THE SPEED INDICATOR.—Four hemispherical cups A are mounted on four radiating arms B, which are secured to a vertical stem C, and adapted to rotate in suitable bearings in a case, which, for convenience in explaining, is not shown.
On the lower end of the stem C, is a small bevel pinion, which meshes with a smaller bevel pinion within the base. This latter is on a shaft which carries a small gear on its other end, to mesh with a larger gear on a shaft which carries a pointer D that thus turns at a greatly reduced speed, so that it can be easily timed.
Fig. 62. Air Pressure Indicator.
AIR PRESSURE INDICATOR.—This little apparatus is readily made of a base A which is provided with two uprights B, C, through the upper ends of which are holes to receive a horizontally-disposed bar D. One end of the bar is a flat plane surface E, which is disposed at right angles to the bar, and firmly fixed thereto.
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 constituted 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