ABSTRACT
Air, as the primary constituent of atmosphere, surrounds us everywhere. We humans tirelessly breathe in and out its tiny portion all the time as long as we live. This chapter describes how we can recognize the presence of atmosphere and also vacuum. The old belief that vacuum space was not possible must have come from the fact that sucking air by a pump seemingly caused the tendency of water to fill the space where air originally existed. The chapter presents an example of the variation of atmospheric air pressure throughout one year in Yokohama. It also presents an experiment to investigate the relationship between the height of air space and that of mercury, which was made by Towneley, Robert Boyle and Robert Hooke. The chapter focuses on how to quantify the moisture in the atmospheric air. It considers a volume of humid air as the mixture of two constituents: one is “dry air” and the other water vapour.
