A little beyond the curves, a very small but perfectly
conspicuous dot is engraved--the intersection of two lines of
construction that it was doubtless desired to efface, but the scarcely
visible trace of which subsists. Upon measuring with the compasses the
distance between the insertion of the thread and this dot, we find
exactly the distance, N P, of our diagram. Therefore there is no doubt
that this dot served as a datum point. The existence of the bead upon
the thread and the use of it as a rude calendar therefore appears to
be certain.
The compass is to furnish us new indications. After dismounting it--an
operation that the quite primitive enchasing of the face plate renders
very easy--we took a copy of it, which we measured with care. The
arrow forms with the line O C-O R an angle of 90 deg. + 8 deg.. The compass
was therefore constructed in view of an eastern declination of 8 deg..
Now, here is what we know with most certainty as to the magnetic
declination of Paris at the epoch in question:
Years. Declinations.
1550. 8 deg. east.
1580. 11.30
1622. 6.30
1634. 4.16
On causing the curve (Fig. 3, No. 3) to pass through the four points
thus determined, we find, for 1612, the declination 81/2 deg.
Pages:
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152