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[ 429 ] 



LXIX. Trigonometrical Height of Inglehorough above the 

 Level of the Sea, Part II. ^j/ John Nixon, £55'. 



[Continued from p. 261, and concluded.] 



Of the Vertical Angles, 



^HE horizon sector was furnished with new arcs and veN 

 niers (the former in February 1827j and the latter in 

 January 1832,) divided by the best engine in London, yet de- 

 cidedly inferior in accuracy to the original graduations by 

 Allan. Formerly the bubble of each cross level was adjusted 

 to its mark when its arch stood parallel to a plumb line sus- 

 pended near it, but latterly the adjustment has been effected 

 hi a superior manner. A block of wood 8j inches high, 

 6 long, and 2j thick, planed with its ends and sides perpen- 

 dicular to its base, was mounted with a spirit-level, by which 

 the base (by the process of reversing) could be set truly level. 

 In this state, either arc when placed in exact contact with 

 that end of the block in which recesses had been made for 

 the protruding vernier, &c. would be vertical. Supposing the 

 inclination of the arcs to be even 1°, an altitude of 2400 feet 

 would be measured only 4 inches in excess, or in the propor- 

 tion of the secant of the angle of inclination to radius. 



In making bisections of pale, dim, or very distant objects, 

 a dot* of about the same diameter as that of the vertical wire 

 (spider's line), and adhering to one side of it a few minutes 

 above the middle of its height, was successfully made use of 

 up to 1832, when the filament mentioned in Lond. and Edinb. 

 Phil. Mag., at page 166 of volume iv., was substituted. When 

 the object was so near or dark that die dot or filament could not 

 be seen distinctly against it, recourse was had to the horizontal 

 wire, which was pointed in the first instance a little too high, 

 and then gradually lowered (by moving a light weight placed 

 within the stand of the sector in the direction of the object,) 

 until the lower edge of the wire and the base of the signal, &c. 

 seemed (from the inflection of light?) suddenly to commence 

 running into each other. Zenith distances thus measured — as 

 the second observation, with the telescope inverted, is made 

 with the opposite edge of the wire and will be equally in defect 

 with the preceding one — require in strictness a minute correc- 



* The substitution of the dot for the horizontal wire arose from the im- 

 possibility of measuring the depression of the horizon of the sea with the 

 latter. [It may be useful to some of our readers to refer in this place to 

 the substitution of a dot for cross wires made by Mr. Gardner, in the tri- 

 gonometrical operations for determining the difference of longitude be- 

 tween Paris and Greenwich, as stated by Capt. Kater in the Philosophical 

 Transactions for 1828, p. 194 — 5, and considered by him to be a very im- 

 portant improvement in the theodolite. — Edit.] 



