INTRODUCTION. 



13 



commonly believed that a bird exerts quite an enormous 

 amount of power as compared with a fish ; a fish exerting a 

 much greater power than a land animal. This, there can be 

 no doubt, is a popular delusion. A bird can fly for a whole 

 day, a fish can swim for a whole day, and a man can walk 

 for a whole day. If so, the bird requires no greater power 

 than the fish, and the fish than the man. The speed of the 

 bird as compared with that of the fish, or the speed of the 

 fish as compared with that of the man, is no criterion of the 

 power exerted. The speed is only partly traceable to the 

 power. As has just been stated, it is due in a principal 

 measure to the shape and size of the travelling surfaces, the 

 density of the medium traversed, the resistance experienced 

 to forward motion, and the part performed by the mass 

 of the animal, when moving and acting upon its travel- 

 ling -surfaces. It is erroneous to suppose that a bird is 

 stronger, weight for weight, than a fish, or a fish than a 

 man. It is equally erroneous tff assume that the exer- 

 tions of a flying animal are herculean as compared with 

 those of a walking or swimming animal. Observation and 

 experiment incline me to believe just the opposite. A flying 

 creature, when fairly launched in space (because of the part 

 which weight plays in flight, and the little resistance expe- 

 rienced in forward motion), sweeps through the air with 

 almost no exertion.^ This is proved by the sailing flight of 

 the albatross, and by the fact that some insects can fly when 

 two-thirds of their wing area have been removed. (This ex- 

 periment is detailed further on.) These observations are 

 calculated to show the grave necessity for studying the media 

 to be traversed ; the fulcra which the media furnish, and the 

 size, shape, and movements of the travelling surfaces. The 

 travelling surfaces of animals, as has been already explained, 

 furnish the levers by whose instrumentality the movements 

 of walking, swimming, and flying are effected. 



1 A flying creature exerts its greatest power when rising. The effort is of 

 short duration, and inaugurates rather than perpetuates flight. If the volant 

 animal can launch into space from a height, the preliminary effort may be 

 dispensed with as in this case, the weight of the animal acting upon the 

 inclined planes formed by the wings gets up the initial velocity. 



