. Machinery and processes of the industrial arts, and apparatus of the exact sciences. fromany elevated point the traveler wishes to measure the depression ofsome object, as of the sea horizon, by which to determine his elevationor distance, knowing one of them; or, knowing the distance of anotherand lower mountain, to determine the difference of elevation. We haveencountered all these difficulties, and also the less frequent one of fallingin with a reef at night with the stars visible, but the sea horizon totallyobscured in darkness. Mr. Davidsons improvement consists in the use of an observi
. Machinery and processes of the industrial arts, and apparatus of the exact sciences. fromany elevated point the traveler wishes to measure the depression ofsome object, as of the sea horizon, by which to determine his elevationor distance, knowing one of them; or, knowing the distance of anotherand lower mountain, to determine the difference of elevation. We haveencountered all these difficulties, and also the less frequent one of fallingin with a reef at night with the stars visible, but the sea horizon totallyobscured in darkness. Mr. Davidsons improvement consists in the use of an observing tubeto which is attached, on the top, a small spirit level, the bubble of whichis seen by reflection, in a manner which Fig. 140 will serve to observing tube is G G, which is supported by the exterior tube C :and this in turn is secured to the sextant by the screw M. which cow. DAVIDSONS SPIRIT-LEVEL SEXTANT. 601 nects with the ordinary telescope support. The ends of the tube G Gare closed by plane glasses; a precaution apparently unnecessary. At Fiff. 140. Fig. W, on the top of the tube, isan opening, which permits thebubble of the level S to be seenby reflection from the mirrorE, placed at an angle of forty-five degrees to the axis of the 6 tube. As the level will be too Davidsons Spirit-level Sextant. near for distinct vision, a convex lens, L, is introduced into the tubebetween the mirror E and the eye; and this, by means of a screw-headexterior to the tube, may be advanced or withdrawn to accommodatedifferent eyes. As the mirror E must not be permitted to cut off theview of the objects under observation, it has a breadth only equal toone-half the diameter of the tube. The lens L, also, is but a the half of the tube next the face of the instrument is clear. is a cross-section, and shows this arrangement. It shows also thatthe spirit level is unsymmetrically placed on the top of the tube, beingdirectly over the outer half, or that
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Keywords: ., bookcentury1800, booksubjectmachinery, booksubjectscientificappa
