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the top of the mercurial column. The vernier is then placed in the position shown and the barometric pressure is read off in the same way as the sextant, the whole and tenths of an inch being read on the scale, and the hundredths of an inch on the vernier, as indicated by a division on the vernier coinciding with a division on the scale.
large watch up to an 8-inch or 10-inch face. They consist of a cylindrical box of metal, with a top of thin elastic corrugated metal. The air is removed from the box. When the atmospheric pressure increases the top is pressed inwards, and when it diminishes the top is pressed outwards by its own elasticity, aided by a spring beneath. These movements of the cover are transmitted and multiplied by a combination of delicate levers that act on an index hand and cause it to move either to the right or left over a graduated scale. These barometers are self-correcting (compensated) for variations in temperature. They are portable, and are so very delicate (when carefully made) that they show a difference in atmospheric pressure when transferred from the upper part of a room to the floor. The instruments should be handled with extreme care, as they are easily injured. A good aneroid barometer, costing from $20 to $30, is of great value to the navigator as a “weather glass” if carefully observed, but its readings are not so accurate as those of a good mercurial barometer.
15. The Meaning of Barometric Changes.-In order to understand the meaning of changes in the atmospheric pressure, as indicated by changes of the mercury column in a barometer, the student is referred to Fig. 7, which represents a weather map, such as is published daily by the United States Weather Bureau. On such map there will, as a general rule, appear one or more approximately circular areas marked “Low,” while other areas of a more irregular outline are marked “High.” The first implies that the reading of the barometer within the area indicated is be/ow the average height; the second, that it is above it. Around the area marked “Low" are drawn lines, each of which has a number attached. These lines are called isobars and the attached numbers are barometric readings; thus, at all points along any one of these lines the reading of the barometer, at the hour represented by the chart, is the same. The point of lowest barometer, or point of least atmospheric pressure, is known as the storm center, inasmuch as it coincides very nearly with the area over which a storm prevails. Furthermore, as we go from the center in any direction it will be noticed that the atmospheric pressure increases between each isobar; in other words, as we recede from the center the barometer will gradually rise, and, conversely, when we approach the center it will gradually become lower. Now, since a storm is always moving, it is evident that whenever the barometer shows a tendency to drop below the average height, the navigator will know that an area of low pressure is approaching, and since this area indicates the presence of a storm of more or less intensity, he is thus warned of the impending change in weather.
From this the student will realize the important function of the barometer and that by noting the changes of the mercurial column an observer is able to foretell, with a tolerable degree of accuracy, any decided change in weather conditions. The use of the barometer as a weather forecaster will be further discussed under the heading, “ Notes on Weather Indications.”
16. Cause of Wind.—As previously stated, the atmosphere surrounding the earth is composed of air. When some portion of this air is put into motion, we say that “a wind is blowing.” Wind, therefore, may be defined as air in motion. Then come the questions, How is wind caused? What powerful fan puts the air into motion and pushes it along at so great a velocity that it sometimes is able to carry with it stones, trees, and houses, the weight of which would tip the scale at several tons?
All winds—the raging hurricane as well as the gentle evening breeze—are caused directly or indirectly by changes of temperature. When, from any cause, two neighboring regions become unequal in temperature, the air of the warmer region, being lighter than the other, will ascend and spread out over the top of the colder air, while the heavier air of the colder region will flow in to supply its place. Thus a motion is produced, the swiftness or velocity of which will depend on the difference in temperature between the two regions. The greater the difference the greater the velocity of the wind; and this wind, or rather these winds—one blowing from the colder regions to the warmer along the surface of the earth, the other from the warmer to the colder in the upper regions of the atmosphere—will continue to flow until equilibrium is restored. Another effect of the warm air ascending to the upper strata of the atmosphere is the formation of clouds. As the air rises it expands and in doing so it is rarefied and cooled. Its vapor is then condensed into clouds or precipitated in rain. When air is at rest, it is said to be in a state of calm.
17. Force of Wind.—The Beaufort scale is commonly used by seamen for recording the force of wind. For the guidance of those unaccustomed to its use the corresponding velocity per hour in statute and in nautical miles is shown in the following table.
Velocity Per Hour
From this table it is seen that the force of wind varies from 0, a calm, to 12, a hurricaner-the greatest velocity it ever reaches. Intermediate forces can be readily estimated by the personal judgment of the student. To obtain accurate results in recording force and direction, the speed and course of the ship or steamer must be considered.
18. Classification of Winds.-Winds are classified as constant, periodical, and variable, and are named according to the direction from which they come, that is. from the true bearing of the wind.