vi CONTENTS. § 15. Physical Properties of Air in Motion § 16. Perimeter, Area, etc., defined. § 22. Laws of Pressure, and Friction of Air in Mines, with MINE VENTILATION. CHAPTER I. 1. THE atmosphere is composed of nitrogen and oxygen, with a trace of carbonic-acid gas. These three gases, essential to the existence of all animal and vegetable life, when taken separately will not support life. A mechanical mixture of these gases, in the proportion of four parts of nitrogen to one part of oxygen, is the air we breathe; which, if mixed with deleterious gases (or, as we say, impure), will cause serious physical disorders, and not unfrequently premature death. Carbonic-acid gas rarely exceeds one part in sixteen hundred of pure air; being present in the atmosphere, so say our best chemists, in the ratio of four parts in ten thousand. The term "atmosphere designates that immense expanse or ocean of gaseous matter which envelops or surrounds our earth, commonly called "air.” It is supposed that this atmos |