The next year the experiment was continued by MM. Hermite and Besançon with balloons of thirty-five cubic feet contents, and about half of those dispatched from Paris were recovered within a radius of one hundred miles. The height to which the balloons could rise is determined by the following considerations: to ascend 18,000 feet, where the atmospheric pressure is one-half that at the earth (see [Plate I].), the balloon when half full of gas must lift itself from the ground; to rise 35,000 feet, where the pressure is reduced to one-quarter, it must be able to start upward when one-quarter filled, and so on. In practice the ascensional force usually diminishes at first from various causes, such as the escape of gas, its cooling, and the deposit of moisture on the outside of the balloon. To penetrate the clouds, therefore, it is necessary to have a considerable excess of ascensional force, but above the clouds, since the heating effect of the sun increases greatly with altitude, the gas in the balloon is warmed much above the surrounding air, and so the theoretical altitudes are exceeded.
Having determined that balloons inflated with one hundred and fifty cubic feet of coal-gas would rise to great heights, simple and light registering instruments, as well as the postal cards, were attached to them. As the pressure diminished, an aneroid barometer traced a line on a smoked glass, and after the descent was placed under the receiver of an air-pump, and the pressure required to reproduce the trace was measured by a manometer. From this the height could be computed approximately. The maximum and minimum thermometer was of the well-known U-form, and instructions appended asked that it be read as soon as found. A slow-match was arranged to detach postal cards successively, so that if they were found and mailed, the track of the balloon could be determined. These balloons at first were called ballons perdus, or lost balloons, but when it was known that most of the fourteen balloons liberated from Paris were recovered, the name ballons explorateurs was given, which was afterwards changed to ballons-sondes, or sounding balloons. The Germans call them, Registrir-Ballons, and in English they have been designated unmanned balloons also. One of these paper balloons having reached a height of nearly 30,000 feet, MM. Hermite and Besançon proceeded to construct a balloon of gold-beater's skin, having a capacity of 3960 cubic feet, in order to carry a better instrumental equipment. The self-recording instruments made by the French firm of Richard Brothers were well adapted for this purpose, and a combined barometer and thermometer, registering in ink on an upright drum that is turned by clockwork inside, is shown in [Fig. 5]. The exhausted pair of boxes B of the barometer actuates the lower pen, while the curved tube C, which is filled with alcohol, by its change of shape moves the upper pen and records the temperature. From the indications of the barometer and the temperature of the mass of air, it is possible by Laplace's formula to calculate the height at any hour of the registration. The balloon mentioned was the first of the so-called Aérophiles, and when inflated with coal-gas it could lift seventy-seven pounds besides its own weight of forty pounds. It carried two of the baro-thermographs described, and a package of information cards arranged to be detached by a slow-match. To mitigate the shock of striking the ground one of the instruments was hung by rubber cords inside a wicker basket that in the first ascent was not screened from the sun. It was decided to liberate these balloons entirely filled with gas (instead of partly full, to allow for its expansion), and to utilize all possible ascensional force at first rather than to weight the balloon with an automatic discharger of ballast ([Fig. 6]). The trial of the Aérophile occurred March 21, 1893, and the next day one of the cards was returned announcing its fall in the department of the Yonne, where the balloon and the instruments were recovered injured. From the blurred traces of the latter it was computed that at an altitude of about 49,000 feet a temperature of -60° Fahrenheit had been met with, both pressure and temperature being the lowest measured in a balloon up to that time. Although the data secured by this ascent were somewhat doubtful, yet the feasibility of exploring the atmosphere by ballons-sondes was proved. It was seen that the enormous velocity of ascent overcame the wind and permitted the path of the balloon to the summit of its trajectory to be followed, the balloon appearing like a meteor visible in daylight, and so its height could be calculated by trigonometrical measurements; while the descent, caused by the escape and cooling of the gas, was gentle and regular, permitting the delicate instruments to be recovered uninjured.
Fig. 5.—Baro-thermograph of Richard.
Fig. 6.—The Aérophile rising. The left-hand picture shows the deformation caused by the resistance of the air to its rapid ascent, and the right-hand one the violent oscillations when first liberated.