第42章

1st.--That winds blowing five to seventeen miles per hour frequently had rising trends of 10 degrees to 15degrees, and that upon occasions when there seemed to be absolutely no wind, there was often nevertheless a local rising of the air estimated at a rate of four to eight miles or more per hour. This was ascertained by watching thistledown, and rising fogs alongside of trees or hills of known height. Everyone will readily realize that when walking at the rate of four to eight miles an hour in a dead calm the "relative wind" is quite inappreciable to the senses and that such a rising air would not be noticed.

2nd.--That the buzzard, sailing in an apparently dead horizontal calm, progressed at speeds of fifteen to eighteen miles per hour, as measured by his shadow on the ground. It was thought that the air was then possibly rising 8.8 feet per second, or six miles per hour.

3rd.--That when soaring in very light winds the angle of incidence of the buzzards was negative to the horizon --i. e., that when seen coming toward the eye, the afternoon light shone on the back instead of on the breast, as would have been the case had the angle been inclined above the horizon.

4th.--That the sailing performance only occurred after the bird had acquired an initial velocity of at least fifteen or eighteen miles per hour, either by industrious flapping or by descending from a perch.

An Interesting Experiment.

5th.--That the whole resistance of a stuffed buzzard, at a negative angle of 3 degrees in a current of air of 15.52 miles per hour, was 0.27 pounds. This test was kindly made for the writer by Professor A. F. Zahm in the "wind tunnel" of the Catholic University at Washington, D. C., who, moreover, stated that the resistance of a live bird might be less, as the dried plumage could not be made to lie smooth.

This particular buzzard weighed in life 4.25 pounds, the area of his wings and body was 4.57 square feet, the maximum cross-section of his body was 0.110 square feet, and that of his wing edges when fully extended was 0.244 square feet.

With these data, it became surprisingly easy to compute the performance with the coefficients of Lilienthal for various angles of incidence and to demonstrate how this buzzard could soar horizontally in a dead horizontal calm, provided that it was not a vertical calm, and that the air was rising at the rate of four or six miles per hour, the lowest observed, and quite inappreciable without actual measuring.

Some Data on Bird Power.

The most difficult case is purposely selected. For if we assume that the bird has previously acquired an initial minimum speed of seventeen miles an hour (24.93feet per second, nearly the lowest measured), and that the air was rising vertically six miles an hour (8.80 feet per second), then we have as the trend of the "relative wind" encountered:

6-- = 0.353, or the tangent of 19 degrees 26'.

17 which brings the case into the category of rising wind effects. But the bird was observed to have a negative angle to the horizon of about 3 degrees, as near as could be guessed, so that his angle of incidence to the "relative wind" was reduced to 16 degrees 26'.

The relative speed of his soaring was therefore:

Velocity = square root of (17 squared + 6 squared) = 18.03 miles per hour.

At this speed, using the Langley co-efficient recently practically confirmed by the accurate experiments of Mr. Eiffel, the air pressure would be:

18.03 squared X 0.00327 = 1.063 pounds per square foot.

If we apply Lilienthal's co-efficients for an angle of 6 degrees 26', we have for the force in action:

Normal: 4.57 X 1.063 X 0.912 = 4.42 pounds.

Tangential: 4.57 X 1.063 X 0.074 = - 0.359 pounds, which latter, being negative, is a propelling force.

Results Astonish Scientists.

Thus we have a bird weighing 4.25 pounds not only thoroughly supported, but impelled forward by a force of 0.359 pounds, at seventeen miles per hour, while the experiments of Professor A. F. Zahm showed that the resistance at 15.52 miles per hour was only 0.27 pounds, 17 squared or 0.27 X ------- = 0.324 pounds, at seventeen miles an 15.52 squared hour.

These are astonishing results from the data obtained, and they lead to the inquiry whether the energy of the rising air is sufficient to make up the losses which occur by reason of the resistance and friction of the bird's body and wings, which, being rounded, do not encounter air pressures in proportion to their maximum cross-section.

We have no accurate data upon the co-efficients to apply and estimates made by myself proved to be much smaller than the 0.27 pounds resistance measured by Professor Zahm, so that we will figure with the latter as modified. As the speed is seventeen miles per hour, or 24.93 feet per second, we have for the work:

Work done, 0.324 X 24.93 = 8.07 foot pounds per second.

Endorsed by Prof. Marvin.