What Keeps a Balloon Up?
A balloon stays up in the air, because of the air in it, together with the weight of the balloon, is less than an equal bulk of the air in which it floats.
In former days of ballooning the balloons were filled with hot air and were then found to rise and stay up until the air inside of the balloon became of the same temperature as that in which it floated. When this stage was reached, the balloon itself would fall because the material of which it was made was denser than air.
Today balloonists fill their balloons with gas which is lighter than air, even when as cool as the air in which they rise and are thus able to stay up a long time.
You, of course, have seen many of the red, white and blue paper balloons which are sent up on the Fourth of July. You will remember that father, or whoever it is that is sending them up, lights the oil-soaked knot of cloth that is attached to the balloon immediately below the opening at the bottom. He first lights this and then holds the balloon for a time with his hands.
Soon, however, you will remember that the balloon starts upward with father still holding it. This is because the air inside the balloon is becoming heated. You will notice also that at first he has to hold out the sides of the top of the balloon with his hands or has some one help him do this, but that even so the balloon does not stand out round and full as it should. When the balloon starts to rise, however, you will notice that it is round and full. This is because the air in the balloon has become heated and is expanding. Soon the balloon is tugging to get away and father lets go and it rises and sails away with the wind. As long as the fire below it burns, and if the wind does not upset it so as to make the paper part catch fire, the balloon will stay up; but, when the fire burns out, the balloon will come down.
The balloon merely rises because the air inside, and held there by the covering of the balloon, is warmer air and lighter than the air on the outside.