only 25 percent will remain in your memory.

This fact should not be discouraging to you, but simply should be considered when you are talking with your fellow workmen.

Psychologists are not in agreement concerning all aspects of the human mind, but one item they are of the same opinion is that individuals think in terms of words. The normal speaking rate is about 125 words a minute, but the brain cells are capable of listening at a much faster pace, possibly as much as 600 words a minute or more. The human mind has no trouble in grasping the 125 words a minute of someone talking, and at the same time has room to wander in all directions. The "wandering" of the mind is the real problem of listening since, by the time it has returned from a side trip, something important may have been said, and we are in trouble.

We are told by authorities in such matters that spoken words should be listened to with a definite regard for their surroundings. The facial expression, the posture, and gestures may say more than the words. What the speaker doesn't say may be more expressive than what he does say.

The fact that individuals are difficult to reach because of competition for their attention is more the reason to give the important business of listening your full attention.

What, then, is good listening? Good listening is understanding and remembering what is heard. How can you become a better listener? A noted speaker and salesman presented the following thoughts on the art of listening: Anticipate what the man is going to say, analyze his thoughts, and then summarize in your mind. By doing this, your retention ability will be about 60 percent greater than before.

Naturally, before you can do much listening, your associates have to talk. This can be developed by a little encouragement on your part. Once encouraged, most men will talk freely about their attitudes and opinions on their jobs, beliefs, fears, frustrations, and other personal problems.

The velocity of an air current is its speed or the rate of travel. It represents the distance that a particle of air travels in a certain length of time.

Velocity measurements are always given in lineal distance (inches, feet) per unit of time (seconds, minutes). Velocities of mine air are always expressed in feet per minute. Always include the unit of time with the statement of distance as for example "300 feet per minute."

Measurement of Air Velocity

Mine air current velocities are determined by means of an instrument known as an anemometer. The anemometer is a small windmill with slanting blades connected through a clutch and gearing to one or more dials, which record the amount of linear feet of air that passed.

Reading the dials of an anemometer is no more difficult than noting the time of day from a watch or clock. Before reading any anemometer the observer should acquaint himself with the dials so he will know the values each one is recording.

Figure 22 is a sketch showing an anemometer having three small dials on the face of the large dial. The large dial records readings from 0 to 100; the small dial to the right indicates hundreds of feet; the lower dial records thousands of feet; and the small dial to the left indicates ten thousands of feet of air.

As indicated in Figure 22 the pointer of the small dial on the left is approximately two-thirds the distance between 0 and 1, which signifies that the total reading will not be greater than 10,000 feet but something less. The pointer of the lower dial is between 6 and 7, meaning that the reading will be somewhere between 6,000 and 7,000. In noting the small dial to the right, we see that the pointer is almost to 8, giving a value close to 800. The large dial is now checked and is seen pointing to 99, which is its true value. The anemometer reading is now taken by grouping all of the readings in their proper order; namely 6,000 from the lower dial, 700 from the right dial, and 99 from the large dial for an anemometer reading of 6,799 The small dial to the left did not enter into this reading since the first 10,000 mark had not been passed.

The anemometer does not measure the velocity of the air; it merely records the feet of air travel in a certain length of time, and

this element must be included in the reading to give its value. The velocity of the air is found by dividing the anemometer reading by the time in minutes that the anemometer was in operation.

Velocity (ft per min.)

=

Anemometer Reading
Time in Minutes

In taking air readings the anemometer should be held at arms length with the instrument face at right angles to the direction of the air flow and moving it during the reading period so as to cover the entire airway area which will provide an average velocity reading for the entire cross-section. The observer should have his back to the direction of air flow and holding the instrument well away from his body, and in such a manner that air will enter the back. The reading should be for a definite period of time.

The measurements should be taken in a section where the flow is near uniform, and where no obstructions are nearby. The walls of the station should be as uniform as possible. Some companies have set up measuring stations where the ribs and roof have been smoothed with cement for some distance so as to create uniform flow of air and thus give accurate air readings.

The anemometer should only be placed in the hands of a competent person who is thoroughly familiar with its operation. It should be handled with care, similar to the care that would be given a good watch. The vanes operate on jewel bearings and the slightest misuse will cause the anemometer to give inaccurate results, Some of the points to consider when handling an anemometer are listed below:

3. Do not repair the anemometer; if it needs any repair, send it to the manufacturer.

4.

Do not expose the anemometer to air currents greater than those for which it was designed.

Quantity of Air

The quantity of air passing through an airway in a certain length of time is the product of the velocity and the cross-sectional area of the airway In ventilation calculations quantity is indicated by "Q", velocity by "V", and area is indicated by "A"; so the quantity equation becomes:

The quantity of air is always expressed as cubic feet per minute or is a volume measurement. Many problems will require the "volume" of air passing through an airway, which is the same as the quantity of air

Consider the following illustration for an explanation of the quantity or volume of air.

The above is a sketch of a section of an airway having the dimensions as shown. The volume of the section will be 5 x 10 x 100 or 5,000 cubic feet. Let us suppose that an anemometer is held at the entrance of the section for a period of one minute and that the anemometer reading is 100. During this one minute period, the air that was in the first section has moved up the passageway a distance of one hundred feet as shown by the dotted outline, which is the amount of linear feet of air travel.

The quantity of air passing through the section would be: Q = A x V = (5 x 10) x 100 = 5,000 cu. ft. per min., thus showing that quantity of air and volume of air are identical.

The following problem will better familarize the student with airquantity calculations: