28 



The Irish NaUiralisi. 



February, 1921. 



the obstacle that it also presupposes cojiditions in the atmosphere widely 

 different from what we have reason to believe to prevail — at least in the 

 higher regions — while it is far from clear that the feat of soaring would 

 be adequately explained by the most elaborate stratification of swifter 

 and slower currents. It is w^orth noting that in the late ]\Ir. F. W. 

 Headley's important work, /' The Flight of Birds " (published in 19 12) 

 the opinion is once more decidedly maintained that soaring can only be 

 effected in ascending currents. Mr. Headley frankly confesses that the 

 " var^dng velocities " theory is unintelligible to him, and he gives the 

 full weight of his authority to the time-honoured view that soaring in a 

 horizontal and uniform wind would be impossible. 



It is the uniformly acknowledged impossibility that Colonel de Villamil 

 puts forward as the only intelligible and perfectly simple solution of the 

 whole riddle. In his view, the time-honoured and hitherto unchallenged 

 opinion that no bird could soar in a horizontal and uniform wind is based 

 on sheer forgetfulness of the principle of the conservation of energy. 

 The soaring bird rises with an energy that it has " trapped " from the 

 wind before commencing its spiral ascent, and continues to " trap " 

 afresh during each down-wind half of the circles it describes in the whole 

 course of the spiral movement. 



Those who wish to follow Colonel de Villamil's quantitative estimates 

 of the various steps in a soaring bird's progress should consult his 

 pamphlet ; but his general conception can be made clear without them. 

 Supposing a horizontal wind with sufficient force acting or " doing work 

 upon " a bird -that is flying with the ordinar}^ " rowing " action a more 

 or less down-wind course, he shows that the bird will soon have acquired 

 a velocity and a kinetic energy much in excess of what it would have 

 possessed but for the work of the wind. Converting some of this 

 superfluous kinetic energy into potential energy of height, it ceases to 

 flap, adjusts its wings at an appropriate angle, and rises for the first half 

 (the upwind half) of the first round of its spiral. The second or down-wind 

 half of the round is a mere gliding descent, in which the wind again works 

 on the bird and returns to it the energy it had expended during the 

 up-wind curve. There is, therefore, nothing to prevent the spiral from 

 'being continued ad infinitum, provided the wind remains uniform. As 

 observers are agreed that in actual soaring the rise is limited to the 

 up-wind half of each round. Colonel de Villamil's explanation certainly 

 seems to stand the test of meeting the known facts of the case with an 

 exactness that seems almost above criticism. It will give an added 

 interest to the fine spectacle of a Heron or a Gull " screwing up " into 

 the air to endeavour to compare its movements with those required by 

 this " simple mechanical solution " — though the full weight of Colonel 

 de Villamil's reasoning can only b,e appreciated by those having some 

 acquaintance with the more powerful soarers — the Vultures, Albatrosses, 

 Adjutant Birds, etc.-^that can be watched in more southern regions. 

 That the author has hit on the real clue to a very ancient puzzle will 

 ficarceb' be denied. 



C. B. Moffat. 



