"For exploration and scientific work the artificial lights are a valuable adjunct, as they make it possible to photograph at any depth and at any time; but, there being so many other details to be taken care of in the taking of a scene under water, we try to do them all in the daytime. With as many as five divers operating in a scene, the divers wearing self-contained suits with no connection with the surface, having the tide and wind and the photographic apparatus and other things to be all worked at the same time, it is better to be working in the daylight, when you can keep your eye on the sharks and take care of the divers." The reproduction of under-sea photographs shown in this book will give the reader some intimation of the "wonders of the deep," but unfortunately the wonderful colors and the play of light and movement cannot be reproduced. Similarly, for scientific purposes as well as those of safeguarding navigation, submarines equipped for hydrographic work will prove of immense value. My work with submarine boats, both in the United States and foreign countries, has taught me that most charts are very unreliable, so far as their recorded depths are concerned. While they may be fairly accurate as to the average depths, they do not record many of the peaks or depressions that exist, especially where the water-bed is formed over a rock foundation. Silt and sand may fill in the depressions between peaks so that the average depth is fairly constant, yet here and there are outcropping peaks or humps that have, in many instances, proved fatal to shipping. The method of charting our coast lines and the estuaries of the sea has been by the use of the sounding lead, taken at points a greater or less distance apart. The depths recorded at these points are plotted by the triangulation method of location from tripods or known structures, or objects on shore, and shown on the chart. These points would need to be taken every few feet to give an accurate topography of the bottom, the cost of which, in time and money, would be prohibitive. Assuming that our coast waters were sounded and depths recorded, at points only fifty feet apart in all directions, even such close soundings would not guarantee that some peak might not project above the bottom and prove disastrous to some ship. I remember some few years ago the battleship _Missouri_ struck such a peak in New York Harbor, seriously injuring her bottom. Thousands of ships of equal draft had passed this vicinity, but none of them had happened to strike this particular spot and no one suspected that such a rock existed in this much-frequented highway. In 1900 the steamer _Rio de Janeiro_ struck an unknown rock in entering the harbor of San Francisco, with a loss of one hundred and thirty-one lives and over two million dollars in property.