Although the first confirmed human flight in a balloon famously took place on 19 October 1783, when a balloon designed by the Montgolfier brothers (Joseph-Michel and Jacques-Etienne) demonstrated the technique to an astonished crowd in Paris, their balloon used hot air to generate buoyancy. The obvious drawback to this technique is that the balloon has to carry a fire on board, and cannot stay aloft once the fire is exhausted. But even in 1783 it was clear from Boyle’s Law (see here) and the investigations of gases made by people such as Henry Cavendish and Joseph Black that a balloon filled with hydrogen ought to rise upwards through the atmosphere. This fired the imagination of another Frenchman, Jacques Charles, who designed a hydrogen balloon made of silk, which he coated with rubber to make it gas-tight.
The first small Charlière (as such balloons became known) was launched from the Champs de Mars (the present-day site of the Eiffel Tower) on 27 August 1783. It was a 35-cubic-metre capacity sphere capable of lifting about 9 kilograms. After a flight northward covering 21 kilometres and lasting 45 minutes, the balloon landed in the village of Gonesse, where the local peasants, thinking it the work of the Devil, hacked it to bits with knives and pitchforks.
Less than two months after the Montgolfier brothers’ first flight, on 1 December 1783, Charles launched a manned hydrogen balloon from the Jardin des Tuileries, carrying himself and Nicolas-Louis Robert, who had devised the method of coating silk with rubber. This hydrogen balloon had a volume of 380 cubic metres, and the ascent was watched by a vast crowd, estimated at 400,000 people – half the population of Paris – and including Benjamin Franklin and Joseph Montgolfier. The balloon reached a height of about 1,800 feet (550 metres) and landed at sunset in Nesles-la-Vallée, 36 kilometres away, after a flight of 2 hours and 5 minutes. Charles and Robert alighted unscathed, just after sunset. Charles then took off again on his own, and reached an altitude of about 10,000 feet (3,000 metres), high enough to see the sun again. But pain in his ears made him release gas and land.
Charles and Robert carried a barometer and a thermometer with them to measure the pressure and the temperature of the air, but these observations were very much a secondary consideration. The first truly scientific balloon ascents were made in 1804, by Joseph Gay-Lussac and Jean-Baptiste Biot, using hydrogen-filled Charlière balloons. Gay-Lussac was thoroughly familiar with the behaviour of gases, and in 1802 had spelled out what is now known as Gay-Lussac’s law, which says that if the mass and volume of a gas are kept constant then the gas pressure is proportional to the temperature (on what is now known as the ‘absolute’ or Kelvin scale, where zero is –273.16 ºC). He also discovered that equal volumes of gases expand by the same amount for the same increase in temperature. This is usually known today as ‘Charles’s law’, because Jacques Charles discovered it in the mid1780s, but as he had not published the discovery, Gay-Lussac found it independently.
Gay-Lussac and Biot made an ascent together from the garden of the Conservatoire des Arts et Metiers, on 24 August 1804. The object of the flight was to measure changes in magnetism and the composition and humidity of the air at different altitudes. They reached an altitude of 4,000 metres, and showed that the Earth’s magnetic field does not vary noticeably with altitude, but did not achieve any other significant scientific results. So Gay-Lussac obtained a larger balloon and made a solo ascent a few weeks later, on 16 September. This time the balloon took him to an altitude of 7,016 metres (some 23,000 feet; for comparison, the height of Mount Everest is 29,035 feet). This was far higher than anybody had previously flown. At this altitude Gay-Lussac recorded a temperature of –9.5 ºC (9.5 ºC below freezing), but stayed to make his measurements of the moisture of the air, magnetism, and so on. The magnetic measurements confirmed that, within the accuracy of his instruments, the magnetic field is constant, even up to this altitude. But he noted that he had considerable difficulty in breathing, and that the air was so dry that his mouth and throat became parched to the point where it was painful to swallow a piece of bread. He also recorded an increased pulse rate. But because Gay-Lussac collected samples of air at different altitudes during his flight, he was able to analyse them in the comfort of his laboratory in Paris, rather than struggling to make all the measurements with frozen fingers while in the basket of the balloon. This analysis showed that the composition of the atmosphere does not change with increasing altitude.