The soaring bird is chiefly held upward by the pressure of the air under each wing, and just as the common center of gravity is 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 vertical line through the CG, and usually above it. Call it CP.
CG1 and CP1 are then, the symbols of CG and CP as referred to the horizontal plane. CG2 and CP2 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 CG1 and CG2. 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 by 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 be 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 but 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 CP2 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 CG2 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 thing we want to know.
It will be very convenient also to have a wing dissected from the body and the wing itself held in about the soaring curve by a bit of light stick balanced on a pencil point, which will give the CG of the wing as distinct from that of the body. However, the three things I principally want, beside a sketch or [p288] photograph of the bird from about its own level coming directly toward or going directly away in soaring flight, are these:
Approximate weight of the bird,—and approximate tracing of its extended wing with the area, so that we can tell the area of the supporting surface relative to the weight, and finally, the distance between CP2, and CG2, which is obtainable by the process which I have explained.
I am afraid that what I have just been describing at such length may have a certain obscurity to you, but if you will give me an opportunity, I shall be pleased to illustrate it with the actual experiment when the bird is hung up by a string, and you will see that it is in reality simple.
Referring to the sketches on page 3 of this communication, a and b correspond to the centers of pressure on either wing where the upward pressure of the air distributed over each wing may be supposed to be gathered in a single point. This, as I have said, is called the center of pressure with reference to the vertical flight, and its symbol is CP2, while the horizontal dotted line between them represents the level of CP2, from the best estimate that I could make when the wings are in their natural position of soaring. It is evident that this line passed far above the body of the vulture, and if (the corresponding symbol for the height of the center of gravity being CG2), the CG2, of the entire bird be taken, it will be found to lie nearly in the point c. Where c is in the present case, I could not determine exactly in my hasty examinations in the live bird, but I assume that it is about 12 way between the central horizontal axial line of the bird’s body and the upper portion. I repeat that it is important to me to know what the vertical distance is between CP2 and CG2 in each specimen of soaring bird. I may observe in illustration that in the common sea-gull, it is nearly as shown in the faint sketch; that is to say, that the corresponding line a b in the soaring gull passes distinctly through the upper part of the body, and the distance down to the CG2 of the whole in the gull is almost nil, while in the buzzard it is very considerable as shown by the corresponding distance in the “John Crow.”
Now, what I want to get from you is the corresponding figures for an average specimen of our Washington buzzard. If you will kindly have one killed and weighed while fresh, and before the rigor-mortis has set in, first noting the position of its wings when soaring in a calm, and (if possible) when coming toward you or going away in about a horizontal plane with your eye, in which position the wings will be elevated and bent somewhat as in the case of the above sketch of the “John Crow”; if you will kindly do this, so as to give me corresponding facts with reference to the buzzard, namely weight, area of extended wing surface, distance between tips as bent up in ordinary flight, distance between extended tips, the quantity CP2−CG2, and also will make such a tracing of the buzzard’s wing as Mr. Manly will show you of the “John Crow’s,” I shall be obliged.
My impression is that the buzzard is a considerably heavier bird than the “John Crow,” without, however, very much greater spread of wing. I may observe that when the wings of the Jamaica bird were spread out, they were spread quite to their utmost extent, and the distance between the tips of the terminal feathers was much greater than when in flight. I wish you would kindly also add the scientific name of the “John Crow,” with any particulars that you would think of interest.