57 



Areograph. An instrument for drawing a circular arc without 

 the use of a central point. [27.] 



Argentometer. A hydrometer for measuring the density of 

 silver solutions. [6, 17, 21.] 



Arithmometer. A comprehensive name for a calculating machine. 

 [17, 20, 21.] 



Armillary Sphere. An ancient astronomical apparatus composed 

 of rings to represent the circles of the celestial sphere. 



Armillary Sphere Sun-Dial. A form of skeleton sphere in which 

 the shadow of the central axis or pole falls on the divided 

 equatorial circle and indicates the time. [15.] 



Armstrong Scale. A drawing scale, containing a set of eight 

 scales, used chiefly by engineers. 



Arrows for Land Chain. Strong wire pins for use with the land 

 or surveying chain. [2, 5, 6, 13, 15, 16, 17, 20, 21, 24, 25, 27.] 



Arsenic Apparatus. Used in the preparation and testing of 

 arsenic. 



Artificial Horizon. An instrument used to give the true hori- 

 zontal level and to determine by means of a sextant the 

 true altitude of a heavenly body. It may consist of (i) a 

 bath of mercury in a suitable vessel ; (2) a piece of silvered 

 or black glass suitably mounted. [2, 5, 6, 13, 15, 16, 17, 



20, 21, 24, 25.] 



Artificial Horizon, George's. A portable form of mercurial 

 artificial horizon for sea and exploring work. [6, 13, 15, 17, 



21, 24, 27.] 



Artificial Horizon, Paget. An improved form of bubble horizon 



for attachment to a sextant. [13, 15.] 

 Ashton's Compass Rule. See Compass Rule. [13, 15.] 

 Aspirator. An apparatus for passing air or gases through or 



over certain liquids or solids. 

 Aspirometer. An aspirator graduated so as to measure the 



volume of air or other gases aspirated. 



Assay Apparatus. Equipment for analysis of metallic ores, etc. 

 Assmann's Psychrometer. See Psychrometer. [6, 17, 21.] 

 Astatic Galvanometer. See Galvanometer. [4, 16, 19.] 

 Astigmatic Test Chart. A diagram with lines drawn in various 



directions for testing astigmatism of the eyesight. [2, 8, 



20, 26.] 

 Astrodicticum. An astronomical instrument by means of which 



many persons are able to view the same star at the same 



time. 



Astrolabe. The ancient form of instrument for taking the alti- 

 tude of a star or heavenly body. It is now superseded by 



the quadrant or sextant. 

 Astrometer. An instrument invented by Sir John Herschel for 



measuring the apparent magnitudes of the stars. 

 Astrometeoroscope. An apparatus for demonstrating by means 



of the optical lantern the effects of persistence of vision. 

 Astronomical Clock. 



