A Description of Helioscopes, and Some Other Instruments

...then use the plumb line original: "Plumbet" at the end of it to align the meridian or twelve o'clock point. Once aligned, you can turn the axis or rod dd using the handle until the index (pointer) ee on the axis points to the hours, half-hours, quarters, or minutes you wish to mark on your sundial. The second index gg will then point directly to the correct corresponding spot on the surface original: "Plain" of the dial itself. However, for sundials on or near a polar plane a dial surface parallel to the Earth's axis, it is helpful to use a small thread stretched from the cross until it touches the surface at the various hours, halves, quarters, minutes, and so on. During this process, the arms of the joint must be fixed so that the axis of the plate crosses the axis of the rod at right angles.

The Universal Joint for all these types of operations, which I did not have time to describe in the previous section original: "last Exercise", I will now explain in more detail. It consists of five separate parts, each of which I will describe with reference to Figures 9 and 10.

The first two parts are the rods and axes A and B. The semicircular arms are fastened to these, and they are joined together so that the movement of one rod transfers movement to the other in a specific ratio—which, for clarity, I call elliptical or oblique.

The next two parts are the two semicircular arms, CC and DD, which are fastened to the ends of those rods. These arms hold the four points of the ball, circle, connecting piece original: "Medium", or cross in the center (labeled X). Each pair of arms has two center-holes into which the sharp ends of the center piece are placed. Through these, the elliptical or oblique movement is transferred steadily, precisely, and very easily from one rod or axis to the other. I call these center-holes the hands.

The fifth and final part is the ball, round plate, cross, or connecting piece (X) in the middle, which is held by the "hands" of both pairs of semicircular arms. For clarity, I will hereafter refer to this as the medium. The two points (marked 1 1) held by the hands of the first axis, I call the points; the other two points (marked 2 2) held by the second pair of arms, I call the pivots.