the relative position of stars most likely to be used for observations in order that no mistakes are made, or uncertainty felt, as to what star is observed. The same applies to the identification of planets used for navigational work.

To facilitate identification of stars, the Hydrographic Office publishes tables known as Star Identification Tables. In these tables are given simultaneous values of the declination and hour angle corresponding to the values of the latitude, altitude, and azimuth ranging from 0° to 88° in latitude and altitude, and from 0° to 180° in azimuth.

Besides these tables, there are various graphic methods of identifying stars. Those proposed by Sigsbee, Rust, Littlehales, and De Aquino are among the best. Nautical supply houses acting as agencies for the sale of Hydrographic Office publications are usually able to furnish descriptions of these methods, with the exception of that by Rust, which is published by the United States Naval Institute, Annapolis, Maryland.

42. In the supplement of the American Nautical Almanac, published by the Nautical Almanac Office, a very useful table is included giving the mean time of transit of not less than 55 stars for every day throughout the year. In selecting stars in transit for latitude determinations, this table will be found very convenient.

43. Importance of Dependable Chronometers.-As the element of time is very important in most methods of determining the position of a ship, particular care should be given to the chronometer and its performance. Its error and daily rate should be frequently noted, especially just before leaving port, and care should be taken to maintain a uniform rate during the trip by means of systematic winding and by preventing the chronometer from being exposed to marked changes in temperature.

44. Every important seaport of the world now has established time signals, displayed at certain defined instants. If time balls are not in use, signals are received by telegraph by which the chronometer may be rated. For the convenience

of vessels under way, time signals are now sent out from government radio stations and vessels equipped with receiving instruments are thus enabled to verify their chronometer error. From the naval radio stations at Arlington, Key West, and New Orleans, time signals are sent daily at noon and also at 10 P. M., 75th meridian time. On the west coast, radio time signals are sent daily except Sundays and holidays by the naval radio station at North Head, Eureka, Point Arquello, and San Diego at noon and at 10 P. M., 120th meridian time. For further particulars about time signals sent by radio, navigating officers of trans-oceanic vessels should apply to the Hydrographic Office, Washington, District of Columbia.

45. Discrepancies in Sights.-The question is often asked, how close may a fix be to the ship's actual position? In other words, what is the average error in the position of a ship as determined by observations of celestial bodies? In answering this question, it must be borne in mind that with the most painstaking work in obtaining the data needed the result will be approximate only. The imperfections of the sextant, a slight inaccuracy in measuring the altitude or in recording the Greenwich mean time, excessive refraction, and errors in estimating the distances run between sights contribute to render the result approximate. What then is the probable limit of dependence? Under favorable conditions and with careful work on the part of the observer and his assistant in noting the chronometer, the ship may be assumed to be within 2 miles of the position calculated; or, the position of the ship may be anywhere within a circle formed by the fix as a center and a radius of 2 miles.

46. In case the fix is determined by the intersection of two position lines, the area within which the ship is located is bounded by four auxiliary lines drawn, respectively, 1 mile on either side of the position lines establishing the fix. The parallelogram thus formed represents the limit of uncertainty. in the ship's position and in shaping his course the navigator will give due allowance for this probability of error in his fix.

However, in actual practice this refinement is applied only when approaching a coast or in passing close to points considered difficult or fraught with an element of risk. In the open sea and with plenty of sea room, the fix obtained from good sights is usually marked on the chart as the actual position of the ship.

47. Refinement in the Working of Sights.-The use of seconds in working out sights is often a matter of divided opinion among experienced navigators. Those advocating the elimination of seconds in picking out logarithms claim that inasmuch as the ship's position even with good sights may be considered in error 1 or more miles, as explained, the use of seconds in working out sights is immaterial and useless. If the errors due to the elimination of seconds had the effect to offset errors due to instrumental and other imperfections, such a contention would be well founded. But as they may have an opposite effect and thus increase the errors of observation, it seems a matter of good practice always to work sights, both for latitude and longitude, with seconds.

If the navigator has at his disposal tables of logarithms giving functions to every 15 seconds of arc, this would be sufficiently close; but in the absence of such tables, it is advisable to follow the practice shown in solution of problems in this text, picking out logarithms corresponding to seconds of arc.

OCEAN METEOROLOGY

WIND AND WEATHER

THE ATMOSPHERE

1. Meteorology and the Nautical Profession.-By reason of the peculiarity of his profession, the navigator should, above all others, possess a good, practical knowledge of meteorology-the science that treats of the conditions and changes of the atmosphere.

A good insight into the navigational phases of this science, particularly the law of storms, will be found not only useful, but a real necessity in the navigation of the high seas. A few of the most important facts and principles of this science will therefore be explained in the following paragraphs.

2. Extent of Atmosphere.-The earth is entirely. enveloped by a gaseous body known as the atmosphere. The height of this atmosphere is far greater than any height that can be reached by ordinary means, such as balloons, etc., but by measuring the thickness of the penumbra that surrounds the shadow of the earth on the moon at the time of an eclipse of the moon, its extent is estimated to be from 50 to 60 miles; it covers everything on the earth's surface with a pressure of nearly 15 pounds per square inch. The density of the atmosphere is a maximum at the surface of the earth, and gradually diminishes until the confines are reached, where the density is zero.

Copyrighted by International Textbook Company.

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Entered at Stationers' Hall, London

3. The atmosphere is composed of air, just as the ocean is composed of water. The chief ingredients of air are oxygen and nitrogen; of these, oxygen is the most important, because its inhalation by human beings and animals is essential to life.

4. Heat.-Heat is not a substance, but may be considered as a form of energy. It is due to the rapid motion of minute particles, called molecules, of which all bodies are composed. Thus, when a person feels cold, he may, by rapid motion, for instance, by running, increase the warmth of his body.

5. Temperature. The different states that a body is in, according to the amount of sensible heat it possesses, are indicated by the word temperature. In meteorology temperature refers to the condition of the atmosphere in relation to its sensible heat and cold.

THE THERMOMETER

6. Explanation of Principles.-The instrument used for making accurate measurements of the temperature of gaseous and other bodies is called a thermometer, or heat measurer. In this instrument, the most common method is to utilize the expansive effect of heat on liquids. The liquids used are mercury and alcohol, the former being used because it boils only at a very high temperature, and the latter because it does not solidify at the greatest known cold produced by ordinary means.

In Fig. 1 is shown a mercurial thermometer with two sets of graduations on it. The one on the left, marked F, is the Fahrenheit thermometer, so named after its inventor, the German physicist Fahrenheit; this thermometer is the one commonly used in the United States and England. The one on the right, marked C, is the centigrade thermometer proposed by the Swedish mathematician Celsius; it is used by scientists throughout the world on account of the graduations being better adapted for calculations.