which it passed, at the same time that an aneroid barometer was photographed on the same sheet. The apparatus is hung vertically below the balloon; in the lower portion of the box is an objective which photographs the ground, and in the upper portion is a second objective which photographs the face of an ancroid barometer placed at the proper distance above. A clock-movement makes exposures every two minutes, and a sensitive film unrolled between the objectives receives the images on each side. If there are known, the focal length of the objective, the distance of two points on the ground, and the distance of two points on the photograph, a simple proportion permits the height of the balloon to be determined at that time, and consequently, from the barometric record, the law connecting the pressure with height can be deduced. The apparatus was successfully used in the voyage of a large balloon with observers, and the accuracy of the determination of height was found to be within. If the apparatus is to be used at great heights it would be necessary to protect the barometer and the camera from the very low temperatures. Besides the use for which it was designed, this apparatus may serve to trace the route of a balloon and to determine the horizontal velocity at the different points of its path.

The exploration of the high atmosphere by

ballons-sondes, which can aid so many investigations, has been utilized by M. Viollé to obtain actinometric measures, that is, to determine the amount of heat given by the sun, or what is called the "solar constant." This has been done on mountains with varying results, due to the changing amount of atmospheric absorption. In regions traversed by the balloon where the pressure of the air is reduced to a few inches of mercury, where there is a complete absence of water-vapour, and at heights to which terrestrial dust does not extend, the measure of the quantity of heat sent by the sun towards the earth is freed from almost all the errors which we encounter on its surface. The actinometer of M. Viollé is, in principle, a sphere of copper, blackened externally, and having inside a thermometric apparatus which registers some distance away. Under the action of the solar rays the sphere is heated, and assumes equilibrium when the loss by radiation and by contact with the air compensates for the gain by the absorption of the direct heat. While at low levels the atmosphere also contributes to heat the sphere, at great heights the sun shines from an almost black sky and alone heats the sphere. Since the balloon follows the wind the apparatus is protected from air currents which would otherwise introduce errors. Each twenty minutes a screen cuts off the

solar rays from the sphere so that it cools to the temperature of the air, which is also recorded. On account of its weight this apparatus has not yet been carried by a ballon-sonde, but it has operated successfully in a balloon with observers.

M. Teisserenc de Bort, who is actively engaged in exploring the air from a meteorological standpoint, has constructed a very sensitive thermometer made of a blade of German silver set in a frame of nickel-steel that does not expand with heat. This may be ventilated by a fan, and, while extremely sensitive to changes of temperature, it is not affected by shocks, and consequently is well adapted for use in ballons-sondes that pass rapidly through airstrata of varying temperature.

From this review of the development of the ballons-sondes it is evident that they offer possibilities of obtaining data in the high atmosphere, perhaps up to fifteen miles or more, which, though subject to inaccuracies, are of great interest to the physicist and astronomer. The meteorologist is chiefly concerned with that portion of the atmosphere which lies within two or three miles of the earth, and he requires, moreover, accurate measurements for his conclusions. The new and most satisfactory way of obtaining these data is by kites, and the remaining chapters will treat of this method. of exploring the atmosphere and the results.

CHAPTER V

KITES HISTORY AND APPLICATION TO METEOROLOGICAL PURPOSES AT BLUE HILL AND ELSEWHERE

KITES are supposed to have been invented four hundred years before the Christian era by Archytas, and at Smyrna the flying of kites remains a national sport to this day. We are told that two hundred years later, a Chinese general, Han Sin, employed kites as a means of communication with the garrison of a besieged town, and there is a legend about their use in Japan to dislodge and carry away a golden ornament from a tower. Whatever may be the truth of these stories, we know that kite-flying in the Malay Archipelago, in China, and in Japan, has been a pastime for all classes during centuries, and that the Asiatic people have always been the expert kite-fliers of the world.

Kites with tails seem to have been introduced into England about two hundred and fifty years

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ago, and Isaac Newton when a school-boy made some improvements in them. Notwithstanding the fact that generations of boys have flown kites and so eminent a mathematician as Euler investigated their theory, until recently kites remained toys unsuited for practical purposes. Since the tailless kite has become a familiar object, it has been said facetiously that kites lost their tails by the same process of evolution which deprived man of his caudal appendage; but as kites without tails have been flown in Asia for centuries, the truth is that the tailed kites were the ones first brought to Europe as playthings. To-day in Holland we see boys flying the English bow-kite and the common kite with crossed sticks, both of which require tails, and by the side of them tailless kites imported from the Dutch colonies in Java. Fig. 7 represents a kite from the east coast of Java, drawn from a model in a museum at Amsterdam, and also a drawing of a Chinese birdkite in the National Museum at Washington. Like most of the oriental kites, they are made flat, but when exposed to the wind the extremities of the wings, which have a frame of split bamboo, bend backward, securing in this way the stability which in our common flat kite is gained by the action of the tail in lowering the centre of gravity and in maintaining the inclination to the wind.