Balloon

balloon is a sack filled with gas. Large balloons filled with gas can rise and stay in flight because of the gas inside the balloon.

How Balloons Stay Up

Archimedes (AR-key-MEE-dees), a Greek scientist of the third century b. c.e., real­ized that an object will float when its weight equals, or is less than, the weight of the fluid (gas or liquid) it displaces, or pushes away. His discovery explains why a ship floats. It also provides an explanation of why a balloon flies, because air displaces in the same way as water. This force is called buoyancy.

A small balloon filled with air is light, but it is not buoyant. It will naturally sink to the ground because the weight of the balloon skin makes it heavier than the air around it. In the 1700s inventors began to experiment with “floating flight.” They were possi­bly inspired by watching smoke from a fire rise into the air. Hot air seemed to hold the key to flight.

This was proved in 1709. In Portugal a priest named Bartolomeu de Gusmao demonstrated a small paper balloon filled with hot air. A small fire, lighted in a dish tied beneath the balloon, heated the air. The balloon rose 12 feet (3.66 meters) toward the ceiling of a room. Nothing more came of this experiment, but it proved that hot air, being lighter than the air around it, can enable an object to rise and stay afloat.