When the atmospheric pressure is decreased it has the same effect on the individual that the breathing of rarefied or diluted air has. In this event the adaptation consists in an increased rate of breathing. It is also asserted that the number of red blood corpuscles is increased, which in turn increases the capacity of the blood to carry oxygen. In the diminished air pressure there is a marked decrease in the amount of oxygen absorption and this accounts for the increase in the respiration and quickened pulse rate. As previously stated, if the change from a normal air pressure to a greatly decreased pressure takes place gradually, thus giving Innate sufficient time to bring about an adaptation, man will be enabled to live in a healthy state in a rarefied air. As the altitude is increased the air pressure is decreased. This is because there is less air above to exert pressure. Naturally the air near the earth has the greatest weight above it to exert compression. The air pressure decreases gradually until, it is assumed, it is gradually thinned into a perfect vacuum. If oxygen is added to the air it is possible to maintain life at a remarkably reduced air pressure. A bird was kept alive by P. Bert in oxygenated air at a pressure of 0.1 of an atmosphere. The =symptoms= produced in diminished air pressure will vary with the general conditions of the individual. People suffering from cardiac disturbances will be affected most by high altitudes. This is possibly the first symptom to be manifested in rarefied air. If there is any inclination toward heart weakness it will be observed when the air pressure is decreased. The effects of reduced air pressure will vary with circumstances. There will be increased and deep breathing, with an increase in the pulse rate. There is likely to be dizziness and ringing in the ears. The sense of hearing, of sight and possibly of sensations, become impaired. There may be drowsiness and the patient may have a strong desire to sleep. The intellectual faculties become dull. Where the change has taken place very suddenly to the decreased air pressure there will be syncope, dyspnea, dizziness, nausea and weakness. These symptoms are commonly known as mountain sickness. Increased Air Pressure The greatest air pressure in nature is at sea level. Even the increased pressure in the deepest mines is so slight that it has no physiological significance; hence man is exposed to increased air pressure only under artificial conditions. This subject will be referred to again under the head of Industrial Hygiene.