Fig. 12.—Meteorograph lifted by Kites at Blue Hill.
The method of making a kite-flight for meteorological purposes at Blue Hill is as follows: a kite, fastened by a long wire to the ring in the main wire, being in the air, and the meteorograph suspended, another kite is attached to the ring by a shorter cord ([Fig. 12]). They are then allowed to rise, and to unreel the wire, until its angle with the horizon becomes low, when, by means of the clamps described, other kites are added, the number depending on the size of the kites and the strength of the wind. After a pause at the highest attainable altitude, the winch is connected with the steam-engine and the kites are drawn down. The pauses at the highest point, and when kites are attached or detached, are necessary to allow the recording instruments to acquire the conditions of the surrounding air; and because at these times the meteorograph is nearly stationary, measurements of its angular elevation are made with a surveyor's transit, while observations of azimuth give the direction of the wind at the different heights. The time of making each angular measurement is noted, so that the corresponding point on the trace of the meteorograph may be found. From the length of the wire and its vertical angle, the height of the meteorograph can be calculated, it having been found that the sag of the wire, or its deviation either in a vertical or a horizontal plane from the straight line joining the kite and the reel, does not cause an error exceeding three per cent. in the height so computed. When the meteorograph is hidden by clouds, the height above the last point trigonometrically determined is computed from the barometer record by Laplace's formula. At night there is only the barometer from which to determine the height; for although an attempt was made to use a lantern to sight upon, yet it soon became invisible, or, when seen, was confounded with the stars. Before and after the flight the meteorograph is hung upon a tripod in the free air, in order that its thermometer and hygrometer may be compared with the standards.
HEIGHTS ABOVE SEA-LEVEL OF KITE-FLIGHTS.
(Blue Hill is 630 feet above the sea)
| Year | No. of Records | Heights in Feet | Percentages of Records above | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Mean of Maximum | Absolute Maximum | 500 m. (1640 ft.) | 1000 m. (3280 ft.) | 1500 m. (4920 ft.) | 2000 m. (6560 ft.) | 3000 m. (9840 ft.) | |||
| 1894 | 2 | 1,860 | 2,070 | 50 | 0 | 0 | 0 | 0 | |
| 1895 | 28 | 1,673 | 2,490 | 59 | 0 | 0 | 0 | 0 | |
| 1896 | 86 | 2,772 | 9,327 | 78 | 28 | 9 | 4 | 0 | |
| 1897 | 38 | 4,557 | 11,716 | 95 | 68 | 45 | 21 | 5 | |
| 1898 | 35 | 7,350 | 12,070 | 100 | 92 | 80 | 66 | 20 | |
Since the use of wire and more efficient kites, the heights have been greatly increased. Thus the average height above the hill attained by the meteorograph in thirty-five flights made during 1898 was more than a mile and a quarter, whereas the average height of all the ascents prior to 1897 was about a quarter of a mile (see [Table]). The average height of the meteorograph above the hill, in all the flights during August 1898, was nearly a mile and a half, and on August 26 the meteorograph was raised 360 feet higher than ever before, its altitude, determined trigonometrically, being 11,440 feet above Blue Hill, or 12,070 feet above the neighbouring ocean. The meteorograph was suspended from the topmost kite, one of the Lamson pattern, having 71 square feet of lifting surface, and this was increased to a total of 149 square feet by four kites of the modified Hargrave type, that were attached at intervals to the wire. The five miles of wire in the air weighed 75 lbs., and the total weight including kites and apparatus was 112 lbs. The meteorograph left the ground at 10:40 a.m., attained its greatest height at 4:15 p.m., and returned to the ground at 8:40 p.m., a feat which it would be difficult for a man to equal on a mountain. The cumulus clouds were traversed three-quarters of a mile from the earth, and above them the air was found to be very dry. On the hill the air temperature was 72°, when it was 38° in the free air 11,440 feet above, and the wind velocity increased from twenty-two to forty miles an hour. These figures give an idea of the change of atmospheric conditions which occurs, but the conclusions deduced from the Blue Hill kite-flights will be discussed in the next chapter. However, the phenomena of atmospheric electricity, which have become noticeable since the use of wire, may be described here. Generally, whenever the kites rise above seventeen hundred feet, the wire becomes strongly charged with electricity, and when the great heights are reached the electricity is discharged in long and brilliant sparks at the reel, often to the inconvenience of the attendants. Usually, the electrical potential increases with altitude, and it is greatest during snow-storms or when the conditions favour thunder-storms. Notwithstanding its intensity, the quantity of electricity in the atmosphere is probably insufficient to make its collection and storage for practical purposes worth while.
It must not be imagined that kite-flying for meteorological purposes is a sinecure. At Blue Hill about two hundred flights have been made in all seasons and in all weathers, with temperatures varying from -5° to +90°, in gales, in rain, and in snow-storms, though not in thunder-storms. Sometimes the kites are invisible from almost the time they leave the earth until their return, but when the upper kites are visible it is necessary to observe them with theodolites every few minutes. Remembering that a high flight occupies ten or twelve hours, and frequently terminates late at night, or even continues until morning, it will be obvious that the work requires skill, energy, and perseverance, which have been shown by my assistants at the Blue Hill Observatory who have conducted the flights.
Occasionally, for lack of wind or from breakage of the line, the kites fall to the ground, usually intact. If they were visible, trigonometrical measurements on the hill enable the place of descent to be located, and then the kites and meteorograph are sent for and the wire is reeled up. But at night, or when clouds hide the kites, the direction in which they fall is not known, because the azimuth of the wire at the reel often differs from that of the kites; so last autumn several hundred miles of road, path, wood, and swamp were traversed before the aërial apparatus, which had been lost during a flight at night, was found comparatively close at hand.
From what has been said it will be evident that a former toy has been proved to be of the greatest importance for meteorological investigation at the Blue Hill Observatory. On account of the success there attained it is coming into use elsewhere for meteorological observations. In 1898 the United States Weather Bureau created seventeen kite stations, chiefly in the Mississippi Valley, with the intention of obtaining data every day, at the height of a mile or more, with which to plot a synoptic weather map similar to the map that is now drawn from the data at the ground. From a knowledge of the weather conditions prevailing simultaneously in the upper and lower air, it was expected that the weather forecasts could be improved, but unfortunately, on account of the light winds during the summer, it was impossible to make enough simultaneous kite-flights to construct the upper-air map, and therefore the scheme was abandoned. However, the data obtained will no doubt furnish valuable information about the vertical temperature gradient, etc., in various conditions of weather. The chief meteorological bureaus of Germany and Russia are equipping stations with kites and balloons, and M. Teisserenc de Bort, who has provided his private observatory near Paris with kite apparatus of the Blue Hill type, has already reached high altitudes. In Scotland too, which was the birthplace of scientific kite-flying, experiments have been resumed by a Scotchman and an American—a happy union of forces.