'Cantinela' or 'cantilena' means 'ballad' or 'song' (Collins) so I can only presume they read in a singsong voice, as if they were singing the words. Given your context though, it doesn't sound very appropriate for the eighteenth century.

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Note added at 1 hr 6 mins (2004-09-17 09:37:09 GMT)
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Anna,

I\'m familiar with the expression - it\'s just that I don\'t know if its entirely appropriate for your context. Perhaps a different term was used in the eighteenth century.

I\'ve just checked the Shorter Oxford and it defines \'sing-song\' as, \"Tone of voice marked by a monotonous rise and fall, with kind of singing effect (1822)\" and \"Chacracterized by a jingling trivialty or monotonous rise and fall (1734)\". As the dates suggest it may be appropriate for your text.