Page:Popular Science Monthly Volume 34.djvu/310

296 two systems of graduation traced upon its outer edge—degrees and points. The bulb is an air-tight ellipsoid, with a conical depression on its nether surface; in this depression is a small brass with four tiny set-screws (R, R)—only two are shown, however—which press and hold steady in place the jewel (Y)—a sapphire hollowed out and smoothed to the utmost degree, so that the highly polished pivot-point upon which it rests may encounter the least friction possible.

The little set-screws are for adjusting the jewel to the very center of the graduated rim. The tubes, which are two of one length and two of another, are about the diameter of a lead-pencil; their extremities are soldered to the under edge of the rim, and in addition the two inner ones are soldered to the bulb K; this gives the rim rigidity, as of itself it is both light and flexible.

In the tubes are placed the magnets—the vital part of the compass. These magnets are prepared with the most scrupulous