202 ON SOARING FLIGHT. 



So deliberate seemed to be their purpose, and so well executed, tbat 

 for the iirst time it occurred to me that these birds might be masters 

 of the situation and able to command the winds almost at their will, 

 and that nature had stored up for their use in the heated strata near 

 the earth inexhaustible stores of energy which they might take and 

 utilize as their needs might require; and that they had learned, what 

 man had never suspected, that the heated air so situated could make 

 its escape from the earth with difficulty, and that the simple maneuver 

 of circling over one spot facilitated its rise and produced an ascending 

 current at the precise locality and time where it was needed. Obser- 

 vation thus first suggested the hypothesis that the bird had it in his 

 power, through the unstable equilibrium of the air, to generate ascend- 

 ing currents; subsequent observation seemed to confirm my supposi- 

 tion, and later numerous experiments seemed to indicate that such 

 currents could be produced artificially. All my observations have gone 

 to show that the difiusion of heated air through cold air is widespread 

 and rapid. A column of smoke from a furnace chimney a few square 

 feet in cross section in an incredibly short time becomes so diffused 

 as to have a cross section of many linndred square feet. In a strong 

 wind we may all have seen dense black volumes of smoke from a loco- 

 motive vanish almost in an instant. A balloon filled with heated air 

 rises steadily and rapidly, because its mass is kept intact, but the same 

 air in ascending through a stable atmosphere is disrupted, diffused, and 

 lost, and can only rise through the slow process of permeation. Fur- 

 tliermore, the more stable the equilibrium of the upper air the more 

 rapidly will diffusion take place and the more slowly will the heated air 

 rise, so that an accumulation of heated air is continually taking place 

 in the lower strata, and the tension is only relieved at intervals with a 

 certain degree of violence by an upheaval of the masses. For the rest, 

 there seems to be no serious objection to the supposition that if the 

 equilibrium is unstable the birds are able to destroy it; for almost any 

 disturbance, and especially any so violent as vigorous flapping, would 

 perhaps be sufficient for the purpose. 



DIRECT PLIGHT IN HIGH WINDS. 



The hypothesis of ascending currents is applicable to spiral flight 

 only, and can not account for direct flight without loss of elevation, for 

 the reason that under the actual circumstances of flight currents of the 

 necessary magnitude and strength can not by any rational supposition 

 be accounted for. Thus, it is incredible that the albatross, while wander- 

 ing back and forth for miles across a stormy sea at an elevation seldom 

 exceeding 100 feet, can be supported by ascending currents like those 

 which, there is good reason to believe, carry the laud birds upward in 

 light winds. Any ascending current which might exist would of neces- 

 sity be swept on hy the winds which enveloped it, as tornadoes are 

 known to be, and if the birds were swept on with it it could only be 



