<P> Solar balloons are thin, large balloons filled with air that is heated by the sun in order to decrease its density to obtain lift . </P> <P> Balloons are often deliberately released, creating a so - called balloon rocket . Balloon rockets work because the elastic balloons contract on the air within them, and so when the mouth of the balloon is opened, the gas within the balloon is expelled out, and due to Newton's third law of motion, the balloon is propelled forward . This is the same way that a rocket works . </P> <P> Balloons filled with hot air or a buoyant gas have been used as flying machines since the 18th century . The earliest flights were made with hot air balloons using air heated with a flame, or hydrogen as the lifting gas . Later, coal gas and later still helium were used . An unpowered balloon travels with the wind . A balloon which has an engine to propel it is called a dirigible balloon or airship . </P> <P> Angioplasty is a surgical procedure in which very small balloons are inserted into blocked or partially blocked blood vessels near the heart . Once in place, the balloon is inflated to clear or compress arterial plaque, and to stretch the walls of the vessel, thus preventing myocardial infarction . A small stent can be inserted at the angioplasty site to keep the vessel open after the balloon's removal . </P>

In which country were the first full size balloons built