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Postcard showing the heliograph at the Puy-de-Dôme Observatory, at an altitude of 1467 m, with an explanation of its function: it records hours of sunlight by carbonization on a ruled cardboard. The postmark of July 16, 1930, is visible. On the reverse, the publisher is indicated as 'Edit. spéc. du Bar des Téléphon. Téléph. de l'Observatoire'.