M>ACTIONS DURING FLIGHT. 



265 



neither the frequency nor the extent ? Again, does not the bird 

 advance in its onward course with variable rapidity ? Shall 

 we not find m the action of its wings a series of impulses, 

 which give to its advancing course a jerking motion ? 



These queries can be answered experimentally in the follow- 

 ing manner. 



Since we have at our disposal the means of sending the 

 signals of movements to a^ distance, and recording them by 

 tracings, when these movements are made to produce a4)res- 

 sure on the membrane of a drum filled with air, we must 

 endeavour to reduce to a pressure of this kind the movements 

 which we desire to study. 



The oscillations which can be effected by the bird in a hori- 

 zontal plane must be made to exert on the membrane of the 

 drum pressures alternately strong or feeble, in proportion as the 

 bird mounts or descends. The same kind of experiment must 

 be made on the variations in its horizontal rapidity. 



The question has been already solved for the vertical 

 re-actions, by means of the apparatus represented in fig. 28, 

 when we were treating of terrestrial locomotion ; a slight 

 modification will allow us to employ the same method to 

 ascertain whether vertical oscillations are produced during 

 flight. 



tiG. 112.— Apparatus intended to transmit to the registering instrument all 

 the vertical oscillations of the bird. 



Fig. 112 shows the arrangement that we have adopted. 

 The mass of lead is applied directly to the membrane ; some 

 wire- work protects the upper surface of the apparatus from 

 the friction of the feathers of the bird, which, without this 

 precaution, might sometimes affect the form of the tracing. 



