_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 HOW THE FIGURES ARE DETERMINED.--The foregoing figures are determined in the following manner: As an example let us assume 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: