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 BesanÁon 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 _AÈrophiles_, 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 _AÈrophile_ 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