Most of the material used in the preparation of the charts above described is obtained from a great corps of volunteer marine observers, who enter their observations at stated hours in forms provided for the purpose and send these records to the establishment in charge of the work at the end of each voyage. The forms furnished by the United States Weather Bureau prescribe only one regular observation a day, to be taken at Greenwich mean noon. Each observation shows the position of the ship, the direction of the wind, the force of the wind on the Beaufort scale, the height of the barometer, the readings of the dry-bulb and wet-bulb thermometers, the temperature of the water at the surface, the state of the weather, and the kind, amount, and movement of clouds. In order to check the accuracy of the barometric readings, the observer is instructed to read his barometer at prescribed hours on three successive days when in port and send the readings to the Weather Bureau. On receipt of these readings the Bureau compares them with those of the nearest meteorological station, and then mails the observer a “barometer tag,” showing the results of the comparison and the error of his instrument. Besides keeping up these routine observations, the observer keeps a record of fog encountered at any hour of the day and makes detailed reports on storms. Many marine observers also report observations at stated hours by wireless telegraphy. The enormous fund of information thus collected over the ocean is applied to several purposes besides the construction of Pilot Charts. Our Weather Bureau and certain foreign meteorological institutions prepare daily charts, showing approximately the instantaneous conditions over great oceanic areas, especially the North Atlantic. These maps are analogous to the daily weather maps published for land areas, but the drawing of each map is, necessarily, delayed for several months after the date to which it refers, in order to allow time for the receipt of as many reports as possible from ships at sea. As a rule such charts are prepared in manuscript only, but, though they cannot be distributed after the manner of ordinary weather maps, they are valuable for studies in the institution itself on the movements of storms and other atmospheric processes. They also enable the meteorological officials to answer inquiries concerning the winds and weather that have prevailed over a particular part of the ocean on any specified date. Such inquiries come from vessel owners, underwriters, and others, and the replies are frequently used as evidence in admiralty suits.