258 THRELFALL AND POLLOCK ON A QUARTZ THREAD GRAVITY BALANCE. 



DESCRIPTION OF PLATES. 

 PLATE 1. 



N.B. The drawing does not show the long inner tube carrying the eye-piece inside the outer microscope 

 tube, otherwise it is a working drawing. 



INDEX TO LETTERING ON THE DRAWING. HORIZONTAL SECTION. 



A. Axle carrying rosette spring. B. Axle carrying vernier arm. C. Rosette spring. D. Wire lever. 



EE. Glass tube carrying platinum wire thermometer wound in a double spiral. 



FF. Main bearings. GGG. One of the three bars holding the bearings together. 



H. Pinion of arrester working into two racks carrying the arrester jaws. I. Jaws of the arrester. 



JJJJ. Copper tube bored to fit the main bearings and rigid with respect to the inner mechanism. 



KKKK. Paper insulation round the copper tube. LLLL. Outer tube of polished brass. 



MMM. Levelling screws forming the supports of the balance on the tripod stand. 



NNNN. Ground and lapped tube carrying the microscope. 



000. Points at which connection is made to the thread by soldering. 



PPPP. Mercury stuffing boxes on the vernier and arrester shafts. 



QQQQ. (Also in the plan of the graduated arc.) Screws attaching the inner mechanism to the copper 



tube. 



RR. (Also in the plan of the graduated arc.) Part of the framework of the webbing of the sextant arc. 

 SS. Cap of mercury stuffing box on vernier axle. 

 T. (Also in plan.) One of the screws fastening the vernier arm to a strengthening piece screwing to the 



vernier axle. 

 U. (Also on the plan of the graduated arc.) Clamping washer and lock nut with left-handed thread to 



insure rigid connection between the vernier arm and the axle. 

 V. Clamp screw of arrester shaft. 

 W. Slow-motion screw used in the preliminary adjustment of the tension of the thread, afterwards 



clamped by the clamp screw and soldered up to an invariable position with respect to the 



frame. 

 Y. Box key on the arrester shaft. This key can be drawn out by unscrewing the stuffing box from the 



end of the balance case. The drawing shows the box key too large to pass through the hole, 



this is wrong. 

 ZZZZ. Screw clamps or clips holding the balance proper to the under frame. 



INDEX TO LETTERING OF THE ELEVATION OF THE MICROSCOPE AND STRIDING LEVEL. 



L. Level. KK. Adjustments for the level tube. C. Cross level. 



VV. The sides of the V grooves of the level carriage. MM. Worked tube carrying iie microscope. 



PLATE 2. 

 Reproduction of photographs of the apparatus. 



