A Description of Helioscopes, and Some Other Instruments

[You] may easily examine whether your instrument makes the subdivisions exactly or not, which will be a great confirmation of the certainty and accuracy original: "truth" of your instrument. But for the second method, using straight screws, the table of subdivision into degrees, minutes, and seconds must be proportioned according to the length of the chords original: "Subtenses" corresponding to the radius, which is the distance from the center of the rollers original: "Rowlers" to the center of the instrument.

Now, because in an instrument of this size it would be quite difficult to turn the whole angle using only the screw on the edge original: "Limb"; the graduated arc of a quadrant or sextant—which I find is also somewhat difficult even in the three-foot radius instrument when the angle is large—to prevent that trouble and to be able to immediately open the instrument to the desired angle, or very near it: the screw l (in the first figure of my Animadversions original: "Animadv."; refers to Hooke's 1674 work Animadversions on the first part of the Machina Coelestis of... Johannes Hevelius) at the end of the movable arm is designed so that, by unscrewing it, the long screw is pulled away from touching the threads original: "threds" on the edge. Once this is done, the arm is at liberty to be moved to any part of the quadrant. Then, by returning screw l, the screw-frame and screw are brought back down again to grip the threads on the edge of the instrument.

The only care to be taken in this action is that neither the pointer original: "Index" e e is moved at all from its position relative to the pointer-frame h h, nor that the pointer 8 is moved at all around the rod of the screw 9 9 9. It does not matter at all if the screw-rod 9 9 9 is turned or moved, provided it is done by the rod o o o and its handle p p, or by the small handle x at the end of the screw-rod, and that pointer 8 (being very firmly fixed to that rod) moves around with it by the same motion without changing its position on the rod. For once it is brought back down by the return of screw l to take hold of the threads of the edge—into which it must be steadily guided by hand—pointer e e will show upon the edge the number of threads or rotations original: "Revolutions" from the beginning, and pointer 8 will show what fraction of a rotation should be added to it.

I hope I shall not need to spend time explaining original: "explicate" how the centers of these telescope tubes original: "Tubes" are to be made, nor how the lenses original: "Glasses" and thread-sights original: "Thred-fights"; crosshairs made of fine thread or wire are to be fixed. Nor do I need to show much about how the tubes may be stiffened to keep them from warping very much;