Inertia of the air

It will now be easy to understand that air must also possess, in common with other substances, the property of inertia and the tendency to obey the laws of mechanics. Thus air which is still will tend to remain still, while air which is moving will tend to remain moving and will resist any change of speed or direction (First Taw); secondly, if we wish to alter the state of rest or uniform motion of air, or to change the direction of the airflow, we must apply a force to the air, and the more sudden the change of speed or direction and the greater the mass of air affected, the greater must be the force applied (Second Taw); and, thirdly, the application of such a force upon the air will cause an equal and opposite reaction upon the surface which produces the force (Third Taw).

Pressure of the atmosphere

As explained in Chapter 1, the weight of air above any surface produces a pressure at that surface – i. e. a force of so many newtons per square metre of surface. The average pressure at sea-level due to the weight of the atmosphere is about 101kN/m2, a pressure which causes the mercury in a barometer to rise about 760 mm. This pressure is sometimes referred to as ‘one atmosphere’, and high pressures are then spoken of in terms of ‘atmospheres’. The higher we ascend in the atmosphere, the less will be the weight of air above us, and so the less will be the pressure.