Orr's Circle of the Sciences: Practical astronomy, navigation, nautical astronomy and meteorology

William Somerville Orr
W.S. Orr and Company, 1856

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Página 174 - Astronomical instrument-making may be justly regarded as the most refined of the mechanical arts, and that in which the nearest approach to geometrical precision is required, and has been attained. It may be thought an easy thing, by one unacquainted with the niceties required, to turn a circle in metal, to divide its circumference into 360 equal parts, and these again into smaller subdivisions, — to place it accurately on its centre, and to adjust it in a given position ; but practically it is...
Página 156 - Rock, which stands on one side of the harbour's mouth, so nearly right ahead that we had not to alter our course above a point in order to hit the entrance of Rio. This was the first land we had seen for three months, after crossing so many seas and being set backwards and forwards by innumerable currents and foul winds.
Página 191 - ... the two compasses adjusted by fixed magnets. If the captain had been aware of the changes which might, and most probably would, take place, when the ship began to strain in a different position from that in which she had been built ; if he had known that the compasses...
Página 77 - ... summary proof to be given of the truth or falsehood of either of the hypotheses. One, for several reasons that will hereafter appear, is much more probable than the other. Indeed the hypothesis of the revolution of the sphere is inadequate, as astronomical science now stands, to solve all the phenomena. We must, however, be content, at present, to take for granted the truth of the hypothesis of the Earth's rotation. If it continues to explain simply and satisfactorily, other astronomical phenomena...
Página 174 - ... part of an inch, a quantity too small to be certainly dealt with without the use of magnifying glasses ; yet one minute is a gross quantity in the astronomical measurement of an angle. With the instruments now employed in observatories, a single second, or the 60th part of a minute, is rendered a distinctly visible and appreciable quantity.
Página 175 - ... his opportunities, and so to familiarize himself with all the causes which may produce instrumental derangement, and with all the peculiarities of structure and material of each instrument he possesses, as not to allow himself to be misled by their errors, but to extract from their indications, as far as possible, all that is true, and reject all that is erroneous.
Página 174 - With the instruments now employed in observatories, a single second, or the 60th part of a minute, is rendered a distinctly visible and appreciable quantity. Now, the arc of a circle, subtended by one second, is less than the 200,000th part of the radius, so that on a circle of 6 feet in diameter it would occupy no greater linear extent than -jJ^th part of an inch; a quantity requiring a powerful microscope to be discerned at all.
Página 191 - He had found, on inquiry, she had been built with her head nearly north-east. Here, then, were the precise circumstances for expecting a change in the ship's magnetic distribution. Having been built with her head to the north-east, she had a certain magnetic distribution...
Página 552 - States in winter. Before the warmth of the Gulf Stream was known, a voyage at this season from Europe to New England, New York, and even to the Capes of the Delaware or Chesapeake, was many times more trying, difficult, and dangerous than it now is. In making this part of the coast, vessels are frequently met by snow-storms and gales which mock the seaman's strength, and set at naught his skill.

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