Alas, he observed, "the balloon is still sinking."

This would unroll as the balloon sank to working altitude of about 300 metres.

This will let you talk about the different densities of the 3 gases, and how the density determines whether the balloon will float or sink in air.

The balloon ceased to struggle, and sank downward.

The balloon sank horrendously under the huge load.

When the balloon sank, more of the rope lay on the round; the balloon, relieved of its weight, rose again.

As the balloon sank, Marko sighted a suitable field.

Nearer and nearer to the surface of the lake sank the balloon.

The balloon sank so rapidly that he knew that something would break if he hit the ground this hard.

Lucy peered down, and the balloon sank gently towards the roof, which was partly gabled with flat areas between.