Unity means oneness—oneness of thought and of purpose. In order for a sentence to have unity
(1) its statements must be closely related,
(2) the relationship of its statements must be clear, and
(3) the statements must compose only one thought.
Do not put unrelated ideas into the same sentence. If the ideas are not closely related, make them separate sentences. If the ideas are not related at all, eliminate one of them.
Alexander Graham Bell invented the telephone, and his invention was the result of many years of scientific study. (The ideas are not closely related.)
Alexander Graham Bell invented the telephone. His invention was the result of many years of scientific study. (correct)
Make sure the relationship between ideas is immediately clear.
We were going to school, and we saw a possum. (relationship not clear)
As we were going to school, we saw a possum. (relationship of time, now clear)
They wanted to maintain good health, and they started an exercise program. (relationship not clear).
To maintain good health, they started an exercise program, (relationship of purpose, now clear).
The length of a sentence does not necessarily affect its unity. A very long sentence may have unity, whereas a short sentence may not.
If we resort for a criterion to the different principles on which different forms of government are established, we may define a republic to be, or at least may bestow that name on, a government which derives all its powers directly or indirectly from the great body of the people, and is administered by persons holding their offices during pleasure, for a limited period, or during good behavior. — James Madison
(Although this sentence contains sixty-eight words, it has unity because its ideas exhibit a oneness of thought and purpose.)
Dressed in a blue suit, he voted against the bill. (This sentence of only ten words lacks unity because its ideas are unrelated.)
Do not introduce so many details that the central thought of the sentence is obscured. If the details are relevant and necessary, put them into other sentences.
Early in the morning of May 1, long before sunrise, Admiral Dewey, with his fleet of eight warships, consisting of the Olympia, which was the flagship, the Boston, the Petrel, the Concord, the Philadelphia, the Baltimore, the Raleigh, and the revenue cutter McCulloch, silently entered the harbor of Manila, and was not detected by the Spaniards until he was well past the forts guarding the entrance. (This sentence contains details which obscure the central thought.)
Long before sunrise on the morning of May 1, Admiral Dewey silently entered the harbor of Manila with his fleet of eight warships. Not until he was well past the forts guarding the entrance did the Spanish detect him. (better)
