A Description of Helioscopes, and Some Other Instruments

which, I hope, will make it sufficiently plain to any artist original: "Artist"; meaning a skilled craftsman or instrument maker to understand.

Thirdly, the central original: "medium" ball or cross X must be made of a size suitable to the arms and cylinders, and great care must be taken that all the ends, points, or handles lie exactly in the same plane original: "plain", and that they be all equally distant from their center—or at least that any two opposite ones be made so, because it is not strictly necessary that all four should be equal, though in most cases it is best. Furthermore, the handles or pivots ought to be exactly round, conical, or cylindrical, and their center lines original: "middle lines" must intersect each other at right angles, or perpendicularly. In general, everything about the said joint should be so designed and manufactured that the axes of the two rods may always intersect each other in the center of the central cross or plate, and that the said center, regardless of any change to the joint, may always remain exactly at the same point without any shifting.

The shape of this central piece original: "Medium" may be either a cross whose four ends each have a cylinder (which is the weakest design; it is illustrated in Figures 9 and 10 by the cross X); or secondly, it may be made of a thick brass plate, upon the edge of which are fixed four pivots which serve as the handles for the iron arms to grip. This is much better than the former design, but lacks the strength and stability of a large ball, which is the method I most approve of, as being strong, steady, and elegant. These are depicted in the aforementioned figures as X x and X x x.

If an elliptical dial a sundial with an elliptical layout is to be drawn by orthographic projection original: "Orthographical projection"; a method of representing three-dimensional objects in two dimensions, the previously mentioned method for describing tangent dials sundials where the hour lines are determined by the tangent of the hour angle provides the lines that divide the ellipse original: "Ellipsis" of the equinox in its true proportions. If you want the lines that divide the ellipse of either tropic, or of any other parallel circle, you must adjust original: "rectifie" the semicircular arms CC of the axis BB to the degree of declination of that parallel. Then, proceeding as before, you will have the lines which, from the aforementioned circle, divide the ellipse of that parallel accordingly. Perpendiculars dropped from the ends of the cross 1 1 also give the true ellipse in the orthographic projection corresponding to that parallel.