Transactions . G F his Alhidada or index with sightes A B the line perpendiculars fromB donnward noted irith 200 psxtes, equal] to the diuisionso! the scale, DECthe semicircle bauing on his Diameter two sightes fixed as was to fore is also to he noted, that the double scale is compound o! two Geometrical]squares, the one seruing for altitudes, tin-other for profundities. The squarewhich the line perpendicular cutteth when the Diameter is directed to any market lying lower than your Station, I call the scale of profundities, the other shall for Fig. V Diggess Semicircle. dis
Transactions . G F his Alhidada or index with sightes A B the line perpendiculars fromB donnward noted irith 200 psxtes, equal] to the diuisionso! the scale, DECthe semicircle bauing on his Diameter two sightes fixed as was to fore is also to he noted, that the double scale is compound o! two Geometrical]squares, the one seruing for altitudes, tin-other for profundities. The squarewhich the line perpendicular cutteth when the Diameter is directed to any market lying lower than your Station, I call the scale of profundities, the other shall for Fig. V Diggess Semicircle. distinction be named the scale of altitudes. This semicircle ought so to beplaced that the centre B hang directly over the centre A, and that the diameterD C with his sightes may be moued vp and downe, and also sidewise whither youlist, ahvayes carying G F about directly under it. You must also prepare a staffepyked at the ende, to pitche on the ground with a flat plate on the toppe to setthis instrument vpon. It is also requisite that within Theodelitus you haue aneedle or fly so rectified, that being brought to his due place the crosse diametersof the Planisphere may demonstrate the foure principall quarters of the Horizon,East, Weste, North and Southe: And this may you do by drawing a right linemaking an angle (with that one diameter of youre instrument representing themeridiane) equal I to the variation of the copasse in your region : which in Eng-land is 11^ grades or neere therabout, and may be redely observed in all placessundrie waves. But thereof I mind
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
