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12for it is clear there is a great difference between the forces with which the several winds act. Inasmuch as the brevity with which the foregoing rules are laid down may prevent their being clearly understood, I have thought it right to add for the clearer understanding thereof two figures, or as the Greeks call them σχήματα, at end of this book. The first shews the precise regions whence the different winds blow. The second, the method of disposing the streets in such a manner as to dissipate the violence of the winds and render them innoxious. Let A be the centre of a perfectly level and plane tablet whereon a gnomon is erected. The ante-meridianal shadow of the gnomon being marked at B, from A, as a centre with the distance AB, describe a complete circle. Then replacing the gnomon correctly, watch its increasing shadow, which after the sun has passed his meridian, will gradually lengthen till it become exactly equal to the shadow made in the forenoon, then again touching the circle at the point C. From the points B and C, as centres, describe two arcs cutting each other in D. From the point D, through the centre of the circle, draw the line EF, which will give the north and south points.
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