To such end the writer in 1892 devised an anemograph for recording simultaneously the speed of the wind and its horizontal and vertical components of direction, while Dr. Langley devised a very light and delicate cup anemometer for recording the variations of wind speed in a horizontal plane, but not the changes of direction. Both instruments were set up in January, 1893, and both investigations were published with the Proceedings of the International Conference on Aërial Navigation of that year; but neither investigation was pushed far enough to prove conclusively the possibility of a particular bird or model soaring in the particular wind recorded. The two together did, however, reveal quite astonishing fluctuations of the wind in both speed and direction, results that have since received ample exemplification in the more extended records of other observers.
Fig. 56.—Universal Anemograph. (The vanes are high above the point indicated by the break in the vertical pipe.)
Fig. 56 shows the recording anemometer for speed and double direction constructed by the writer in 1892. A large weather vane was firmly strapped to a vertical pipe which turned freely on ball bearings and, by means of a small crank actuating a chronograph pencil, recorded its fluctuations on a long sheet of paper winding on the drum from a roll behind. On top of the pipe and about fifteen feet from the ground, was mounted a carefully balanced horizontal vane, from which a fine steel wire ran down the axis of the pipe to a fixed pulley, thence to a second recording pencil. A third pencil recorded the beats of a pendulum, thus standardizing the speed of the paper. A fourth pencil, not shown, was designed to record the turns of an anemometer mounted near the top of the pipe. The records of the wind speed thus secured are omitted for lack of standardization, as the experiments were prematurely terminated.
Fig. 57.—Records of Wind Variation in Horizontal and Vertical Direction.
Typical records of the wind direction are shown in Fig. 57 in which the circles represent the paths swept by the wind-vane cranks that operated the corresponding pencils. Both vanes, as shown by their diagrams, veered quite frequently ten degrees in a short interval of time, and not seldom twenty to thirty degrees. Frequently, also, it was observed, in scanning the various records, that a rise or lull in the wind speed was accompanied by a corresponding variation in direction; but the observations were not sufficiently numerous and extended to establish this phenomenon as a general occurrence. But as it can be shown theoretically that a horizontal stream of air of constant cross section and uniform velocity at each section, can not greatly fluctuate in velocity from point to point, without more pronounced changes of density than the barometer records, it naturally follows that the stream must broaden where the air speed lags, and narrow where it accelerates; in other words, it follows that there must be some change in direction. The records were taken in the middle of a clear open space of two hundred acres at Notre Dame University on a sunless day in January, 1893, when the temperature was 24° F., and the wind eight to twelve miles per hour. Their application to the theory of soaring need not be considered here.
Further studies of the wind pulsations were made by use of a toy balloon attached to a long thread. The first trials are thus recounted in the paper above cited:
“After some preliminary tests from the top of the Physical Laboratory of the Johns Hopkins University, during the Easter vacation of 1893, I ascended the Washington Monument at Baltimore, where I paid out the exploring line at a height of 200 feet. The wind was blowing toward the southeast at the speed of 25 to 35 miles per hour, and the sky, which had remained clear till 3 o’clock, was rapidly darkening, with indications of approaching rain. The balloon, when let forth, immediately fell to a depth of 30 or 40 feet, being caught in the eddy of the monument, then presently encountering the unbiased current, sailed in it toward the southeast, approximately level with the spool end of the thread. After the balloon had drawn out 100 feet of thread I checked it to observe the behavior of this much of the exploring line. The balloon rose and fell with the tossing of the wind, but did not flutter like a flag, as it would do if formed of irregular outline. Neither did the thread flutter, nor do I believe there is ever a tendency in a line greatly to flutter in a current as does a flag or sail. Presently I paid out 300 feet of the exploring line, whereupon the waves in the thread became quite remarkable. The thread then, as a rule, was never approximately straight. Sometimes it was blown into the form of a helix of enormous pitch; at other times into the form of a wavy figure lying nearly in a single vertical plane; and again, the entire exploring line should veer through an angle of 40° to 60°, either vertically or horizontally. The balloon, of course, seldom remained quiet for more than a few seconds at a time, but tossed about on the great billows like a ship in a storm. Quite usually the billows could be seen running along the line from the spool to the balloon, and, as a rule, several different billows occupied the string at one time.
“The observations just delineated, however curious they may be, afford no adequate conception of the behavior of the air currents over an open plane, nor at a great height above the earth, because the Washington Monument at Baltimore stands but 100 feet above the surrounding buildings, which undoubtedly send disturbances to a greater height than 200 feet. To supplement these explorations, therefore, I determined to have them repeated from the top of the Washington Monument at Washington and the Eiffel Tower at Paris.”
Some months later in the year, the experiment was repeated at the top of the Washington Monument in Washington, at a height of five hundred feet. The balloon, with a stone attached, was paid out from the north window of the monument till it reached the ground. Then the stone was removed by an assistant who drew the balloon well away from the huge eddy of the great shaft, and let it fly toward the east, drawing the thread after it like a mariner’s log in the wake of a ship. When six hundred feet of the thread had been let out, it was observed to veer in all directions under the varying surges of the wind. These variations seemed larger than could be expected from the wake of the shaft alone near its summit, where it measures about thirty feet in thickness.