Most people would have been discouraged by such a failure, but the brothers Montgolfier guessed the cause of it, and proceeded to manage things better. They built another balloon in the shape of a box, with its opening downwards, and this time they employed silk instead of paper. They held it over a fire of straw and wool, thinking that such a fire developed a kind of gas which was very light, or which was repelled by the earth. They found, however, after a while, that the nature of the fire did not matter so long as it was hot enough. They found, in fact, that it was the hot air which really caused the balloon to rise, and that a flame burns upwards simply because it is lighter than air. How this should be so is quite clear from the principle of Archimedes, according to which a body loses as much in weight as the weight of the fluid it displaces. We have seen already that the weight of the air inside an ordinary sized room is about a hundredweight. If therefore we construct a balloon of the same bulk as a room, and fill it with a gas much lighter than air, the balloon will weigh less than its ordinary weight by about a hundredweight. If the balloon together with a light gas which fills it weighs 60 lbs., it will be able to lift 52 lbs., or rather it will require 52 lbs. to keep the balloon down on the ground. By making the air inside the balloon hot, about 45 lbs. of air can be made to balance the pressure of the outside air, so that 60 or 70 lbs. may be the weight of the balloon itself. But by using hydrogen the weight of the gas can be reduced much further. For eight pounds of hydrogen exert the same pressure as a hundredweight of air, so that these eight pounds of hydrogen can lift 104 lbs. The silk balloon constructed by the brothers Montgolfier rose up into the air, but fell down again as soon as it cooled. They therefore made a new balloon 40 feet high, constructing the envelope of packing-cloth covered with paper. The lower end was open, and under it was hung an iron cradle filled with moist straw and wool, which gave a slow fire, and helped to keep up the heat of the balloon. The new balloon rose into the air in June 1783, and soon sailed out of sight, bearing a message up to the clouds that mankind was about to enter their territory.