246 Recent Literature. [kv% 



for soaring, but it is an undisputable fact that soaring is possible when 

 there exists, so far as it is possible to determine, a perfect calm. 



With the idea that the sun's heat possibly caused ascending currents in 

 the air Dr. Hankin made extended observations along this line, with the 

 result that the time of appearance of " heat eddies " indicating upward 

 air currents was found to coincide almost exactly with the time of the 

 beginning of soaring. 



He found, on the other hand, however, that " heat eddies " not directly 

 caused by the sun had no relation to the " soarability " of the air and 

 that when the solar energy that causes "heat eddies " was held back by 

 thin clouds, soaring continued, uninterrupted. 



Ordinary ascending air cm-rents from "heat eddies," therefore, seemed 

 not to be the basis of soaring nor did they seem sufficiently powerful, and 

 he concludes " if soarability is due to ascending currents caused by the sun's 

 rays, these currents must resemble heat eddies in being widely and appar- 

 ently uniformally distributed," but " they must differ in containing a great 

 deal more energy and in being as yet undiscovered." 



Mr. William Brewster's observations on Gulls sailing into the teeth of 

 the wind, near an advancing vessel were duplicated in Dr. Hankin's ex- 

 perience. He says the Gulls were observed in the usual " soarable area " 

 on the leeward side of the stern and also " gliding ahead of the ship in a 

 head wind, keeping the same speed for minutes together. Sometimes they 

 kept at a distance of only a few feet from the bridge and so under the best 

 condition for observation and yet no trace of any movement of the wings 

 could be observed." He adds " were these cases of the soarable area being 

 greatly extended or was the air uniformally soarable under the tropical 

 sun? " Mr. Brewster's observations go to disprove the latter suggestion 

 as they were not in the tropics. 



The evidence that Dr. Hankin has gathered seems to indicate that 

 " besides the effect of the air disturbance caused by the motion of the ship 

 another factor of importance is the nature of the wind .... some winds 

 are soarable and other winds are not soarable. Apparently in both cases 

 some unknown factor affecting soarability is involved." 



Dr. Hankin's observations are not limited to soaring birds but cover the 

 whole field of flight as the following chapter headings will show : ' Prelimi- 

 nary Description of Soaring Flight ' ; ' Preliminary Account of the Condi- 

 tions Necessary for Soaring Flight ' ; ' Preliminary Account of Directive 

 Movements in Gliding Flight ' ; ' On Conditions Affecting Sun Soarability ' ; 

 'A Further Description of Steering Movements'; 'Metacarpal Descent'; 

 'Arching ' ; ' Functions of the Tail ' ; ' Flapping Flight ' ; 'Lateral Stability' ; 

 'Position of Centre of Gravity' ; 'The Flight of Bats ' ; 'The Flight of Flying 

 Fishes ' ; ' The Flight of Sea Gulls ' ; 'Ascending Cm-rents ' ; ' Wind Soar- 

 ability'; 'Soaring in Stormy Winds'; 'Colour Phenomena of Soaring 

 Flight ' ; ' Relative Efficiency of Different Wing Forms in Respect to Soar- 

 ing Flight ' ; ' On the Flight of Dragon-flies ' ; ' Glossary ' . 



In turning over the pages of Dr. Hankin's volume one is astonished at 



