Star-land; being talks with young people about the wonders of the heavens (1899)
Ball, Robert Stawell, Sir, 1840- [from old catalog] · Boston and London, Ginn & company · leaf 247
seemsto have been about six or seven inches in diameter, and 234 STAR-LAND. nearly an inch thick. Though copper is such a toughsubstance, and though tin is also tough, yet when meltedtogether to make speculum metal, as this alloy is called,they produce an exceedingly hard and brittle material.When… … Fig. (>9. —The Mirror. be more brittle than a bit of glass of the same dimen-sions, and when the speculum is cast, unless it is cooledvery carefully, it will fly into pieces. Herein lay oneof the difficulties that Herschel encountered. Speculummetal must be put into an oven as soon as the…
Astronomy
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