lent exercise, a man consumes from eight to nine times the amount of oxygen required when at rest; or, say 128 to 144 cu. in. per min. The exhaled breath may then contain 6.6 per cent. carbon dioxide and only 14.3 per cent. oxygen.

Depletion of Oxygen in Air, Effect on Life.-Air containing 3 per cent. carbon dioxide can be breathed without discomfort, even when the oxygen content has been reduced to 16 per cent.; but 5 per cent. carbon dioxide causes headache, dizziness and nausea, after a short time. When no carbon dioxide is present in the air the oxygen content may fall as low as 14 per cent. before much difficulty is experienced in breathing; but air containing but 10 per cent. is no longer breathable; but will cause death quickly by suffocation.

Composition of Air.-Normal air is composed chiefly of oxygen and nitrogen, which are invariably mixed in the following proportions expressed as percentage by volume and by weight of each of these gases:

TABLE SHOWING COMPOSITION OF NORMAL AIR

Air also contains 0.04 per cent. of carbon dioxide (CO2), together with smaller amounts of argon, ammonia and water vapor. Atmospheric air, it may be said, is never absolutely dry or free of moisture. The term "dry air" in respect to the atmosphere is only a relative expression, meaning that such air is comparatively dry.

Weight of Dry Air.-The weight of dry air, per unit volume, varies directly with the pressure it supports, and inversely as its absolute temperature. There are two formulas for finding the weight of 1 cu. ft. of air, one being expressed in terms of the barometer (B), in inches, and the other in terms of the pressure (p) in pounds per square inch.

Moisture in Air. This subject is fully treated under "Hygrometry," and it is sufficient here to say that the water absorbed or held by the air is an invisible vapor that resembles a gas in its behavior, until a sufficient amount is present to fully saturate the space it occupies. This point of saturation is called the "dew point," because at that point any excess of vapor condenses and appears as a mist or cloud. The condensation is more rapid in contact with a cold surface.

Normal Air. The term "normal air" in respect to its composition refers to air containing a normal percentage of oxygen (20.9 per cent.) as given above. When the percentage of oxygen present is less than normal the air is said to be "depleted" of its oxygen. This frequently occurs in poorly ventilated places in mines. The depletion of oxygen is the result of the various forms of combustion or oxidation that are constantly taking place in mines, and is also caused by the absorption of oxygen from the air by the coal.

Mine Air. Except when diluted with other gases, the air in a well-ventilated mine never shows any appreciable depletion of its oxygen content. Even in poorly ventilated places it is exceptional to find less than 20 per cent. of oxygen except where other gases are being generated in considerable volume whereby the air is diluted and the percentage of oxygen correspondingly diminished. This fact has been well established by innumerable tests of mine air made at different mines and under varying conditions of ventilation.

THE ATMOSPHERE

The atmosphere is the aërial envelope surrounding the earth. The term is also used to describe the air or gaseous mixture filling any given space; as, for example, the mine atmosphere is the air and gases filling the mine or any portion of the workings.

Atmospheric Pressure. The weight of the air surrounding the earth causes a pressure, which decreases as the height above the surface increases; and the density of the air decreases in like manner, with the elevation above sea level.

Variation of Atmospheric Pressure.-Atmospheric pressure at any given place varies irregularly with the condition in respect to storms; the storm center being always an area of lower pressure than that surrounding the storm. In this country, a variation of 2 in. of mercury (say 1 lb. per sq. in.) in atmospheric pressure, in 48 hr., is not uncommon.

There is also a regular daily variation, the pressure attaining a maximum about 10 o'clock and a minimum at 4 o'clock, morning and evening. There is, likewise, a yearly variation, the general pressure reaching a maximum, in the northern hemisphere, in January and a minimum in July.

THE BAROMETER

The Mercurial Barometer.-The pressure of the atmosphere is measured by the height of mercury column it will support against a vacuum. The mercurial barometer is a glass tube. about 36 in. long, closed at one end. This is first filled with mercury and then inverted. The open end being immersed in a basin of the same liquid, the mercury in the tube will fall to a height above the surface of that in the basin, such that the pressure of the atmosphere acting on the surface of the liquid in the basin will support the mercury column in the tube.

Barometric Pressure. The pressure of the atmosphere expressed in inches of mercury is called the barometric pressure. For example, at sea level, the atmospheric pressure will commonly support 30 in. of mercury column; or is equivalent to a barometric pressure of 30 in.

Calculation of Barometric Pressure.-One cubic inch of mercury (32°F.) weighs 0.49 lb. A barometric pressure of 30 in., therefore, indicates an atmospheric pressure of

which is the normal pressure at sea level.

Calculation of Water Column. The height of water column, in feet, the atmospheric pressure will support is found by multiplying the pressure (lb. per sq. in.) by 2.3; or dividing the same by 0.434. Or the barometric pressure, in inches,

multiplied by one and one-eighth will give the equivalent water column, in feet. For example, at sea level,

Vacuum

Principle of the Barometer.-In the mercurial barometer the pressure of the atmosphere supports the column of mercury in the tube. The weight of the atmosphere counterbalances the weight of the mercury column, which rises as the atmospheric pressure increases and falls as it decreases. The height of the mercury column is therefore a true index of the pressure of the atmosphere at the surface of the earth, at the moment of taking the observation.

The principle of the balance pressure between the air and the mercury is clearly illustrated in Fig. 1, where a glass tube, closed at one end, is shown supported in a basin of mercury. The surface of the liquid in the basin is shown as divided into imaginary squares, by lines one inch apart; and the small arrow-heads represent the pressure of the atmosphere exerted on each square inch of surface.

FIG. 1.

Suppose for a moment, that the column of mercury in the tube is exactly one square inch in cross-section; it is evident, in that case, that the mercury column takes the place of the atmospheric pressure on one square inch of surface; and, since there is perfect equilibrium, its weight is equal to the pressure of the atmosphere per square inch.

Furthermore, whatever the sectional area of the mercury column, it is clear that its weight will always equal the atmospheric pressure for the same area of surface. Hence, the area of mercury column is not important, but its height only.

If the weight of one cubic inch of mercury (0.4911 lb.) be multiplied by the observed height of the column of mercury measured in inches, the product will be the pressure of the atmosphere, in pounds per square inch, at the place where the observation was taken. This assumes, that the barometric reading has been reduced to a standard reading, at a temperature of 32 deg. (Fahr.), which must be done when making accurate determinations.

Standard Barometric Readings.-Owing to the fact that the mercury in the tube expands and contracts more rapidly than the glass of the tube, the reading of the barometer will vary slightly for the same pressure, at different temperatures.

In comparing barometric readings taken at different times and at varying temperatures, it is necessary to carefully note the temperature when the reading was taken and reduce the observed reading to a so-called standard reading at 32 deg. F.

Calling the standard reading H, the observed reading h and the temperature t (Fahr.), the corrected reading is found by the formula,

For example, the standard reading corresponding to 30 in. of barometer, observed at a temperature of 60 deg. is = 29.64 in.

30 (1 - 0.0002 × 60)

It is even possible, owing to the more rapid expansion or contraction of the mercury than of the glass, that an observed fall of barometer may correspond to an actual rise in atmospheric pressure, or vice versa, within about 0.4 in.

Description of the Instrument. In the illustration, Fig. 2, is shown the common form of the standard mercurial barometer. The glass tube that contains the mercury column is here inclosed in the metal case A, to the bottom of which is attached a somewhat larger casing B. The latter holds a glass cylinder G terminated at the bottom with a chamoisskin bag, the whole forming the basin that holds the mercury. The entire case AB is hung in a truly vertical position, supported on a substantial base, as shown in the figure. The top