107 



Hygroscope. An instrument for indicating the degree of moisture' 

 in the atmosphere. See also Polymeter. [2, 15, 16, 17, 21, 

 25, 26, 27.] 



Hyman's Range-Finder. See Range-Finder. 



Hyperbolic Curves. Geometrical curves for the use of draughts- 

 men. [15. 24.] 



Hyphydroscope. A periscope as used in submarines. See also 

 Alti-hyphydroscope. [26.} 



Hypsometer, An instrument for determining altitudes by 

 observing the boiling point of water, the boiling point being 

 lowered consequent on the reduction of atmospheric pressure 

 at higher altitudes. [5, 6, 13, 15, 17, 18, 21, 24, 25, 27.] 



Hysterisimeter. An instrument for determining or measuring 

 the effect of strain in materials. 



Ice Calorimeter. See Calorimeter. 



Ichnograph. A term used to designate a ground plan. 

 Illuminator, Parabolic. A parabolic mirror for illuminating 



opaque objects with the microscope. [2, 3, 10, 20, 21, 22, 



26.] 

 Illuminator, Vertical. A reflector used above the object-glass 



of a microscope tor illuminating opaque objects with high 



powers. [2, 3, 10, 20, 21, 22, 26.] 

 Immiscb. Thermometer. See Thermometer. [20, 21, 26.] 

 Impact Testing Machine. For testing materials to fracture by 



the force of impact. In some machines fracture is caused 



by a single heavy blow, in others by a series of lighter blows. 



[4, 16, 24.] 

 Inclinometer. An apparatus to determine the inclination of 



the earth's magnetic force to the plane of the horizon. 



Called also inclination compass-dip circle. [15, 16, 17, 21, 



27.] 



Incubator Thermometer. See Thermometer. [2, 5, 15, 17, 21, 26. \ 

 Index. (i) In astronomy, a brass rule accompanying the plane- 

 table, furnished with perpendicular sight-vanes at each 



extremity. (2) The line or arrow from which the reading 



of a divided scale is made. [15, 27.] 

 Index Gauge. An instrument having jaws whose distance apart 



is indicated by a pointer and dial or other means. 

 Index Glass. The silvered mirror or speculum fixed to the index 



bar of a sextant, the movement of which enables any object 



up to a certain angle to be reflected into the horizon mirror, 



and thence to the eye of the observer. [2, 10, 13, 15, 16, 17.] 

 Index Thermometer. See Thermometer. [4, 15, 16, 21, 25.] 

 Indicator. A finger working in the field of a microscope to 



point out a special object within the field of view. [2, 5, 20.] 

 Indicator, Aeroplane Speed. An instrument for indicating the 



speed of revolution of an aeroplane engine. 

 Indicator, Crosby. An improved form of engine indicator 



(q.v.). [13, 15, 16, 17.] 



