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THE POPULAR SCIENCE MONTHLY. 



two systems of graduation traced upon its outer edge — degrees 

 and points. The bulb is an air-tight ellipsoid, with a conical de- 

 pression on its nether surface ; in this depression is a small brass 



cap {T, Fig. 7), with four 

 tiny set -screws {R, R) — 

 only two are shown, how- 

 ever—which press and hold 

 steady in place the jewel 

 (F) — 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 grad- 

 uated rim. The tubes, 

 which are two of one 

 length and two of another, 

 are about the diameter of 

 a lead -pencil ; their ex- 

 tremities 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 



Figs. 4, 5.— United States Navy Compass. . ,-, t -, , t n ■-, -, 



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 



7.— United States Navt Compass. 



