The exploration of the high atmosphere by _ballons-sondes_, which can aid so many investigations, has been utilized by M. Violle 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. Violle 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 air-strata 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