The other end of the bar has a lateral pin to serve as a pivot for the end of a link F, its other end being hinged to the upper end of a lever G, which is pivoted to the post C, a short distance below the hinged attachment of the link F, so that the long end of the pointer which is const.i.tuted by the lever G is below its pivot, and has, therefore, a long range of movement.

A spring I between the upper end of the pointer G and the other post B, serves to hold the pointer at a zero position. A graduated scale plate J, within range of the pointer will show at a glance the pressure in pounds of the moving wind, and for this purpose it would be convenient to make the plane E exactly one foot square.

DETERMINING THE PRESSURE FROM THE SPEED.-- These two instruments can be made to check each other and thus pretty accurately enable you to determine the proper places to mark the pressure indicator, as well as to make the wheels in the anemometer the proper size to turn the pointer in seconds when the wind is blowing at a certain speed, say ten miles per hour.

Suppose the air pressure indicator has the scale divided into quarter pound marks. This will make it accurate enough for all purposes.

CALCULATING PRESSURES FROM SPEED.--The following table will give the pressures from 5 to 100 miles per hour:

Velocity of wind in Pressure Velocity of wind in Pressure miles per hour per sq. ft. miles per hour per sq ft 5 .112 55 15.125 10 .500 60 18.000 15 1.125 65 21.125 20 2.000 70 22.500 25 3.125 75 28.125 30 4.600 80 32.000 35 6.126 86 36.126 40 8.000 90 40.500 45 10.125 95 45.125 50 12.5 100 50.000

HOW THE FIGURES ARE DETERMINED.--The foregoing figures are determined in the following manner: As an example let us a.s.sume that the velocity of the wind is forty-five miles per hour. If this is squared, or 45 multiplied by 45, the product is 2025. In many calculations the mathematician employs what is called a constant, a figure that never varies, and which is used to multiply or divide certain factors.

In this case the constant is 5/1000, or, as usually written, .005. This is the same as one two hundredths of the squared figure. That would make the problem as follows:

45 X 45 = 2025 / 200 = 10.125; or, 45 X 45 - 2025 X .005 = 10.125.

Again, twenty-five miles per hour would be 25 X 25 = 625; and this multiplied by .005 equals 2 pounds pressure.

CONVERTING HOURS INTO MINUTES.--It is sometimes confusing to think of miles per hour, when you wish to express it in minutes or seconds. A simple rule, which is not absolutely accurate, but is correct within a few feet, in order to express the speed in feet per minute, is to multiply the figure indicating the miles per hour, by 8 3/4.

To ill.u.s.trate: If the wind is moving at the rate of twenty miles an hour, it will travel in that time 105,600 feet (5280 X 20). As there are sixty minutes in an hour, 105,600 divided by 60, equals 1760 feet per minute. Instead of going through all this process of calculating the speed per minute, remember to multiply the speed in miles per hour by 90, which will give 1800 feet.

This is a little more then two per cent. above the correct figure. Again; 40 X 90 equals 3600.

As the correct figure is 3520, a little mental calculation will enable you to correct the figures so as to get it within a few feet.

CHANGING SPEED HOURS TO SECONDS.--As one- sixtieth of the speed per minute will represent the rate of movement per second, it is a comparatively easy matter to convert the time from speed in miles per hour to fraction of a mile traveled in a second, by merely taking one-half of the speed in miles, and adding it, which will very nearly express the true number of feet.

As examples, take the following: If the wind is traveling 20 miles an hour, it is easy to take one-half of 20, which is 10, and add it to 20, making 30, as the number of feet per second. If the wind travels 50 miles per hour, add 25, making 75, as the speed per second.

The correct speed per second of a wind traveling 20 miles an hour is a little over 29 feet. At 50 miles per hour, the correct figure is 73 1/3 feet, which show that the figures under this rule are within about one per cent. of being correct.

With the table before you it will be an easy matter, by observing the air pressure indicator, to determine the proper speed for the anemometer.

Suppose it shows a pressure of two pounds, which will indicate a speed of twenty miles an hour. You have thus a fixed point to start from.

PRESSURE AS THE SQUARE OF THE SPEED.--Now it must not be a.s.sumed that if the pressure at twenty miles an hour is two pounds, that forty miles an hour it is four pounds. The pressure is as the square of the speed. This may be explained as follows: As the speed of the wind increases, it has a more effective push against an object than its rate of speed indicates, and this is most simply expressed by saying that each time the speed is doubled the pressure is four times greater.

As an example of this, let us take a speed of ten miles an hour, which means a pressure of one- half pound. Double this speed, and we have 20 miles. Multiplying one-half pound by 4, the result is 2 pounds. Again, double 20, which means 40 miles, and multiplying 2 by 4, the result is 8.

Doubling forty is eighty miles an hour, and again multiplying 8 by 4, we have 32 as the pounds pressure at a speed of 80 miles an hour.

The anemometer, however, is constant in its speed. If the pointer should turn once a second at 10 miles an hour, it would turn twice at 20 miles an hour, and four times a second at 40 miles an hour.

GYROSCOPIC BALANCE.--Some advance has been made in the use of the gyroscope for the purpose of giving lateral stability to an aeroplane. While the best of such devices is at best a makeshift, it is well to understand the principle on which they operate, and to get an understanding how they are applied.

THE PRINCIPLE INVOLVED.--The only thing known about the gyroscope is, that it objects to changing the plane of its rotation. This statement must be taken with some allowance, however, as, when left free to move, it will change in one direction.

To explain this without being too technical, examine Fig. 63, which shows a gyroscopic top, one end of the rim A, which supports the rotating wheel B, having a projecting finger C, that is mounted on a pin-point on the upper end of the pedestal D.

_Fig. 63. The Gyroscope._

When the wheel B is set in rotation it will maintain itself so that its axis E is horizontal, or at any other angle that the top is placed in when the wheel is spun. If it is set so the axis is horizontal the wheel B will rotate on a vertical plane, and it forcibly objects to any attempt to make it turn except in the direction indicated by the curved arrows F.

The wheel B will cause the axis E to swing around on a horizontal plane, and this turning movement is always in a certain direction in relation to the turn of the wheel B, and it is obvious, therefore, that to make a gyroscope that will not move, or swing around an axis, the placing of two such wheels side by side, and rotated in opposite directions, will maintain them in a fixed position; this can also be accomplished by so mounting the two that one rotates on a plane at right angles to the other.

_Fig. 64. Application of the Gyroscope._

THE APPLICATION OF THE GYROSCOPE.--Without in any manner showing the structural details of the device, in its application to a flying machine, except in so far as it may be necessary to explain its operation, we refer to Fig. 64, which a.s.sumes that A represents the frame of the aeroplane, and B a frame for holding the gyroscopic wheel C, the latter being mounted so it rotates on a horizontal plane, and the frame B being hinged fore and aft, so that it is free to swing to the right or to the left.

For convenience in explaining the action, the planes E are placed at right angles to their regular positions, F being the forward margin of the plane, and G the rear edge. Wires H connect the ends of the frame B with the respective planes, or ailerons, E, and another wire I joins the downwardly-projecting arms of the two ailerons, so that motion is transmitted to both at the same time, and by a positive motion in either direction.

_Fig. 65. Action of the Gyroscope._

In the second figure, 65, the frame of the aeroplane is shown tilted at an angle, so that its right side is elevated. As the gyroscopic wheel remains level it causes the aileron on the right side to change to a negative angle, while at the same time giving a positive angle to the aileron on the left side, which would, as a result, depress the right side, and bring the frame of the machine back to a horizontal position.

FORE AND AFT GYROSCOPIC CONTROL.--It is obvious that the same application of this force may be applied to control the ship fore and aft, although it is doubtful whether such a plan would have any advantages, since this should be wholly within the control of the pilot.

Laterally the ship should not be out of balance; fore and aft this is a necessity, and as the great trouble with all aeroplanes is to control them laterally, it may well be doubted whether it would add anything of value to the machine by having an automatic fore and aft control, which might, in emergencies, counteract the personal control of the operator.

ANGLE INDICATOR.--In flight it is an exceedingly difficult matter for the pilot to give an accurate idea of the angle of the planes. If the air is calm and he is moving over a certain course, and knows, from experience, what his speed is, he may be able to judge of this factor, but he cannot tell what changes take place under certain conditions during the flight.

For this purpose a simple little indicator may be provided, shown in Fig. 66, which is merely a vertical board A, with a pendulum B, swinging fore and aft from a pin a which projects out from the board a short distance above its center.

The upper end of the pendulum has a heart- shaped wire structure D, that carries a sliding weight E. Normally, when the aeroplane is on an even keel, or is even at an angle, the weight E rests within the bottom of the loop D, but should there be a sudden downward lurch or a quick upward inclination, which would cause the pendulum below to rapidly swing in either direction, the sliding weight E would at once move forward in the same direction that the pendulum had moved, and thus counteract, for the instant only, the swing, when it would again drop back into its central position.

_Fig. 66. Angle Indicator._

With such an arrangement, the pendulum would hang vertically at all times, and the pointer below, being in range of a circle with degrees indicated thereon, and the base attached to the frame of the machine, can always be observed, and the conditions noted at the time the changes take place.

PENDULUM STABILIZER.--In many respects the use of a pendulum has advantages over the gyroscope.

The latter requires power to keep it in motion. The pendulum is always in condition for service. While it may be more difficult to adjust the pendulum, so that it does not affect the planes by too rapid a swing, or an oscillation which is beyond the true angle desired, still, these are matters which, in time, will make the pendulum a strong factor in lateral stability.

_Fig. 67. Simple Pendulum Stabilizer._

It is an exceedingly simple matter to attach the lead wires from an aileron to the pendulum. In Fig. 67 one plan is ill.u.s.trated. The pendulum A swings from the frame B of the machine, the ailerons a being in this case also shown at right angles to their true positions.

The other, Fig. 68, a.s.sumes that the machine is exactly horizontal, and as the pendulum is in a vertical position, the forward edges of both ailerons are elevated, but when the pendulum swings both ailerons will be swung with their forward margins up or down in unison, and thus the proper angles are made to right the machine.

STEERING AND CONTROLLING WHEEL.--For the purpose of concentrating the control in a single wheel, which has not alone a turning motion, but is also mounted in such a manner that it will oscillate to and fro, is very desirable, and is adapted for any kind of machine.

_Fig. 68. Pendulum Stabilizers._

Fig. 69 shows such a structure, in which A represents the frame of the machine, and B a segment for the stem of the wheel, the segment being made of two parts, so as to form a guideway for the stem a to travel between, and the segment is placed so that the stem will travel in a fore and aft direction.

The lower end of the stem is mounted in a socket, at D, so that while it may be turned, it will also permit this oscillating motion. Near its lower end is a cross bar E from which the wires run to the vertical control plane, and also to the ailerons, if the machine is equipped with them, or to the warping ends of the planes.

_Fig. 69. Steering and Control Wheel._