Sample paragraph

The following paragraph can be analysed to show its structure and the functions of the 8 sentences. (The numbers are inserted only to identify the sentences)

(1) The Ultra Long Duration Balloon is a super-pressure, or "closed" balloon, which is not vented to the atmosphere like conventional balloons. (2) Usually fabricated from stronger materials such as polyester, super-pressure balloons are inflated like their zero-pressure counterparts and then sealed. (3) Once a super-pressure balloon reaches the desired altitude, the sun's heat forces the internal pressure to rise until it exceeds the outside ambient pressure. (4) As a result, the differential pressure between the inside and the outside increases. (5) At night when the gas cools, the differential pressure drops, but if enough gas has been put into the balloon the differential cannot drop below zero. (6) In this way, the balloon remains full and at a stable altitude without having to drop ballast. (7) So long as the balloon remains impervious to helium or hydrogen molecules, it can stay aloft. (8) Accordingly, super-pressure balloons can be used for flights of far greater duration than zero-pressure systems._

I. Steve Smith, Jnr. & James A. Cutts "Floating in Space" Scientific American Vol 281 No 5 November 1999


(1) is the topic sentence - contrasting the Ultra Long Duration Balloon with conventional ones

(2) - (7) explain how the ULDB works. (2) describes the special features of its construction. (3) and (4) explain how it operates in the air, with (5) adding what happens at night. (6) and (7) summarise its operation.

(8) provides an overall conclusion - the ULDB can stay in the air longer than conventional balloons

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