Emmanuel Liais and the cœlostat: Notes on a forgotten instrument Maria Lucia de Niemeyer Matheus Loureiro Museu de Astronomia e Ciências Afins – MAST (Rio de Janeiro, Brazil) marialucia@mast.br Vitor Luiz Silva de Almeida UFRJ/PPGHC - Universidade Federal do Rio de Janeiro Programa de Pós-Graduação em História Comparada vitoralmeida83@gmail.com ABSTRACT In 1895, Gabriel Lippmann describes a new instrument, the cœlostat, an improvement on the siderostat, which would give a fixed image of the entire sky. In 1874, more than two decades before Lippmann, French astronomer Emmanuel Liais, the then director of the Imperial Observatory of Rio de Janeiro, designed and ordered the construction of an apparatus (also called a cœlostat) for the same purpose, which the great advantage was its potential to photograph the sky. Taking all its parts into account, the cœlostat would take up more than 15 metres in length, making impossible for it to be assembled at the time, since the observatory was located on cramped premises on Castelo Hill. Apparently, Liais’s cœlostat was never used. It is not known what happened to the instrument, and its disappearance probably resulted from the lack of any suitable space for its use and conservation. Keywords: cœlostat, Emmanuel Liais, Imperial Observatory of Rio de Janeiro RESUMO: Emmanuel Liais e o celóstato: Notas sobre um instrumento esquecido Em 1895, Gabriel Lippmann descreve um novo instrumento, o celóstato, um aperfeiçoamento do siderostato, destinado a proporcionar uma imagem fixa do céu. Em 1874, mais de duas décadas antes de Lippmann, o astrônomo francês Emmanuel Liais, então diretor do Imperial Observatório do Rio de Janeiro, projetou e supervisionou a construção de um aparato (também chamado eRittenhouse Vol. 24 (2013) 1