A Description of Helioscopes, and Some Other Instruments

and consequently measure any angle to an accuracy of five seconds; for, by moving the frame to the next division that is slightly smaller than the angle desired, and then using the eyepiece original: "Glass" to fix one of the arms (the one holding the plate) exactly over the hole or point of division, the remaining portion of the angle could be exactly measured using the screw.

As for the method of calibrating original: "dividing" any of these, the best way will be to measure a distance of 1,000, 1,500, or 2,000 feet in length upon some level plane. This should be done using two rods, each twenty feet long, or else by wires pulled taut with weights (a method which I shall describe shortly). Beginning from the very center of the instrument, and at? the far end of the measured distance, set up a series of pine boards original: "Deal-boards"; boards made of fir or pine wood joined end-to-end in a straight line. Alternatively, stretch a fairly thick line that intersects the measured line of distance from the center of the instrument at right angles. Then, using a table of natural tangents—with the distance from the center of the quadrant acting as the radius—set and mark the divisions of degrees and minutes upon those boards or lines. Use compasses or rulers original: "Rules" to do this as accurately as possible, marking them so that the degrees may be distinguished very clearly from the minutes.

Then, having adjusted the instrument so that you can see the beginning of those divisions through both tubes at once, set both pointers original: "Indices" to 0 (the beginning of the divisions). Next, keep the lower of the two tubes fixed in the same place, so that it still aligns with the same starting point of the divisions upon the boards or line. With the help of the rod, turn the screw or roller original: "Rowl" until you find the upper tube aligns with the first minute, and then the first degree, and so on, until you see the last minute of the five whole degrees (or whatever angle you intend the instrument to cover).

Then (for the first and third methods mentioned previously), calculate how many whole rotations original: "Revolutions" and what fraction of a rotation are required to make up that entire angle. Subdivide this measurement using a small table into minutes and seconds. You will quickly find through this trial that you will be able to calibrate with an extraordinary accuracy upon those boards by using your tubes and screw, even at a distance of 1,000, 1,500, or 2,000 feet. The precision will almost match the divisions made with your compasses when you were standing right at the boards. And by this method you