. Transactions. ADDITIONAL RBMARKfi OH 3URVBY1NG-IN8TRUMBNT8. The Gri bb-1 i w [fi M ine-Si rvbt ing I>ial. This recently introduced instrument i> Bhown in Fig. 2. Ithas an exterior horizontal divided circle with verniers, like ;•theodolite, reading to either three minutes or one. The v<deal Bemicircle, graduated up to (.,(| , is read with a vernierupou an upward-projecting arm, with clamp and tangent-Bcrew motion: and with a uew form of Bight, invented byMr. Grubb, which is Baid to be a greal improvement upon the Fig. Thorntons Surveying Dial-Circumferknt<»r. usual form. The s
. Transactions. ADDITIONAL RBMARKfi OH 3URVBY1NG-IN8TRUMBNT8. The Gri bb-1 i w [fi M ine-Si rvbt ing I>ial. This recently introduced instrument i> Bhown in Fig. 2. Ithas an exterior horizontal divided circle with verniers, like ;•theodolite, reading to either three minutes or one. The v<deal Bemicircle, graduated up to (.,(| , is read with a vernierupou an upward-projecting arm, with clamp and tangent-Bcrew motion: and with a uew form of Bight, invented byMr. Grubb, which is Baid to be a greal improvement upon the Fig. Thorntons Surveying Dial-Circumferknt<»r. usual form. The semicircle is screwed to the outside of thecompass-box, and magnetic bearings are taken in the usualway. The instrument is mounted with a modified form ofHoffmans patent joint and with leveling-screws; and is some-times constructed without a circular compass, becoming thena very superior circumferentor, with a long trough magneticcompass below the horizontal circle, for determining the mag-netic meridian. 824 ADDITIONAL REMARKS ON SURVEYINO-INSTRUMENTS. Micron btbio Brough, in his further discussion,2 argues that neitherGascoigne (1639), Buygene (1659), Malvasia (1662), nor Au-zout and Picard(1667), contemplated the direct measurementof distances on the earth by means of the visual angle, withthe micrometer; an idea, however, clearly contemplated, in his Fia. 2.
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