. Machinery and processes of the industrial arts, and apparatus of the exact sciences. rom the objectseen by reflection will begin to be cut offby the prism e. When this is the case, thesextant is turned with the limb uppermost,and the higher angles are read upon adifferent part of the limb. In the circlethis expedient cannot be employed exceptby abandoning one of the verniers. If, forinstance, in the figure, the upper index armshould be removed, the lower arm couldtraverse nearly the entire limb. As thesextants are constructed, all angles up to140 degrees are measured with the instru-ment in
. Machinery and processes of the industrial arts, and apparatus of the exact sciences. rom the objectseen by reflection will begin to be cut offby the prism e. When this is the case, thesextant is turned with the limb uppermost,and the higher angles are read upon adifferent part of the limb. In the circlethis expedient cannot be employed exceptby abandoning one of the verniers. If, forinstance, in the figure, the upper index armshould be removed, the lower arm couldtraverse nearly the entire limb. As thesextants are constructed, all angles up to140 degrees are measured with the instru-ment in the usual position. For angles; between 110 degrees and ISO degrees the Pistor & Martenss Circle. instrument is inverted, and the graduationbegins at a new point about GO degrees farther along the limb, readingin a direction opposite to the first. Where the angle approaches ISOdegrees, the head of the observer begins to interfere. This case isprovided for by adding a diagonal eye-piece to the telescope, which per-mits the observer to put himself out of the plane of EEFLECTING INSTRUMENTS SEXTANTS. 599 LAURENTS SEXTANT FOR STELLAR OBSERVATIONS. Another modification of the sextant deserving of attention was exhibited by Captain Albert Laurent, formerly of the French naval service,and at present commander of the transatlantic steamer ImperatriceEugenie. Captain Laurent observes that navigators lose, for the mostpart, the benefit of observations of stellar altitudes, in consequence ofthe difficulty of making the contacts of the image of the star with thehorizon. His remedy for this difficulty is to elongate the image of thestar in a direction perpendicular to the plane of the observation, wherebythe object is made more conspicuous, and the facility of observation isgreatly increased. The elongation of image here spoken of is producedby placing a cylindrical lens with its axis of figure parallel to the planeof the instrument, and its optical axis also parallel to the s
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Keywords: ., bookcentury1800, booksubjectmachinery, booksubjectscientificappa
