Sir John Murray, considered to be the father of modern oceanography, discusses the variations in the depths of oceans, and the methods and instruments used to measure such depths. He discusses many properties of oceanic waters including salinity, compressibility, pressure, colour, viscosity, and the presence of dissolved gases. The author addresses changes in density of water and its impact on the motion of water while describing ocean currents. He gives details of a variety of marine organisms based on the ocean depths at which they are found. Finally, he talks about the types of sediments and deposits obtained from the floor of the ocean, from different depths and latitudes. - Summary by Divya Sharma