A Description of Helioscopes, and Some Other Instruments

First, regarding the Rods: they may be made to whatever size you see fit, depending on the purpose for which you design the Instrument. The only care needed when making them is: first, that they must be perfectly cylindrical in the sections that move within the Collars; and second, that their Axis—the middle line of the rod—must intersect the other exactly at a single point. This point must not change when the Joint is adjusted, whether by increasing or decreasing the angle between the rods, by tilting the Semicircular-arms to a specific angle obliquity, or by rotating the entire Instrument. Constructing them requires a very skillful and knowledgeable Craftsman original: "Artist" to ensure they move with the necessary precision.

Let a b represent one of these Rods and c d a second. These are turned on a lathe to be perfectly cylindrical inside the Collars e, f, g, and h. These Collars are positioned and fixed to a frame so that the middle line or axis of both these cylinders intersects at point i. If the necks of the rods and the collars are made accurately, their Axes (the middle lines) will always intersect at that same point, no matter how they are rotated within their Collars. This point i must remain fixed regardless of how the two Axes are angled toward each other. Even if c d is moved to position l m or n o—forming a wider original: "obtuser" or sharper original: "acuter" Angle—point i must remain the center of the Connecting Piece original: "Medium", where both Axes meet and intersect.

Secondly, the Semicircular-arms may be made to any size, thickness, or strength required for the task. For example, if they only need to move the hand of a Clock to match the shadow on a Standard Sundial original: "Common Dial"—whether designed using Orthographic, Stereographic, or Horological projection—then very little strength is needed. Similarly, if they are meant to show the yearly movement of the Sun along the Ecliptic the sun's apparent path across the sky or the Equation of Time The mathematical difference between time kept by a clock and time shown by a sundial., a small amount of strength is sufficient.

However, if they are intended to rotate a large Quadrant an astronomical instrument for measuring angles, like the one I described previously, they must be made much stronger and more solid. One must also take care that tilting the arms to any angle does not shift the center of the Ball or Cross away from the point where the two Axes intersect. Both these