of Underwater Sound", which was first published in 1946.