236 ANIMAL MECHANISM. 



Let there bo two lines, x x, forming the axis of tlio 

 aKsrissiv, and y y that of the ordinates. Let us assume, that 

 all which is above the Hue of zeros, iu the full curve that is 

 to say, that which corresponds with a movement iu a forward 

 direction, ought to point to the right of the line y y. In- 

 versely, that all which is below the zeros, in the full curve, 

 will point to the left of the axis of y y. The position with 

 reference to this axis will be reckoned, parallel to it, by 

 means of milliuietric divisions. 



On tlto other hand, the different measurements taken on 

 the dotted curve (that which expresses the upward motion of 

 the wing) must point to the corresponding elevation, reckoned 

 above or below the line x x, according as these points in the 

 curve of the elevations are removed a certain number of 

 millimetres either above or below the ZL.TO line. 



Let us take as our point of departure, in the construction 

 of the new curve, the point c (fig. 107), chosen on the dotted 

 line, at one of the times when the wing has arrived at one of 

 its anterior limits. 



This point, according to the millimetric scale, shows us 

 that the wing is depressed 13 divisions beneath the horizontal 

 line. Let us follow the vertical line which passes through 

 the point c, till it meets with the curve of movement in the 

 nntero-posterior direction : the intersection of this vertical 

 lino with the curve shows us that the wing at this moment 

 had been carried forward 26 divisions; on the new curve, 

 therefore, the point a ought to bo marked at a well-ascertained 

 position c, which will be found at the intersection of the thir- 

 teenth division below the axis x JT, with the twenty-sixth to the 

 right of the axis y y, which according to what we have as- 

 Kiimcd, corresponds with 26 divisions iu the forward direction. 



To determine a second point in our curve, let us proceed, 

 in reading the tracings, one millimetric division farther to 

 the right; wo shall find, as before, the intersection of the 

 vertical at this point with the two curves, and we shall thus 

 h.ive a second point in the new construction determined. 



The scri' s of succps.-ive points obtained in this manner 

 form a carve which shows the course of the wing; the arrow 

 indicates the direction of the movement. 



