Marques, João Pedro, The Sounds of Silence- Nineteenth-century Portugal and the Abolition of the Salve Trade.
https://www.cepese.pt/portal.%26%22%26quot/pt/publicacoes/obras/volume-7-number-2-winter-2009/artigos/marques-joao-pedro-the-sounds-of-silence-nineteenth-century-portugal-and-the-abolition-of-the-salve-trade/view
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Marques, João Pedro, The Sounds of Silence- Nineteenth-century Portugal and the Abolition of the Salve Trade.