452 THE POPULAR SCIENCE MONTHLY. 



x x, which corresponds to the zero of pressure on the dynamometer. 

 At the instant of the beginning of the translation the apparatus shows 

 an energetic pressure (a) on the dynamometer ; this is the initial vari- 

 able resistance caused by the inertia of the air against which the 

 screen pushed. Very soon afterward the curve falls, announcing that 

 the resistance of the air has diminished, although the motion of the 

 disk had remained uniform. This is owing to the fact that the air had 

 then partly acquired the motion of the screen. The pressure falls 

 thus to the level b, which marks the constant resistance of the air 

 during the whole remaining period of the translation. 



Finally, when the apparatus is suddenly stopped, we see that the 

 trace of the recording-lever is suddenly depressed at the point c ; this 

 is due to the variable terminal condition : it consists in the carrying 

 of the screen forward by the column of air already set in motion by it. 

 This negative resistance gradually ceases, and the tracer returns to zero. 



We were not able to determine with this rough apparatus the ab- 

 solute value of the resistance of the air corresponding to different in- 

 stants of uniform translation, but we can readily see that there exist 

 two variable states, of which one precedes and the other follows the 

 constant resistance of the air. The studies of physicists have hereto- 

 fore been directed only to the determination of the constant resistance 

 corresponding to different velocities. 



Second Series of Experiments. Increase of the resistance of 

 the air to the downward movement of the wing of a bird, caused by the 

 horizontal translation of the bird. 



In making the above determination, I have roughly imitated the 

 construction of the bird, by reducing each of the wings to a thin and 

 rigid plane of a metre in length, and -^ a metre in breadth. These 

 two wings are simultaneously depressed by the action of a spring. 



A constant amount of work is thus employed for each blow of the 

 wings. The translation of the artificial bird takes place in gliding 

 along an horizontally-stretched iron wire. Two large wheels, one of 

 them furnished with a crank-handle, move an endless cord parallel to 

 the iron wire. The apparatus with wings is attached to this cord, 

 and can thus be moved horizontally with greater or less velocities. 



It is now necessary to determine with precision the velocity of 

 translation and the duration of the depression of the wings. The 

 graphic method gives readily these two measurements : 



1. Measurement of the Velocities of Translation of the Apparatus. 

 This velocity is evidently the velocity of a point on the endless cord, 

 to which the winged apparatus is attached. This cord passes around 

 a little pulley, whose revolutions are counted and registered on a re- 

 volving cylinder by means of a lever which is worked pretty much as 

 the lever in the registering apparatus of Morse's telegraph. 



The little pulley which serves to measure the velocities is exactly 



