28                   The Meafzure of the Earth.

         After this preparation it will be to the purpofe to faflen the Ob-
       je6t Glafs apart in a Coipper Box pierced through its two enlds, and
       rerfc(ly turned round ; in which, neverthelets, it vnuft have a little
       play in fuch Ibrt that one may a little thrufm it from one fied to
       t'other by three Screws with their heads cut off to hold it 1leadxr;
       and this Box being exadtly enchafed ilito the Objelive Pinnule,
       one may make it turn upon its Cenrer, mean w h ile the xw hole body
       of the Telefcope remains immoveable ; and one may ob erve, thad
       if in making the Obje&t Glafs fo to turn, the Telekcope alv, Avs
rc,,
       mains pointed to the fame Objed, otherwife the Objet Glafs mnut
       be moved either to the one fide or the other.
          We thought it neceflary to give all thefe dif ring ways of veri-
       fication, to the end that there might remain no doubt as to the great
       exaatnefs which one ought to look after in Telefcopes uetd for Pin-
       tiules or fights of Inflrumcnts.

                             A R T I C L E       X.

       IF the rucafure of the Earth requires precife and exaAt Obferva-
          tion) it is principally for that which concerns the diWfrence of
       Latitudes, becaufe the error of one Minute only amounts to 95I
       Toyfes, which is multiplyed upon the whole as many times as the
       diflance meafured is corftained in the whole Circumference of the
       Earth.
Plate 4Obi. For approaching as much as is pollible to the exac~nefs requifite,
Ift and 2d,.
Figure. the great Initrument reprefented in ihe fourth Plate was caufed to
       be made; it is of Iron ftrengthened Nilith pieces upon the Arda of
       it, as the Quadrant, and covered with Copper at the places neceffi-
       ry. The Limb, which contains not above the zoth part of a Circle
       of ten Foot Radius, is divided by Dragonal Lines even to thirds of
       Minutes very diflindtly.
          A Telefcope of ten Foot ferves for Pinnules or Sights to this In.
       firument. And becaufe that in file obscurity of the Night one
       could not fee the Filaments that were in the Telefcope, they were
       enlightened by the upper end of the Telefcope, or by a hole made
       on the fide.
          The Plumb or Perpendicular was fecured in a Pipe of Tin, w hich
       kept it intirdly covered from the Wind, befide that they always ob-
       ferved in a clofe place, of which the cover or roof was purpofely
       pierced.
          For determining with this Initrument the diffirences of the La-
       titude of Malvoijine, of Sourdon, and of Amiens, the Star called the
       knee of Cajiopea was made choice of, which comes to the Meridi-
       an at 9 or IO Degrees of diftance from the Zenith towards the
       North, about z8' 46" of time after the Polar Star.  A Star more
       near to the Zenith would have been more difficult to be well
       obferved. And if otherwife it fliould have been placed between two
       Zeniths, the error of the iftrument which might not poffibly be
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