NO. 3 APPENDIX 281 



I must preface what follows by a little statemenl of the things which partic 

 ularly interest me here and which are nol a naturalist's ordinary concern. 



First, I want to know the CG of the bird when in flight. You will nnder 

 stand thai though there is bul one center of gravitj (here symbolized as CG), il 



may be considered (1) with reference to its position on the horizontal plan of 

 the bird with wings extended, when it will always lie Found somewhere in the 

 medial vertical plane, passing' through the body, and usuallj nearly at a certain 

 point with reference to length, the position thus considered being called CG, or 

 (2) the position of the same CG with reference to a vertical plane passing trans 

 versely through the medial line, the position thus considered being called ( 

 In the latter case you will understand that the CG which is that of the wh 

 body, wings and all, will be carried more or loss upward when the wings are 

 thrown high up, and will be carried temporarily downward when (lie wings are 

 at their lowest point of the stroke. It would have a certain position when the 

 bird was at rest and another position when it was soaring and the wings wen- 

 above the body. 



The soaring bird is chiefly held upward by the pressure of the air under 

 each wing, and just as the common center of gravity i> a point where all the 

 efforts of gravity are supposed to be centered, so there is a common center of 

 pressure, or one point where all the efforts of the upper pressure of the air 

 may be supposed to be centered, and it will be clear, on very little consideration 

 that this latter point must be always nearly in a \ erl Leal line through the CG, and 

 usually above it. Call it CP. 



CG, and CP, are then, the symbols of ( 'G and CP as referred to the horizontal 

 plane. CG 2 and ('!', are the symbols for the corresponding ones when referred 

 to their position in the vertical plane. 



I shall be glad to explain to you, if you are not familiar with it, the simple 

 method of finding the CG, and CG 2 . It consists in bending the wings into just the 

 position that they would ordinarily occupy above the body in plain soaring flight, 

 keeping them there by a very light bent stick or wire, then hanging the bird up bj 

 a line attached to the tip of one wing, and see where this line would pass through 

 the body of the bird, for the CG will he somewhere in this line. After marking 

 then, on the body of the bird its position, hang it up a second time by the head or 

 tail and note again where the new vertical line runs in the new position. There 

 is hut one CG and but one point in which two straight lines can cross, and that will 

 be the CG necessarily. Note with all care just where this is above or below the 

 center of the body of the bird. 



As for the CP for either wing, that may be nearly found by tracing the wing- 

 on a flat piece of thick paper or cardboard strong enough not to bend much 

 cutting out the tracing and balancing it well on the point of a pencil— the point 

 about which it balances is very near CP, or the center of pressure in the vertical 

 plane. There is such a point of course in each wing, and when they are thrown 

 up in the actual position that they have in calm soaring flight, we may suppose a 

 horizontal line drawn between them, and it is the distance from this horizontal 

 line to C(l. compared with the area of the wings or with the distance between 

 their extended tips which we want to know, which gives the vertical distance 

 which the CG is below the CP, the thin-' we want to know. 



It will be very convenient also to have a wing dissected from the body and 

 the win<>- itself held in about the soaring curve by a hit of light stick balanced on 

 a pencil point, which will give the CG of the win- as distinct fro,,, that ol the 

 body However, the three things I principally want, beside a sketch or photo 



