The second ascent of this Aérophile was its last, for, after falling in the Black Forest, it was burned by children. However, M. Besançon, not discouraged, constructed the Aérophile II. of 6300 cubic feet, and improved the instruments as experience suggested. The records had often been interrupted by freezing of the ink, so the pen was replaced by a needle marking with less friction on smoked paper surrounding the record drum. To avoid heating of the thermometers by the sun, they were placed in a wicker cylinder open at both ends and covered with bright metallic paper. This was hung below the balloon with its axis vertical, in order that the draught through the cylinder when the balloon was rising or falling should counteract the insolation, and in the next ascent, at about the same altitude, a temperature lower by 36° Fahrenheit indicated the effect of the protection. To secure an independent record of the lowest temperature an ingenious device was used, consisting of a thermometer tube filled with alcohol and having black divisions. The lowest point to which the alcohol sank was recorded on photographic paper placed behind the tube, the whole being enclosed in a metallic box that was automatically closed on striking the ground, and so was preserved against the meddling of curious persons. Up to the middle of 1898 ten voyages had been made by the Aérophiles, which were now constructed of varnished silk to hold 16,000 cubic feet of gas.
One of the objects sought was the collection of samples of air at great heights, but this was not accomplished until recently. In the first apparatus for this purpose, an aneroid barometer at a predetermined pressure turned the cock communicating with an exhausted receiver that filled with air and was then closed. The cock leaked, so next the ingenious device of generating heat chemically to seal the glass tube was tried. This, too, failed, but finally, an apparatus of M. Cailletet solved the problem. It is advisable to control the height deduced from the barometric records by direct observations so long as the balloon remains visible, and for this purpose micrometric measures were made with a telescope as soon as the balloon left the ground. There was also used a species of registering theodolite which, when kept pointed at the balloon, automatically traced on paper its azimuth and angular altitude. These records, when combined with the barometric height at a known hour, permitted the horizontal distance traversed, and hence the velocity, to be calculated, or, with two such instruments at ends of a base line, the height of the balloon could be found.
The first experiments with ballons-sondes in France were soon repeated in Germany, where a balloon of rubber-fabric holding 8700 cubic feet was obtained by the German Society for the Promotion of Aërial Navigation. When inflated with coal-gas it had a lifting force of about two hundred and ninety pounds, in excess of its envelope, etc., weighing ninety-three pounds, and the meteorological apparatus weighing six pounds. The Cirrus, as it was called, burst on its first trial, but in July 1894 it made a remarkable voyage from Berlin to the boundary of Bosnia, a distance of seven hundred miles, at an average speed of sixty-two miles an hour. A maximum height of 54,000 feet and a minimum temperature of -63° Fahrenheit were recorded. The Cirrus on its third voyage was accompanied by manned balloons in order to have simultaneous observations at different levels, and this time it travelled eighty-three miles an hour and rose 61,000 feet. The lowest temperature of -88° Fahrenheit was supposed to be too high, for the reason that whereas the ventilation of the thermometers in a rapidly ascending or descending balloon might be sufficient to counteract solar radiation, this would not be the case when the balloon was approaching its culminating point with a diminishing speed. Therefore, Dr. Assmann, under whose supervision the German experiments were conducted, employed the thermometer, which in the captive balloon was aspirated electrically, but now was driven by a weight, and later, because the ink froze, the registration was made photographic. The efficacy of the aspirator was seen in the ascent referred to, for, when its action stopped, a higher temperature was recorded though the balloon continued to ascend. In April 1895 the Cirrus rose to the extraordinary height of 72,000 feet, or more than thirteen and a quarter miles, where the barometric pressure was reduced to one and a half inches of mercury. (In [Plate I]. this extreme and possibly excessive height is not shown as the height of the ballon-sonde, but the average of the three highest ascents of the Cirrus is indicated.) The comparative warmth (-50° Fahrenheit) recorded has led Dr. Assmann himself to doubt the accuracy of the usual methods of registering temperature at such extremely low pressures. [Plate VII]. shows the heights in metres, and the temperatures in degrees Centigrade, during eight voyages from Berlin prior to June 1897.
Notwithstanding the rivalry and difference of opinion between the Germans and French as to the methods of exploring the high atmosphere, there was also a sincere desire to co-operate, and the International Meteorological Conference which was held at Paris in September 1896 furnished an opportunity to make the arrangements. Resolutions were adopted favouring ascents with manned balloons, as well as simultaneous ascents of ballons-sondes in the different countries. The successful use of kites at Blue Hill to lift self-recording instruments more than a mile into the air led to the wish that similar experiments should be tried elsewhere. An International Committee was appointed to carry out these resolutions, of which Professor Hergesell of Strassburg is president, and the veteran Parisian aeronaut and journalist, Wilfrid de Fonvielle, is secretary.
It was agreed to make a night ascent and to use identical instruments, in order that the observations might be made everywhere under the same conditions. Accordingly, on the early morning of November 14, 1896, five balloons manned by observers, and three ballons-sondes with recording instruments, were liberated in France, Germany, and Russia. By means of the automatic diagrams from the ballons-sondes, and the direct observations in the manned balloons, it was sought to determine the decrease of temperature with height in vertical sections of the atmosphere connecting the various centres from which the balloons started. Seven such sections were available by connecting Paris and Strassburg, Berlin and St. Petersburg, Warsaw and Munich, etc., but, unfortunately, observations in the highest strata were generally lacking.
Three more international ascents were made during the year 1897, which were participated in less extensively. At this time it was necessary to decide questions that had arisen, and to make plans for the future, consequently a meeting of the International Committee was held at Strassburg in 1898. Many technical questions were settled, but the chief result accomplished was the dissipation of misunderstandings and prejudices, not only between French and Germans, but between the German representatives themselves, for no doubt personal intercourse is the greatest good of such conferences. Although it was not a surprise, nevertheless it was regretted that no one came from Great Britain, where, since Glaisher's epoch-making balloon ascensions, little has been done to explore the air. The beneficial results of the Conference were apparent at the fifth international ascent, which occurred in the early morning of June 8, 1898. Austria and Belgium joined Germany, France, and Russia, and the field of atmospheric survey was extended over a good part of Europe. A veritable aeronautic fleet was launched from Paris, Brussels, Berlin, Warsaw, St. Petersburg, Strassburg, Munich, and Vienna, consisting of twenty-one balloons, of which thirteen carried observers, who all used the aspiration thermometers, and eight were equipped only with self-recording instruments. Some of the latter balloons reached altitudes of 50,000 feet, and the former attained extreme heights of one-third this. On the day selected the atmosphere was in a state of repose, with light variable winds, except high up, where they blew, as is usual, from the west or south-west. These observations were sufficiently numerous to form a synoptic chart at a considerable height above Europe for comparison with the usual chart drawn from the surface observations.