[1] A tail will prevent any kite from turning over, or "diving," because its weight keeps the lower end down while the pressure of the wind on the tail also pulls the lower end backward and maintains the necessary angle of the kite to the wind, the most efficient angle being about 22 degrees. Bending back the ends of the cross-stick gives stability to a kite because, when, on account of the eddies in the wind, a stronger pressure is exerted on one side of the kite, this side is driven backward, thereby presenting less effective surface to the wind, while as the other side comes forward more nearly at right angles to the wind, it receives greater pressure than before. In this way the equilibrium about the central stick is automatically maintained, the required inclination to the wind being secured by the greater surface presented to the wind below the point of attachment of the bridle.
In 1891 Mr. Eddy lifted a minimum thermometer by several of these kites flown tandem, and proposed to obtain in this way data to forecast the weather. In the Proceedings of the Aeronautical Conference, held in connection with the Chicago Exposition, Prof. M. W. Harrington, then Chief of the U. S. Weather Bureau, quoted Mr. Eddy's estimate of the cost of exploring the air by means of kites flown in series, and advocated their use.
Up to this time it does not appear that self-recording instruments—that is to say, those which make continuous graphic records—had been raised by kites. In the days of the early experimenters such instruments were too heavy and cumbersome to be lifted by the more or less unmanageable kites, but within the past few years M. Richard of Paris has made the simple and light recording instruments described in connection with balloons, which can be attached to kites. In this way it is possible to obtain simultaneous records at the kite and at a station on the ground, and from them to study the differences of temperature and humidity, and this seems to have been done first at Blue Hill Observatory. In August 1894 Mr. Eddy brought his kites to Blue Hill and with them lifted a Richard thermograph, which had been partly reconstructed of aluminium by Mr. Fergusson so that it weighed but 1¼ lbs., to the height of 1500 feet, and so the earliest automatic record of temperature was obtained by a kite. During the next summer, Mr. Eddy assisted again in the experiments at Blue Hill, and secured photographs of the Observatory and the hill by a camera carried between his kites to the height of a hundred feet or more.
Now that the possibility of lifting self-recording meteorological instruments to considerable heights had been demonstrated, an investigation of the thermal and hygrometric conditions of the free air was undertaken by the staff of the Blue Hill Observatory, who had already made an investigation of the movements of the clouds by the methods described in the second chapter.
The development of the kite and its accessory apparatus, and the acquisition of the knowledge how to use them, required much time, and resulted in the damage or loss of many kites. Two meteorographs, as the combination of two or more self-recording instruments is called, were dropped from a great height and no trace of them was found. When, however, by the breaking of the line both kites and instrument are carried away, the kites act as a parachute and bear the instrument gently to the ground, where both are usually recovered uninjured; to facilitate their return should they fall at a distance, the name and address are marked on each. It would be tedious to relate the ups and downs of scientific kite-flying at Blue Hill before the wind was successfully harnessed to the service of science, and the kites were prevented from kicking over the traces, or from breaking away, so only a brief account of the progress of the work will be given, and then the methods at present used will be described. At first the Eddy, or Malay kites, as they are also called, covered with paper or with varnished cloth, were coupled tandem to secure greater safety and lifting power. The principle of attaching kites at several points on the line was early adopted at Blue Hill, for although it can be demonstrated theoretically that a greater height is possible by concentrating all the pull at the end of the line, yet in the case of a line which is not infinitely strong the best results are got by distributing the pull, and in this way, too, kites can be added as the wind conditions aloft warrant. To obviate the frequent breaking of the bowed cross-piece, Mr. Fergusson made it in two pieces, each being held in a metal socket on the central stick, the two pieces forming a dihedral angle towards the wind. It had the advantage also of being readily taken apart for transportation. This kite, shown in [Fig. 8], flew at a high angle above the horizon and through a considerable range of wind velocity, but it could not be kept permanently in balance, or made to adjust itself to great variations in wind velocity, and therefore it was discarded.
Fig. 8.—Eddy tailless Kite.