RE-ACTIONS DURING FLIGHT. 



265 



neither the frequency nor the extent ? Again, does riot the bird 

 advance in its onward course with variable rapidity ? Shall 

 we not find in the action of its wings a series of inipulses, 

 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 

 r-ignals of movements to a distance, and recording them by 

 tracings, Avhen these movements are made to pi'oduce a pres- 

 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 iu 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 



FIG. 112. Apparatus intended to transmit to the registering instalment all 

 tiic vertical oscillations of tbc bird. 



Fig. 112 shows the arrangement that we have adopted. 

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

 Mire-work protects the upper surface of the apparatus from 

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

 precaution, might sometimes affect the form, of the tracing. 



