The Nautical Almanac and Astronomical Ephemeris, for the Year 1769. - Softcover

9781130349139: The Nautical Almanac and Astronomical Ephemeris, for the Year 1769.
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1768 Excerpt: ...before and after, according as the Weather (hall permit. The Observation of next Importance after the Contacts, is that of the nearest Distance of Venus from the Sun's Centre, which is best done by the new divided Object Glass Micrometer sitted to the End, either of a reflecting Telescope, or the new achromatick refracting Tesescope of 3s Feet above mentioned; other Distances of Venus from the Sun's Centre may also have their Use. But the Observations of the Contacts alone may be sufsicient for the Generality of Observers to attend to. It has been remarked above, that dark Glasses should be used to desend the Eye from the Intens1ty of the Sun's Light. Transparent Glasses smoaked over the Flame of a Candle or Lamp-and applied to the Telescope, will give a more distinct and agreeable Vision of the Disks of the Sun and Venus than any tinged or coloured Glasses will do. Provide some Pieces of clear Glass, not too thick (the common Crown Glass used for Windows may do as well as any) cut two of them into equal Rectangles of convenient Lengths-and wipe them clean and dry, and warm them a little by the Fire, if the Weather be cold, to prevent their Cracking when applied to the Flame of the Candle; then draw one of them gently, according to its whole Length, through the Flame, and Part of the Smoke will adhere to the Glass; repeat the fame Operation, only leaving a little Part at one End now untouched; repeat the Operation-leaving a further Part at the fame End untouched; and so each Time leave a further Part at the fame End untouched, till at last you will have tinged the Glass with several Dies,increasing gradually in Blackness from one End to the other; smoke the other Glass in like Manner, and apply the two Glasses one against the other, only separated rated by ...

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  • PublisherRarebooksclub.com
  • Publication date2012
  • ISBN 10 1130349136
  • ISBN 13 9781130349139
  • BindingPaperback
  • Number of pages28

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