The amount of power which a land animal has to exert is always a fixed and definite quantity. If an animal weighing 100 lbs. has to ascend a hill 100 feet high, it always means the development of 10,000 foot-lbs. With a bird, however, there is no such thing as a fixed quantity. If a bird weighing 100 lbs. should raise itself into the air 100 feet during a perfect calm, the amount of energy developed would be 10,000 foot lbs. plus the slip of the wings. But, as a matter of fact, the air in which a bird flies is never stationary, as I propose to show; it is always moving either up or down, and soaring birds, by a very delicate sense of feeling, always take advantage of a rising column. If a bird finds itself in a column of air which is descending, it is necessary for it to work its wings very rapidly in order to prevent a descent to the earth. I have often observed the flight of hawks and eagles. They seem to glide through the air with hardly any movement of their wings. Sometimes, however, they stop and hold themselves in a stationary position directly over a certain spot, carefully watching something on the earth immediately below. In such cases they often work their wings with great rapidity, evidently expending an enormous amount of energy. When, however, they cease to hover and commence to move again through the air, they appear to keep themselves at the same height with an almost imperceptible expenditure of power. [Illustration: Fig. 3.--While in the Pyrenees I often observed eagles balancing themselves on an ascending current of air produced by the wind blowing over large masses of rock.] Many unscientific observers of the flight of birds have imagined that a wind or a _horizontal_ movement of the air is all that is necessary to sustain the weight of a bird in the air after the manner of a kite. If, however, the wind, which is only air in motion, should be blowing everywhere at exactly the same velocity, and in the same direction--horizontally--it would offer no more sustaining power to a bird than a dead calm, because there is nothing to prevent the body of the bird from being blown along with the air, and whenever it attained the same velocity as the air, no possible arrangement of the wings could prevent it from falling to the earth. It is well known that only a short distance above the earth’s surface, say 30 or 40 miles, we find an extremely low temperature sometimes referred to as interstellar temperature or absolute zero. In order to illustrate the extremely low temperature of space, I would cite the following instance:--