Transactions . ed a dividedeirele of nietal. gave an inde-pendent motion to the instru-ment by means of a rack-and-binion, and also placed insidethe compass-box a vernier which,, t. ~ , D. instQ r Magnetic Compass of btone s liion, 1 7zo. read to 3 minutes of arc, and was used like a theodolite. The metal ring placed inside thecompass-box and upon the upper thin base-plate (for therewere two, one revolving upon the other), was provided withtwo sets of divisions, one on the inner edge, for needle-read-ings, and the other on the outer edge, for plane-sights revolved horizont
Transactions . ed a dividedeirele of nietal. gave an inde-pendent motion to the instru-ment by means of a rack-and-binion, and also placed insidethe compass-box a vernier which,, t. ~ , D. instQ r Magnetic Compass of btone s liion, 1 7zo. read to 3 minutes of arc, and was used like a theodolite. The metal ring placed inside thecompass-box and upon the upper thin base-plate (for therewere two, one revolving upon the other), was provided withtwo sets of divisions, one on the inner edge, for needle-read-ings, and the other on the outer edge, for plane-sights revolved horizontally with one of the base-plates and vernier. This form of the miners dial was thencalled a circumferentor, and is made to-day, under the samename, in precisely the same form. Sissons Theodolite.—Since Mr. Scott wrote his paper, thewriter has also discovered that Fig. 16 in that paperf is notafter the general style of Jonathan Sisson. Fig. 4 repre-sents the original form of Sissons theodolite, which was some-. * Stones English translation of Bion, London, 1723, p. Trans., xxviii., 696. 7:;o REMARKS UPON SURVEYING INSTRUMENTS. what peculiar in construction. This instrumenl is illustratedin an old English work,* and described as follows: 11 For the more perfect understanding of what I would say on this head, it willbe requisite that I give a description of one of the best sort of Theodolites everyet invented for the use of a Surveyor in measuring and mapping made and con-trived by Mr. Jonathan Sisson at the corner of Beaufort Buildings in the Strand ;(a figure whereof is exhibited in the Map fronting the Titel-page of this Book).The Ball and Socket of this instrument are so contrived that by the help of fourScrews placed at right angles to one another below the Centre, the whole is veryrapidly fixed horizontal as well as steady ; and thereby not liable to any Motionor Variation upon moving the Index. There is a double Sextant of Equal Radiusto the Limb (with a Spirit Leve
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
