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The Barometer

Next to the mariner's compass and chart, the barometer is the most important aid to navigation ever invented. Many persons know that a barometer is an instrument for recording changes in the weather. The student of physics is taught that this is done by measuring the weight or pressure of the atmosphere. A rising barometer denotes the approach of good weather; a falling barometer, the reverse. A sudden fall warns the mariner to be on the look-out for a severe storm.

The barometer was invented during the seventeenth century by Torricelli. The ship's barometer, which is kept in the chart room, is very different from the original device. It traces a barometer chart, recording the atmospheric pressure throughout the voyage.

The Gulf Stream

The Gulf Stream is by far the most important, as well as the best known of the great ocean currents. It derives its name from the Gulf of Mexico, out of which it flows between Cuba and the Bahamas on the one side and the Florida Keys on the other. In its narrowest portion the Gulf Stream is about fifty miles wide, and there it has a velocity at times of as much as five miles an hour. Flowing in a North-Easterly direction along the American coast, its current gradually widens and its velocity diminishes.

Reaching the banks of Newfoundland it turns and sweeps across the Atlantic; then, dividing into two portions, it sends one arm down toward the Azores and the coast of Morocco, while the other passes near the shore of the British Isles and on to Norway.

As it emerges from the Gulf of Mexico it has a temperature of 84°F in summer, higher than that of the ocean at the equator. Even by the time it has reached mid-Atlantic it has not fallen more than 14°F. The effect of the stream upon the climate of Great Britain and the North-West coast of Europe 4,000 miles away from the Gulf, is to raise the winter temperature about 30°F above what would be the normal temperature of those latitudes.

Nautical Distances / Speed

A Caveat

The information given in this section formed part of Appendices added to the cruise souvenir books printed for the guidance of passengers on board. The actual source being the Great African Cruise book of 1950. Since that time, particularly with the advent of International Standard of Units (SI), the values may have been amended.

Distances At Sea

Miles at sea are understood to be nautical miles. In the U.S.A., the nautical mile is defined to be one-sixtieth part of the length of a degree on a great circle of a sphere whose surface is equal in area, to the area of the surface of the earth.

In France, Germany and Austria the nautical mile has a length of 6,076.23 feet. In the U.K., it is 6,080 feet.

It is no more necessary to say “nautical” miles when speaking of a sea distance than to say “statute” miles when speaking of a land distance.

Knots: A Measure of Speed

Landsmen are apt to confuse knots with nautical miles. A knot is not a measure of distance but a measure of speed, and the only word to express this in the English language.

• When speaking of a vessel that travels, say at 19.11 knots, we mean that it's travelling at a speed of 19.11 “nautical miles per hour” or just over 22 miles per hour
• the actual distance covered may be one nautical mile or a thousand, depending upon the length of time the 19.11 knots are maintained
• In official documents, Knots are usually recorded to 2 decimal places
• Only landsmen use the expression “knots per hour.” The “per hour” addition is superfluous and incorrect

Distance / Speed Conversion

Nautical Distance & Speed Converter (JS)

Type numbers into either box to show
its speed or distance counterpart:

Speed in Knots / Nautical Miles :

Speed in M.P.H. / Statute Miles :

The calculator uses the 1.15157 multiplier of the period

Variations of Time

Local time is determined by the relation of the meridian of a place to the Sun. Noon at any place is defined as the moment when the mean sun passes the meridian of that place. Travelling eastward toward the rising sun, noon arrives earlier each day. Consequently timekeepers have to be advanced. Travelling westward the reverse operates.

There is a difference of five hours between the meridians at Greenwich and New York. If a ship travels the distance in five days she gains an hour each day going eastward, or loses the same time if westbound. As, however, comparatively few ships make such speed, on most liners the day's length varies and the only good guide for setting watches is the ship's clock, which is corrected each day, usually at midnight. The amount of correction necessary each day is posted on a special bulletin board.

Visibility at Sea

Visibility at sea depends on atmospheric conditions and the height above sea level from which the object is viewed. Under normal atmospheric conditions visibility is as under:

Distance to Horizon Table

Height above Sea Level in ft.	Approx Distance Nautical Miles	Height above Sea Level in ft.	Approx Distance Nautical Miles
5	2.55	45	7.67
10	3.61	50	8.08
15	4.43	60	8.85
20	5.11	70	9.56
25	5.71	80	10.22
30	6.26	90	10.84
35	6.76	100	11.43
40	7.23