The two diagrams illustrate the main parts of a hot air balloon and indicate how it ascends and descends in flight. Summarise the information by selecting and reporting the main features, and make comparisons where relevant.

There are

two

pictures that illustrate how an

air

balloon

ascends and descends depending on its structure. The

air

balloon

is constructed with a

balloon

envelope

hanging a blanket for passengers. In the blanket,

two

gas tanks are put on the floor linked burners at the junction of the

envelope

and the blanket.

Besides

, there is a

valve

cord inside the

envelope

connected to the

valve

on the top of the

envelope

. When the

air

balloon

ascends, the valves of

two

gas tanks are opened and the

valve

on the

envelope

is closed. With burners opened, the

envelope

will expand because hot

air

is blown into it, pulling the

air

balloon

to the sky.

By contrast

, when the

air

balloon

descends,

two

gas tanks and burners are closed, and a person pulls the

valve

cord to open the

valve

on the top so hot

air

will be released through it.

As a result

, the

air

balloon

will lose the force to rise and back to the land.

Overall

, the

air

balloon

is a complex machine and its movement is predominantly decided by the quantity of hot

air

in its

envelope

.