CHAPTER XVI. GENERAL INFORMATION. The publications of the United States Hydrographic Office for the use of mariners, comprise charts, sailing directions, and light lists, also special books and pamphlets, issued from time to time. They are subject to frequent correction, for information relative to changes in natural and artificial features received subsequent to the date of publication. The scheme of chart publication embraces three general classes of charts: 1. General or Sailing Charts, which cover a large area, and are, therefore, on a comparatively small scale. These are used for laying down routes and proceeding along them. They are for general cruising purposes. 2. Coast Charts, which cover less area than the general or sailing charts, and are commonly on a larger scale. They are used for coasting, and for making and leaving the land. 3. Harbor Charts are special charts of localities, and are intended for piloting, and for various other local purposes. For example: A vessel at sea and out of sight of land would use a general or sailing chart. On sighting land, the coast chart would be resorted to, and for entering harbor, the harbor chart would be used. Sailing directions give additional information, to supplement that contained on the charts, and in greater detail. Light lists also supplement the charts by giving fuller information relative to lights and fog signals. There are also buoy lists published, which give, according to the latest information, the position and character of buoys, beacons, and daymarks. These lists serve as checks upon the correctness of the charts. The effort of the Hydrographic Office is to issue charts which shall be correct up to the date of issue, so far as information at hand permits. In the use of all Hydrographic Office publications, the date of issue should be considered. Light Lists, published by the Hydrographic Office, which have been corrected for the latest information, are also issued. In cases where light lists published by other departments or nations are issued, the Hydrographic Office is not responsible for their correctness. Sailing Directions can not, from their nature, be kept fully corrected by the Hydrographic Office by the insertion of slips, etc. Their date of issue should always be carefully considered, and where they differ from charts of later issue, the chart should be taken as the guide. Notice to Mariners. In order that charts, sailing directions, etc., may be corrected for information received at the Hydrographic Office subsequent to the date of issue, weekly publications, styled "Notices to Mariners," are issued by the Hydrographic Office to the public free of charge. The Notices to Mariners contain brief itemized statements of special information received. The items are in such shape that they may be cut from the Notices to Mariners and pasted in books, on charts, and in the Light Lists. The Notices to Mariners also mention the publications that are affected by the several items of information. A list of charts issued and cancelled by the Hydrographic Office, and of the books published, is given in the first notice of each month. It is seen that this scheme of supply and information assumes that navigators will keep themselves supplied with Notices to Mariners, Supplements, Light Lists, etc., in order that they may keep publications already in their possession corrected for the latest information. Notices to Mariners are supplied from the main Hydrographic Office, in the Navy Department, at Washington, D. C., or from any of the Branch Hydrographic Offices, of which there are twelve. The Branch Hydrographic Offices upon the Great Lakes are in the Masonic Temple at Chicago, and in the Arcade Building at Cleveland. For the correction of Sailing Directions, supplements to the several volumes are published from time to time. THE USE OF CHARTS. never be greater than the A correct chart can not be It is obvious that the value of a chart can value of the survey from which it is made. made from an incorrect survey. Given an accurate survey, the appearance or character of a chart may be varied to suit the taste or need of those interested in its use. For mariners, all needless detail is commonly discarded in order that the information contained upon the chart may be impressed quickly upon the mind. Although the charts of the Great Lakes, as a rule, proceed from painstaking surveys, this is not true of all charts, especially certain charts of foreign coasts. To the experienced eye, the appearance of a chart affords a good basis of judgment as to its trustworthiness. In scanning a chart to judge of its value, certain points should be noticed, viz: the date of the survey and by whorn the survey was made; the date of issue of the chart; the date at which it has received general correction, and the date at which it has received special corrections. As a rule it may be said that recent surveys are more accurate than those made many years ago. In many places the character of the bottom undergoes change, making resurveys necessary. For such places an old chart THE USE OF CHARTS. 109 should be accepted with caution. Where charted soundings are distributed evenly over a certain area, but are well opened out one from another, it does not mean necessarily that the soundings shown are all that were taken during the survey. The Hydrographic Office engraves upon its charts only characteristic soundings, and discards for the purpose of chart making soundings which are superfluous. If, on a chart intended to show details, there are no soundings placed over certain areas where they would ordinarily be expected to appear, it may mean that no survey has been made of the vacant areas. It is often the case, however, that soundings are not shown beyond a certain depth of water. Even in the best surveys a detached pinnacle of rock or other submerged danger may not be discovered. This applies especially to rocky coasts and to the vicinity of outlying rocks. As a rule, therefore, rocky shores and patches should be given a reasonable berth. In using a chart, the notes printed upon it should be carefully read. It may be that charts published by different offices are based upon different plans. For example, one may refer to magnetic courses, and the other to true courses; one may give the bearings of an object on shore as taken from the vessel, and the other the bearing of the vessel from the object on shore. In respect to seaboard charts, one may give depths for mean low water, while the soundings on the other may refer to low water of spring tides, etc. It should also be noted whether the soundings indicate fathoms or feet. In some cases fathoms only are used; in others, only feet; while in other cases feet are designated to depths of three (3) fathonis, beyond which fathoms are shown. In the last case the water areas in depths less than three (3) fathoms or eighteen (18) feet, are dotted or "sanded." DISTORTION OF CHARTS. Charts printed from copperplates are subject to distortion. They are printed necessarily on dampened paper. By reason of the dampness the paper has expanded, especially in one direction; that is to say, in the direction in which the fiber of the paper runs. The dimensions of the printed chart coincide with those of the plate immediately after leaving the press, but, in drying, the paper contracts to its original dimensions, thus distorting the printed matter. Different Different degrees of dampness produce different degrees of distortion. It may be, therefore, that charts printed from the same chart plate at different times and under different conditions of dampness, will not coincide in all their parts if one is superimposed on the other. When this distortion takes place the compass and graduated scales on the chart are distorted in the same ratio as other matter, hence, for the purposes of navigation, no harm is likely to result from the use of plate-printed charts. As a rule, a chart of the largest scale available should be used for coasting and piloting. For pursuing an extended route with plenty of sea room, there is an advantage in using a chart covering greater area on which both the port of destination and of departure are shown. It should be remembered, however, that in laying down a position from compass bearings, a small error of observation, when plotted, is likely to result in less displacement of position on a chart of a large scale than on one of a small scale. In one case it may reach only yards, and in the other a considerable fraction of a mile. In laying off compass bearings on the chart for fixing a position, bearings on near objects should be used in preference to those on remote objects, because an error in observing the bearing of an object by compass, would have greater effect if continued on the chart through a long distance than through a short distance. Light Lists should always be referred to, as well as the chart, in order to obtain full details of lights. Buoy Lists of the latest issue should also be used to note, when visiting a strange port, if the buoys have been plotted upon the chart according to the latest position assigned to them. In using a Light List it should be remembered, in respect to the range of visibility of a light, as given therein, that it is true only for a certain height of the eye above the water. This is commonly about fifteen (15) feet. If the eye is higher, the light should be seen farther in clear weather, and if the eye is lower, the reverse should be the case. Commonly, the meaning is, that with the eye fifteen (15) feet above water, the light will dip below the horizon when the observer is a greater distance from the light than that of the tabulated range of visibility. Sometimes, however, a light may be so high above the water that the rule as to the range of visibility would give a range beyond the carrying power of the light itself. In such cases the range of visibility is oftentimes assigned on the basis of the power of the light. A good idea of the power of the light may be formed from noting its order as given in the Light List. FOG SIGNALS. In respect to fog signals, it is almost impossible to lay down any rule as to the range of audibility. The intensity of the sound and its apparent direction are very much influenced by the wind, and to such an extent that the signal may be heard from a remote distance and yet be inaudible or very faint when nearer. Mariners should beware of placing implicit confidence in fog signals. VESSEL'S POSITION. Good nautical practice requires that the master of a vessel shall know continuously the position of his vessel. In well-known channels or lanes of travel, simple visual observation may give him this information close enough for practical purposes; otherwise, he must resort to the use of his nautical charts and instruments. It is especially important that a |