[Illustration: FIG. 81--Sand bars at Cape Charles, Va. South of the town at the right is the harbor, into which a tug-boat is towing a barge. West of the town (left) is a belt of light-colored beach sand and sand bars ending at the south against the breakwater, north of the harbor. The sand bars appear somewhat dim because they were photographed through water. They are under water 1 to 6 feet deep at low tide, or 4 to 9 feet at high tide. Scale, about 1:6,000.] “aerial photo-hydrography.”[12] On the other hand, Volmat reports the successful use of air photography for similar purposes on the French coast, where photographs of objects down to a depth of 17 meters (about 56 feet) were found useful in several ways--among others, the discovery of points of rock which had [Illustration: FIG. 82--Bars, channels, beaches, and marsh near Far Rockaway, Long Island, N. Y., as photographed from a height of 7,000 feet at 11 A.M., September 15, 1920. In order from the bottom of the picture upward are: East Rockaway Inlet with a shoal to the left and the sand hook at the end of Long Beach to the right of it; the beach south of Far Rockaway with streets and houses; a group of boats in the inlet at the right of the beach; an area of salt marsh that is filling the lagoon behind the barrier beach; and a small section of the village of Far Rockaway. Scale, about 1:12,000.] escaped attention during very detailed surveys. He states that with proper plates and ray filters the presence of objects invisible to the eye is revealed by the camera.[13] Similar use of air photographs has been made by the English in charting reefs, shallows, and harbors. Thomas says: “In 1917 aeroplane photography was successfully used for charting the harbor of Rahbeg on the Arabian coast.”[14] It is a well-known fact that, under proper conditions, objects submerged to a considerable depth under clear water can be seen from points high above the surface. During the war, submarines were detected and followed by observers in airplanes, and sunken vessels, mines, and other submerged objects have been located by observation from the air. Illustrations in this paper show the possibility of using this method of observation, to some extent at least, in detecting and mapping shoals, channels, and other features under water.