LAKE NAVIGATION

(PART 5)

CURSORY TREATMENT OF THE CAUSE OF STORMS AND THE BAROMETRIC CHANGES ATTENDING THEM

THE ATMOSPHERE

1. The earth is entirely enveloped by a gaseous body known as the atmosphere. The height of this atmosphere is far greater than any height to which we can attain; yet its approximate limit has been determined with a tolerable degree of accuracy to be from 50 to 60 miles. The atmosphere covers everything on the earth, and has a pressure on each square inch of the surface of nearly 15 pounds. The density of the atmosphere is a maximum at the surface of the earth and gradually diminishes until the confines are reached, where it is zero.

2. 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.

3. 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.

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4. Temperature. The different states that a body is in, according to the amount of sensible heat it possesses, are indicated, or denoted, by the word temperature. We thus say that the temperature of a body is high or low, according as the body is hot or cold.

C

THE THERMOMETER

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

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

The expansion or contraction of the mercury, by applying or withdrawing heat from the body with which the bulb of the glass tube is in contact, causes the highest point of the mercury column to rise and fall, and, since for equal changes of temperature the mercury rises and falls equal distances, this instrument, when properly FIG. 1 made and graduated, indicates any change in temperature with great accuracy.

6.

How the Thermometer Is Graduated. - In order to graduate a thermometer, it is placed in melting ice, and

the point to which the mercurial column then falls is marked freezing. It is next placed in the steam rising from water boiling in an open vessel, and the point to which the mercurial column rises is marked boiling. Two fixed points are thus established, the freezing point and the boiling point. If it is desired to make a Fahrenheit thermometer, the distance between the points is divided into 180 parts, called degrees. The freezing point is marked 32° and the boiling point 212°; 32 parts are marked off from the freezing point downwards, and the last one is marked 0°, or zero. In graduating a centigrade thermometer, the freezing point is marked 0°, or zero, and the boiling point 100°; the distance between the two is divided into 100 equal parts.

When there is any doubt as to the thermometer used, the first letter of the name is placed after the degree of temperature. Thus 183° F. means 183° above zero on the Fahrenheit instrument, and 183° C. means 183° above zero on the centigrade instrument. In Russia and a few other countries another instrument is used, called the Réaumur. The freezing point is marked 0°, or zero, and the boiling point 80°, the distance between these two points being divided into 80 equal parts. 62° R. means 62° on the Réaumur thermometer.

8. Since air has weight it is evident that the enormous quantity of air that constitutes the atmosphere must exert a considerable pressure on the earth. As previously stated, this air pressure decreases with the increase of altitude, because there is less air above us the higher we ascend. The pressure of the atmosphere would be practically the same. at all places having a common altitude above the level of the sea if it were not for the disturbing influence of the solar heat and of the movement of the air caused by its unequal heating at different places on the earth.

9. The instrument used for measuring the pressure of the atmosphere is called the barom

10. The mercurial barometer is represented in Fig. 2. The principle of this instrument may, with the aid of Fig. 3, be explained as follows: A glass tube a of a length of, say, 3 feet and inch in diameter, and closed at one end, is filled with mercury. Then, closing the open end with a finger, the tube is turned over and inserted into a vessel, or cup, b. Some of the mercury will now flow, downwards, out of the tube into the cup until the weight of the mercury remaining in the tube is equal to the pressure of the air on the surface of the mercury in the cup. The space above the mercury in the tube will now be practically a vacuum, and consequently there will be no pressure on the top surface of the mercury in the tube. It is evident, then, that when the pressure of the atmosphere on the

b

FIG. 3

surface of the mercury in the cup increases, the mercury in the tube is forced upwards; and when the pressure decreases, the mercury in the tube will fall. It is on this principle that the barometer shown in Fig. 2 is constructed. The tube and cup at the bottom are protected by a brass or metal casing. At the top of the tube is a graduated scale that can be read to 10 inch by means of a vernier, which is quite sufficient for nautical purposes. Attached to the casing is an accurate thermometer for determining the temperature of the outside air at the time the barometric observation is made. This is necessary, since mercury expands when the temperature is increased and contracts when the temperature falls; for this reason, a standard temperature is assumed, and all barometer readings are reduced to this temperature. This standard temperature is usually taken at 32° F., at which temperature the height of the mercurial column is 30 inches.

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11. The barometer, being a very delicate instrument, requires careful handling. 31When suspended for use, it is important that it should hang freely in a vertical position and in a place where it is protected from the rays of the sun and from all other local sources of heat or cold.

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a

FIG. 4

12. How to Read Off a Mercurial Barometer. - In reading off a barometer, 29 the lower edge of the vernier attached to the instrument should be brought into contact with the uppermost point of the mercury when the eye is at an equal height and looking horizontally at the tube. let ab, Fig. 4, represent a portion of the scale of a barometer, cd the vernier, and e 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

For instance,