If the air column in the tube is 24 inches long, the sounding may be read from any scale graduated for tubes of that length. If of a different length, a special scale must be prepared; its graduations, compared to those of the 24-inch scale, will be proportional to the comparative lengths of the two tubes.

If certain precautions are taken, these tubes will give results which compare favorably with commercial tubes. The paper should be inserted uniformly in the tube, and its upper end, or a mark from which the measurement is taken, should coincide with the top of the air column. Metal tubes have the advantage of uniform bore, but if metal tubes are used the paper, in order to insure uniformity, should be fastened at the upper end when that end is being sealed and then stretched lightly at the bottom. The depth should always be read from the dry portion of the paper, as the wet portion is subject to considerable change in length.

GENERAL INFORMATION-PORTO RICO AND VIRGIN ISLANDS.

The information contained in this volume relates to the coast and waters of Porto Rico and adjacent islands; and of the Virgin Islands. It also includes a description of Mona Passage, Vieques Sound, and Virgin Passage.

Climate. These islands lie entirely within the tropics and they have the usual notheast-trade type of climate. The heat is tempered by the sea breezes and they are generally healthy. The mean monthly temperature at San Juan varies from 75.0° to 81.3° F. in February and August, respectively. The islands are well watered though the streams are small and of no importance to navigation, All the higher hills are heavily wooded and the islands have a general tropical appearance.

Winds.-Porto Rico and the Virgin Islands lie within the limits of strong northeast trades. Except when disturbed by some atmospheric depression, near or remote, these winds blow with the greatest regularity during the entire year, varying in direction between NE. and SE., winds from north of east prevailing from November to April and from south of east from May to October. On and near the island the trade wind usually blows from about 9 a. m. until sunset, attaining its greatest force in the afternoon. During the night it is replaced by a land breeze, blowing outward in all directions from the island and which is more pronounced on the leeward side.

Occasionally, during the period from November to April, the regular trade is interrupted by a period of calms or light variable winds, sometimes persisting for several days, such interruptions being coincident with the passage of an extensive barometric depression to the northward. Such a period is apt to be followed along the northern coast of the island by a heavy sea from northward, and this, in turn, after an interval varying in different cases from a few hours to two days, by a norther, after which the regular trades are resumed, blowing with extra force.

From July to October is the hurricane season, during which there are sometimes heavy gales. See WEST INDIA HURRICANES.

Rainfall. The rainfall in Porto Rico and the Virgin Islands varies greatly in different localities. The greatest amount is found in the interior of Porto Rico, on the northern side of the mountains, in a belt extending from Port Yabucoa and Cape San Juan to Mayaguez and Point Jiguero. The fall diminishes toward the north coast, where the average rainfall is 55 to 75 inches per year. On the south side of the island, from Aguirre to Boqueron Bay, the rainfall is not only less in places than on the north coast but is more erratic, droughts being so frequent as to render irrigation necessary. January, February, and March are the months of minimum rainfall, while there are tw maxima, generally June and October. Most of the rain falls in showers, and days of continuous rain are rare.

Fog does not occur around the islands, and except for rain squalls, mists, and haze, there is no thick weather. The mountains on Porto Rico, however, are very often obscured by clouds.

Anchorages are numerous, except on the north coast of Porto Rico, the first requirement under ordinary conditions being shelter from

the easterly trade winds. Strong northerly winds or a heavy sea from northward may occur from November to April. During the hurricane season gales occur from any direction.

Hurricane harbors.-The best hurricane harbors are San Juan, Guanica, Guayanilla, Jobos, Great harbors, and St. Thomas. The hurricane season is from July to October.

System of buoyage.-In conformity with section 4678 of the Revised Statutes of the United States, the following order is observed in coloring and numbering buoys in United States waters, viz:

In approaching the channel, etc., from seaward, red buoys, with even numbers, will be found on the starboard side.

In approaching the channel, etc., from seaward, black buoys, with odd numbers, will be found on the port side.

Buoys painted with red and black horizontal stripes will be found on obstructions, with channel ways on either side of them, and may be left on either hand in passing in.

Buoys painted with white and black perpendicular stripes will be found in mid-channel, and must be passed close-to to avoid danger. All other distinguishing marks to buoys will be in addition to the foregoing, and may be employed to mark particular spots.

Perches, with balls, cages, etc., will, when placed on buoys, be at turning points, the color and number indicating on what side they shall be passed.

Nun buoys, properly colored and numbered, are usually placed on the starboard side, and can buoys on the port side of channels.

Day beacons (except such as are on the sides of channels, which will be colored like buoys) are constructed and distinguished with special reference to each locality, and particularly in regard to the background upon which they are projected.

Aids to navigation. The lighthouses and other aids to navigation are the principal guides, and mark the approach and channels to the important ports. The buoyage accords with the system adopted in United States waters. The principal coast lights are described in the text of this volume. For a complete description of all lighted aids, see the Light List, Atlantic and Gulf Coasts of the United States, published by the Lighthouse Service, which can be obtained from the Superintendent of Documents, Washington, D. C., price 20 cents, or from the agents listed in the weekly Notice to Mariners.

Pilotage is compulsory for certain vessels. There are pilots at the principal ports, who come off in small boats to vessels making signal outside the entrance. For pilot regulations and rates of pilotage see Appendix.

Towboats.-There is a seagoing towboat at San Juan. In some of the harbors there are large gasolene launches which sometimes assist lighters and vessels when inside.

Harbor control.-A captain of the port is appointed for each of the harbors of San Juan, Ponce, and Mayaguez, and they have charge of the anchorage and berthing of vessels in their respective harbors. The harbormaster has charge of the berthing of all vessels at St. Thomas. For harbor regulations see Appendix.

Supplies. Provisions, ice, lumber, and some ship chandler's stores. can be obtained at San Juan, Ponce, Mayaguez, and St. Thomas. Some provisions can be obtained at other places.

Water can be conveniently obtained at San Juan, Mayaguez, Ponce, Aguirre Central, and Ensenada, Guanica. Water can be obtained also from streams at many places, but it is more or less polluted and should be boiled before drinking.

Coal and fuel oil for vessels can be obtained at San Juan and St. Thomas.

Repairs. There are machine shops at San Juan and Ponce, and ordinary repairs to machinery can be made. There are also small machine shops at other points in Porto Rico. The nearest dock is at St. Thomas; this is a floating dock, length 250 feet, length of keel 300 feet, inside breadth 72 feet, greatest draft 21 feet, capacity 3,000 tons.

The following special signals for surveying vessels of the United States employed in hydrographic surveying have been prescribed:

A surveying vessel of the United States, under way or at anchor in a fairway and employed in hydrographic surveying, may carry where they can best be seen, but in any case well above the rigging lights prescribed by law for preventing collisions, three lights in a vertical line one over the other and not less than 6 feet apart. The highest and lowest of these lights shall be green, and the middle light shall be white, and they shall be of such a character as to be visible all around the horizon at a distance of at least 2 miles. In the case of a small vessel the distance between the lights of such private code may be reduced to 3 feet if necessary.

By day such surveying vessel may carry in a vertical line, not less than 6 feet apart, where they can best be seen, three shapes of not less than 2 feet in diameter, of which the highest and lowest shall be globular in shape and green in color, and the middle one diamond in shape and white.

Lighthouse tenders when working on buoys in channels or other frequented waters may display a red flag (international signal-code letter B) and a black ball at the fore as a warning to other vessels to slow down in passing.

The wire drags, some of which are over 2 miles long, used by the Coast and Geodetic Survey in sweeping for dangers to navigation, may be crossed by vessels without danger of fouling at any point except between the towing launches and the large boys near them, where the towline approaches the surface of the water. Steamers passing over the drag are requested not to pass close to the towing launch; also to change course so as to cross the drag approximately at right angles, as a diagonal course may cause the propeller to foul the supporting buoys and attached wires.

Storm warnings are displayed by the United States Weather Bureau on the coasts of the United States and the Great Lakes.

Small-craft warning.-A red pennant indicates that moderately strong winds are expected. No night display of small-craft warnings is made.

Storm warning.-A red flag with a black center indicates that a storm of marked violence is expected.

The pennant displayed with the flag indicates the direction of the wind-white, westerly; red, easterly. The pennant above the flag indicates that the wind is expected to blow from the northerly quadrants; below, from the southerly quadrants.

By night two red lights, one above the other, for winds beginning from the northeast; a single red light for winds beginning from the southeast; a red light above a white light for winds beginning from the southwest; and a white light above a red light for winds beginning from the northwest.

Hurricane warning.--Two red flags with black centers, displayed one above the other, or two red lights with a white light between displayed at night, indicate the expected approach of a tropical hurricane, or of one of those extremely severe and dangerous storms which occasionally move across the Great Lakes and northern Atlantic coast. These warnings are displayed at all stations on the Atlantic and Gulf coasts of the United States and on the following islands in the Atlantic: Jamaica, Santo Domingo, Turks Island, Bermuda, Haiti, Curacao, Porto Rico, St. Kitts, Dominica, Barbados, Trinidad, and Cuba.

The following are the hurricane warning display stations within the limits covered by this volume:

PORTO RICO.-Aguadilla, Arecibo, Arroyo, Canovanas, Culebra, Fajardo, Guanica, Humacao, Jobos, Mayaguez, Ponce (Playa Town), San Juan (Signal Tower Weather Bureau), Vieques.

VIRGIN ISLANDS.-St. Thomas, Fort Christian Signal Station, Cowell Pt.

WEST INDIA HURRICANES.

These are cyclonic storms with a center of lowest barometer, around which the wind blows in a more or less circular course (spirally) in a direction contrary to the hands of a watch. At the same time the storm field advances on a straight or curved track, sometimes with great velocity and sometimes not more than a few miles an hour, occasionally appearing to come to a pause in its onward movements. The estimated velocity in the West Indies is 15 to 20 miles per hour and on the Atlantic coast between Hatteras and the island of Cuba 5 to 15 miles per hour. They cover simultaneously an approximate circular area from 150 to 500 miles in diameter. At the center, the area of lowest barometer, which is from 10 to 20 miles in diameter, comparative calm prevails; the seas within this center are violent and confused and, combined with the sudden shifts of wind which are encountered as the vessel passes through the center, makes this the most dangerous part of the hurricane and the one to be avoided.

Hurricanes form eastward of the Windward Islands or in the Caribbean Sea and take a westerly or northwesterly course. Some curve gradually northward, passing north of the island of Cuba and northeasterly along and eastward of the Atlantic coast of the United States. Others pass over or southward of Cuba and enter the Gulf of Mexico, and while in the Gulf usually curve northward or northeastward so as to strike the coast somewhere between Tampa, Fla., and the Rio Grande. Tracks of hurricanes are shown on pilot charts of the North Atlantic Ocean, published monthly by the United States Hydrographic Office.

The months during which hurricanes are usually encountered are June to November, the months of their greatest frequency are August, September, and October. During these months mariners should be on the watch for indications of a hurricane and should frequently and carefully observe and record the barometer.

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