This produces an opening and closing of the edges symmetrically with respect to the telescope axis.
To avoid such error Dawes used double wires, not spider webs, placing the image of the star symmetrically between these wires, as in fig.
One double web, fixed in the box, is pointed symmetrically, as in fig.
These examples show that Napier was in possession of all the conventions and attributes that enable the decimal point to complete so symmetrically our system of notation, viz.
The cups are placed symmetrically on the end of the arms, and it is easy to see that the wind always has the hollow of one cup presented to it; the back of the cup on the opposite end of the cross also faces the wind, but the pressure on it is naturally less, and hence a continual rotation is produced; each cup in turn as it comes round providing the necessary force.
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