12 SCIENCE PROGRESS 



Chicago, for 191 1 (vol. xix. pp. 696-745) but not noticed in my 

 review of the work of that year. This same paper also contains 

 a restoration of Nyctosaurus. 



In connexion with the above may be noticed a very in- 

 teresting article by Messrs. E. and A. Harle, published in Bull. 

 Soc. Ge'ol. France, vol. xi. pp. n 8-21, on the means by which the 

 giant pterodactyles of the American Cretaceous were enabled 

 to fly. For permission to reproduce, in a somewhat condensed 

 form, the following abstract of this most interesting article, I 

 am indebted to the editor of The Field. 



Some of these pterodactyles had a wing-expanse of at least 

 from 21 to 24 ft., whereas the largest flying birds of the present 

 day, such as the albatrosses, condors, lammergeiers, and mara- 

 bout storks, have not more than about half the bulk of the 

 former, although they have probably attained the maximum 

 size compatible, under present physical conditions, with the 

 power of flight. For studies of the flight of birds and insects 

 in connexion with the theory of aeroplanes have demonstrated 

 that the power necessary to propel animals through the air 

 varies per unit of weight approximately as the sixth root of 

 the weight ; that is to say, as the square root of their dimensions. 

 Accordingly, the power required increases more rapidly than 

 the weight and the dimensions. If, for instance, the dimensions 

 be quadrupled a power is required per unit of weight equal to 

 that originally sufficient multiplied by the square root of four ; 

 that is to say, the power must be doubled per unit of weight. 

 From this it is evident that a limit to the weight, and conse- 

 quently to the size, of animals capable of flight must be reached. 

 But the pterodactyles of the Cretaceous greatly exceeded these 

 limits, yet, from the situations in which their skeletons are 

 found in Kansas, it is evident that they were able to fly dis- 

 tances of at least 100 miles from the shore. Probably they 

 performed skimming flights above the waves in pursuit of 

 surface-swimming fish, for the capture of which the structure 

 of the skull and beak seems adapted. Again, if we go back in 

 time to the Carboniferous period of France, we find gigantic 

 dragon-flies with a wing-expanse of from 28 to 32 in., which it 

 is certain would be unable to fly, from lack of sufficient pro- 

 pelling power, under present physical conditions, as they are 

 fully three times the size of the biggest of their existing 

 relatives. 



