104 



Houston's Apparatus. For the bacteriological examination of 

 milk. [13.] 



Hudson's Horse-Power Computing Scale. See Horse-Power 

 Computing Scale. [5, 15, 21, 24.] 



Hull Calliper. See Calliper. [15.] 



Huyghenian Eyepiece. See Eyepiece. [2, 3, 17, 18, 20, 21, 22, 

 23, 26, 27.] 



H.S. Range-Finder. See Range-Finder. [5, 20.] 



Hyalograph. An instrument for tracing a design on a transparent 

 surface. 



Hydraulic Gauge. See Gauge. [5, 15, 17.] 



Hydro- Barometer. An instrument for determining the depth of 

 the ocean by its pressure, generally called depth gauge 

 (q.v.). [6, 13, 15, 17.] 



Hydrodynamometer. An instrument to measure the velocity of 

 a liquid current by the force of its impact. 



Hydrogen Purity Meter, Shakespear. An instrument for deter- 

 mining the purity of hydrogen. It is calibrated directly in 

 percentages of hydrogen, readings being given by the 

 deflections of a pointer. 



Hydrograph. An instrument used in making Mercator or sea 

 charts for giving the varying scale of latitude. [15, 17.] 



Hydrometer. An instrument for determining the specific gravity 

 of liquids. Attributed to Archimedes, but not much used 

 until it was reinvented by Robert Boyle. It usually takes 

 the form of a narrow sealed instrument of cylindrical section 

 and consists of three parts the counterpoise at the bottom ; 

 the bulb containing air ; and the stem with the scale, at 

 the top. Made of glass or gilt brass. In the latter case the 

 hydrometer is usually provided with weights, which are 

 slipped over the stem and alter the buoyancy of the instru- 

 ment so as to adapt it to liquids of various specific gravities. 

 See also Alcoholometer, Ammoniameter, Saccharometer. 

 [5, 6, 9, 13, 15, 16, 17, 21, 24, 26, 28.] 



Hydrometer, Accumulator or Battery. A hydrometer for taking 

 the specific gravity of liquids in an accumulator. Usually 

 made with flat bulb so that the hydrometer can float 

 between the plates, but sometimes consists of a small hydro- 

 meter contained in a glass pipette, into which the acid can 

 be sucked by means of a rubber bulb. Usual range, 1,100- 

 1,250. [5, 6, 13, 15, 17, 21.] 



Hydrometer, Alcoholometer, Gay-Lussac's. For testing alcoholic 

 solutions. Scale shows percentage per volume, o-ioo per 

 cent. 



Hydrometer, Ammoniameter. For testing ammonia solutions ; 

 scale, o-4O. (To convert to s.g., multiply by 3 and deduct 

 from 1,000.) 



Hydrometer, Barktrometer. For tanning liquor. Scale, o-8o. 

 [16.1 



