In this classic, Athelstan Spilhaus reports on his progress on a prototype of the mechanical bathythermograph, a project that C.-G. Rossby encouraged him to pursue. Although now most often seen in museums or on movie sets (they look a bit like rockets), bathythermographs were extremely influential from the later 1930s into the 1950s. These remarkable, durable instruments allowed continuous traces of temperature versus depth (previously measured as bottle temperatures at typically 50-m separations), and so opened up an entirely new vision of what ocean thermal structure actually looks like. Spilhaus further provides some actual data...