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(Notice the phrases marked a and b.)
The words and indications for the use of the portamento in each of these last four examples are by the respective composers, and as printed in the published editions.
A portamento should never be sung so slowly as to convey the idea of a badly executed chromatic scale; and, as a rule, it is best not to use one between any lesser interval than a third, unless for some particular effect, or at the close of a slow movement, as in the aria “He was despisèd,” in The Messiah:
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It is also effective in connecting syllables in phrases of a smooth, lyric character:
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