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i 1 LIBRARY ^hSSACHt,^^^ 1895 TEXT-BOOK OCEAN METEOROLOGY, COMPILED FROM THE SAILING DIRECTORIES FOR THE OCEANS OF THE WORLD, BY ALEX. GEO. FINDLAY, F.R.G.S. EDITED BY STAFF-COMM^ W. R. MARTIN, R.N., Lecturer on Meteorology at the Royal Naval College, Greenwich. LONDON: PUBLISHED BY RICHARD HOLMES LAURIE, 53, FLEET STREET, E.G. 1R87. O ENTERED AT STATIONERS' HALL. — PREFACE. In the course of delivering my annual series of Lectures on Ocean Meteorology, at the Royal Naval College, I have frequently been asked to recommend some Text-Book on the subject which would give a compre- hensive view of the various Winds and Currents of the Ocean, and which—without, on the one hand, giving the details necessary for Sailing Directions, or, on the other hand, being a mere cram-book for examina- tions—should form a fitting introduction to this highly interesting and very important part of a Sailor's education. Finding that the information in the Admiralty Sailing Directions was scattered through too many volumes for easy reference, and that the information in Findlay's Directories was too voluminous to answer the purpose, I proposed to the Proprietors of the latter well-known Works that they should issue, in a small volume, certain selected portions of those larger Works, and to these I have added various Notes from my own Lectures. I have omitted almost all theoretical questions, and merely brought together material to illustrate what is the practical issue of the subject the Art of making Passages from Port to Port. I hope that this Book may prove useful for the purpose designed of attracting more attention towards a somewhat neglected but highly useful branch of Naval education. W. R. MARTIN. Royal Naval College^ Greemvich. September 2)Olli, 1887. CONTENTS. CHAPTER I. CHARACTERISTICS OF THE ATMOSPHERE.—THE BAROMETER, ITS DAILY AND ANNUAL CHANGES.—CENTRES OF HIGH AND LOW BARO- METRIC PRESSURE.—WIND FORCE.—GENERAL WIND SYSTEMS OF THE WORLD.—MOISTURE OF THE ATMOSPHERE.—CLOUDS.—RAIN. CHAPTER II. CHARACTERISTICS OF THE OCEAN.—CURRENTS.—DRIFT AND SURFACE CURRENTS.—ICEBERGS, ICE ISLANDS, AND DRIFT ICE.—NORTHERN ICE.—SOUTHERN ICE. CHAPTER III. THE NORTH ATLANTIC OCEAN, ITS WIND AND CURRENT SYSTEMS.— THE MEDITERRANEAN SEA, ITS WINDS AND CURRENTS.—THE SOUTH ATLANTIC OCEAN, ITS WIND AND CURRENT SYSTEMS.— PASSAGES ACROSS THE ATLANTIC OCEAN. CHAPTER IV. THE INDIAN OCEAN, ITS WIND AND CURRENT SYSTEMS DURING EACH MONSOON.—PASSAGES ACROSS THE INDIAN OCEAN. CHAPTER V. THE INDIAN ARCHIPELAGO AND CHINA SEA, THEIR WIND AND CURRENT SYSTEMS DURING EACH MONSOON.—PASSAGES AT DIFFERENT SEASONS OF THE YEAR. CHAPTER VI. THE NORTH PACIFIC OCEAN, ITS WIND AND CURRENT SYSTEMS.— THE SOUTH PACIFIC OCE.\N, ITS WINDS AND CURRENTS.-' PASSAGES ACROSS THE PACIFIC OCEAN. CONTENTS. CHAPTER VII. HURRICANES, CYCLONES, OR REVOLVING STORMS.—PROGNOSTICS.— ESTIMATION OF DIRECTION AND DISTANCE.—LAW OF STORMS.— ILLUSTRATIONS OF NORTH ATLANTIC HURRICANES.—STATISTICS OF REVOLVING STORMS IN VARIOUS PARTS OF THE WORLD. CHAPTER VIII. DEPTH AND TEMPERATURE OF THE OCEAN.—DEEP SEA SOUNDINGS AND TEMPERATURES.—SURFACE TEMPERATURES & ISOTHERMAL CHARTS.—SALTN ESS AND DENSITY OF THE OCEAN.—CIRCULATION OF OCEAN WATER. ILLUSTRATIONS. The Prevalent Surface Currents of the Ocean .... Frontispiece. Crossing the Equator at different Seasons. No. 3 Square To face page 92. Courses of various Hurricanes in the West Indies, &c „ 208. Wind Systems and Sailing Ti-acks, for the month of February At end of Book. Wind Systems and Sailing Tracks, for the month of August ( vii ) ERRATA. Pase 3, OCEAN ]\IETEOROLOGY. CHAPTER I. THE ATMOSPHERE. The subject of Ocean Meteorology, or an examination of the prevailing Winds and Currents of the Ocean in their relation to Passages made Eemarks.'*^ ^^°^ P°^^ *° P<^^*' ^^^ always been one of the highest interest to the practical sailor; nor did that interest lessen with the introduction of steam-vessels of great speed, for though navigation became then less dependent on Winds and Currents, yet the competition at the same time introduced led to a closer examination of those circumstances under which the length of voyages could be shortened, even by a very small amount. In the Royal Navy, in times of peace, most vessels-of-war, partly from their construction and partly from a necessary economy of coal consumption, can neither be classed with sailing vessels nor swift steamers ; but, in time of war, circumstances will doubtless arise when passages will have to be made under conditions, similar to the Mercantile Marine. The following pages are designed as a general introduction to the subject, in the hope that a sketch of the leading Phenomena of Ocean Meteorolo'''y may stimulate readers to pursue the study further, and add much-needed contributions to our stock of information on the points touched upon. These points are very fully enlarged on in the various Directories for the different Oceans, to which the general reader desiring fuller information is referred. Ocean Met. b 2 OCEAN METEOROLOGY. As these larger Directories will be in the hands of those actually navigating ji ship, the question of making Passages (which is the practical outcome of Ocean Meteorology) will here be illustrated by a description of only a few typical, and a few peculiar Passages over each of the Oceans. As a science, based on well-established facts. Ocean Meteorology could hardly exist before the intelligent use of Barometers and Thermometers at sea,^ or before the invention of the Chronometer and the consequent improvement in ascertaining a ship's position. In fact, it is only since the Brussels Conference of 1853, and the investigations of Maury, that really rapid progress has been made in assigning limits to Ocean Winds and Currents at different seasons of the year. Before entering on the detailed description of these, a few pages will be devoted to noting some of the general characteristics of the Atmosphere, and of the Ocean. THE ATMOSPHERE. The Atmosphere, considered as an Aerial Ocean at the bottom of which we live, must be regarded as consisting of dry air and aqueous vapour intimately mixed, and both generally existing as invisible gases. Theoretically, this Atmosphere extends to a height of several hundred miles, but, practically, almost all Meteorological Phenomena occur below a height of about 10 miles.^ In round numbers, 13 cubic feet of dry air weigh 1 lb. Av.,"^ and in all respects air follows the same laws as ordinary gases, the density being greatest near the surface of the earth, and decreasing upwards, so that at an elevation of 7 miles it is reduced to one-quarter, at 14 miles to one- sixteenth, and at 21 miles to one-sixty-fourth of what it is at the surface.* Aqueous Vapour also follows precisely the same laws, so long as it exists in the gaseous form. It is constantly given off from the surface Aqueous ^^ water at all temperatures, and even from snow and ice, mixing with the dry air in a proportion depending upon the temperature. Thus dry air at 32° F. can sustain, in an invisible form, a one- hundred-and-sixtieth part of its own weight of aqueous vapour, at 59° F. one- 1 The Barometer was invented in 1643, but not used at sea until 50 years later. The Thermometer appears to have been first used at sea in Anson's voj'age. '^ If a globe 16 inches in diameter be taken to represent the earth, the greatest known depth of the ocean and height of the highest mountain would be represented by a one- hundredth part of an inch, and the height of the Atmosphere, on a similar scale, by about one-fiftieth part of an inch. 3 More accurately, 1 cubic foot at 32° F. weighs 563 grains, and at 90° F. 502 grains. * In the celebrated balloon ascent by Glaisher, iu 1862, at an elevation of 37,000 feet, the Barometer had fuUen to 11-53 inches. THE ATMOSPHERE. 3 eightieth, and at 86° F. one-fortieth, its capacity being thus cloublcd every 2r F.i It is important to observe that dry air is heavier than aqueous vapour, in the proportion of 8 to 5, and hence a cubic foot of moist air weighs less than a cubic foot of dry air at the same temperature. The dry air of the Atmosphere must be regarded as constantly endeavour- ing to evaporate from any water-surface as much aqueous vapour as it can contain,^ Saturation being defined as that point at which a volume of air contains the greatest possible quantity of aqueous vapour, and the Dew Point as that temperature at which the Aqueous^ VaDoiur^ Dew Point. ... ... „ begins to be precipitated as water.^ In the gaseous form, aqueous vapour occupies about 2,000 times the space it does when condensed to ivater, and the frequent change from one form to another is a probable cause of many great atmospheric disturbances. A decrease of temperature renders aqueous vapour visible as fog, clouds, or rain; but while present in ihe gaseous form, it only becomes indirectly apparent through the increased visibility of distant objects. Note.— Careful analyses of the air show that out of every 100 parts by measure^ Nitrogen forms 77*5 parts. Oxygen 21, Aqueous vapour 1-42 (on an average), and Carbonic Acid 0*08. Maury says : —The mean annual fall of rain on the entire surface of the earth is estimated at about 5 feet. To evaporate water enough annually from the ocean to cover the earth, on the average, 5 feet deep with rain ; to transport it from one zone to another ; and to pre- cipitate it in the right places, at suitable times, and in the proportions due, is one of the offices of the grand atmospherical machine.

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