Gravesande, Willem Jacob 's - An essay on perspective

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20098An ESSAY fore the before-mention’d Perſpective will be an
Hour-Line of a Dial, drawn upon the Plane
H L M I, and whoſe Style is G F.

The ſame may be demonſtrated of the Re-
preſentations of other Hour-Lines, which form
a Dial upon the Plane H L M I.

We now proceed to lay down the beſt way of
determining the ſaid Repreſentations.

Prob. I.

122. To draw Vertical Dials.

Draw the Line E O, thro’ E, the Foot of the
11Fig. 66. Style of the Horizontal Dial A B F D, equal
to the Length of the Style of the Dial to be
drawn, and making an Angle with the Meri-
dian C. XII, equal to the Plane’s Declina-
tion.

This Angle muſt be aſſum’d towards the
Point D, when the Plane’s Declination is South-
Eaſt, as here; towards F, when the Declination
is Weſtward; towards A, when it is North-Eaſt-
wardly; and towards B, when it is North Weſt-
wardly.

Now, thro’ the Extremity O of the ſaid Line,
draw the Line I H, perpendicular thereto; and
the Line C P, thro’ the Center of the Dial, pa-
rallel, and equal to E O; thro’ whoſe Extre-
mity, P, draw the Line P S, parallel to H I.

Then, to make the Dial, draw the Line h i,
22Fig. 67. in which prick down the Diviſions of the
Line H I; and in the Point o, (which is the
ſame as O) raiſe the Perpendicular o p, equal to
the Length of the Style of the Horizontal Dial
A B D F. This being done; draw a Parallel,
h i, thro’ the Extremity of this Perpendicular;
on which, prick down the Diviſions of the

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Scan	Original
131	62
132	63
133	64
134
135
136
137	65
138	66
139	67
140	68
141
142
143
144	69
145	70
146	71
147	72
148
149
150
151	73
152	74
153
154
155
156	75
157	76
158
159
160

Scan	Original
131	62
132	63
133	64
134
135
136
137	65
138	66
139	67
140	68
141
142
143
144	69
145	70
146	71
147	72
148
149
150
151	73
152	74
153
154
155
156	75
157	76
158
159
160

Scan	Original
131	62
132	63
133	64
134
135
136
137	65
138	66
139	67
140	68
141
142
143
144	69
145	70
146	71
147	72
148
149
150
151	73
152	74
153
154
155
156	75
157	76
158
159
160