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John Russell on Selenographia, and its earth globe

John Russell gives an account of the instrument in A description of the selenographia: an apparatus for exhibiting the phenomena of the moon (London, 1797). The last paragraph of the work says "The instrument contrived to move these globes was invented by degrees, chiefly in the year 1796; and it is hoped will fully answer the purpose intended by it, namely, that of exhibiting, completely, all the effects of libration, rotation, and elongation, on the surface of the moon" (p. 27).

The terrestrial globe is discussed in the following terms: "In front of the lunar globe, and fixed to the bottom of the brass hemisphere, there is an arm which supports a small terrestrial globe, to render the effects of the moon's parallax, that is, the diurnal and menstrual libration, familiar to those who may not have considered these things maturely. For the convenience of observation, this terrestrial globe is made somewhat larger than the earth really appears to be from the centre of the moon, but converging lines reduce it to its proper size" (p. 9).

Russell later explains these converging lines: "The angle which the earth subtends from the centre of the moon, is not more / than two degrees of the moon's circumference; but, for the convenience of observation, a size much larger than two degrees of the lunar globe is here adopted, that the effects may be better observed upon the small terrestrial globe. This globe, however, is reduced by two converging lines, which are tangents to the terrestrial globe, to two degrees at the periphery of the lunar globe and the whole is so placed that the line which bisects the angle formed by the tangents, would, if produced, pass through the centres of both globes." (p. 20-1)

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