Gravesande, Willem Jacob 's
,
An essay on perspective
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fore the before-mention’d Perſpective will be an
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Hour-Line of a Dial, drawn upon the Plane
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H L M I, and whoſe Style is G F.</
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<
s
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xml:space
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">The ſame may be demonſtrated of the Re-
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preſentations of other Hour-Lines, which form
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a Dial upon the Plane H L M I.</
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<
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<
s
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xml:space
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">We now proceed to lay down the beſt way of
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determining the ſaid Repreſentations.</
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<
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. I.</
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style
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s
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">Draw the Line E O, thro’ E, the Foot of the
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<
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">Fig. 66.</
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Style of the Horizontal Dial A B F D, equal
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to the Length of the Style of the Dial to be
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drawn, and making an Angle with the Meri-
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dian C. </
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">XII, equal to the Plane’s Declina-
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tion.</
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<
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<
s
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xml:space
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">This Angle muſt be aſſum’d towards the
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Point D, when the Plane’s Declination is South-
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Eaſt, as here; </
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<
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">towards F, when the Declination
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is Weſtward; </
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<
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">towards A, when it is North-Eaſt-
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wardly; </
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<
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xml:space
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">and towards B, when it is North Weſt-
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wardly.</
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<
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">Now, thro’ the Extremity O of the ſaid Line,
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draw the Line I H, perpendicular thereto; </
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<
s
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xml:space
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">and
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the Line C P, thro’ the Center of the Dial, pa-
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rallel, and equal to E O; </
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">thro’ whoſe Extre-
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mity, P, draw the Line P S, parallel to H I.</
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<
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<
s
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xml:space
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">Then, to make the Dial, draw the Line h i,
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<
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in which prick down the Diviſions of the
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Line H I; </
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xml:space
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">and in the Point o, (which is the
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ſame as O) raiſe the Perpendicular o p, equal to
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the Length of the Style of the Horizontal Dial
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A B D F. </
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<
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">This being done; </
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h i, thro’ the Extremity of this Perpendicular;
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</
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<
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