The American practical navigator : being an epitome of navigation and nautical astronomy (1888)
Bowditch, Nathaniel, 1773-1838, United States. Navy Dept. Bureau of Navigation, Cooper, Philip Henry, 1844-1912 · Washington : Govt. Print. Off. · leaf 106
ld be made of plate-glass,with perfectly parallel faces. If they areat all prismatic, the observed altitudewill be erroneous. The error may beremov£d by observing a second altitudewith the roof reversed, and, in general,by taking one-half of a set of observations VIG. 53. with the roof in one… … 74 IJJSTRUMEKTS EMPLOYED IN NAUTICAL ASTRONOMY. Instead of the mercurial horizon, a glass plate is sometimes used standing upon three screws, by means o!which it is levelled, a small spirit-level being used to test its horizontality. The lower surface of the plate is black-ened, so that the…
Navigation · Nautical astronomy
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