628 Our Surroundings 



Horse latitudes: High pressure areas near the tropics of Cancer and 

 Capricorn; caused by the descending air from the belt of equatorial 

 calms. 



Horse power: The rate of doing work based on what it was thought 

 the average horse could do. One horse power is equivalent to the 

 power necessary to raise 33,000 pounds one foot in one minute. 



Humidifier (hu-mid'i-fi-er) : A device for moistening air. 



Humidistat (hu-mid'i-stat) : A device for regulating automatically the 

 humidity of indoor air. 



Humidity (hu-mld'i-ti) : The degree of moisture in the air. 



Humus (hu'mws) : The dark part of soil formed by the decay of vege- 

 table and animal matter. 



Hurricane: A violent whirlwind of great width generally accompanied 

 by rain, thunder and lightning; usually originates in the tropics. 



Hydro-airplane (hi'dro) : An airplane equipped with boat-like floats for 

 landing on and taking off from the surface of water. 



Hydrocarbon: A combustible compound containing hydrogen and car- 

 bon, such as benzine. 



Hydrogen (hi'dro-j<?n) : A colorless, odorless, tasteless, inflammable gas. 

 The lightest substance known. One of the elements. 



Hygiene (hi'ji-en) : The science of health. 



Hygrometer (hi-grom'e-ter) : An instrument for measuring the degree 

 of moisture in the air. 



Hypocotyl (hi-po-kot'H) : The first root and stem of a sprouting seed. 



Hypothesis (hi-poth'e-sis) : A principle not proved but assumed for the 

 purpose of argument. 



Igneous rocks (ig'ne-ws) : Rocks cooled from a molten mass. Granite 



is an example. 



Immunity (i-mu'm-ti) : The state of being not susceptible to a disease. 

 Incandescent (in-kan-des'^nt) : Glowing or luminous with intense heat. 

 Incandescent lamp: A glass bulb containing a thin wire filament which 



becomes heated and gives off light when an electric current passes 



through it. 



Incised wound (in-sizd') : A cut, usually made by a knife or glass. 

 Incisor (in-sl'zer) : A tooth with a sharp, chisel-like edge, well adapted 



for cutting. 



Inclined plane: A sloping surface. One of the simple basic machines. 

 Induced current: A current of electricity generated in a coil of insu- 

 lated wire by rotating it within the magnetic field of a magnet, or 



by moving a magnet within the coil. 

 Induced magnetism: Magnetism created in a piece of iron by contact 



with, or nearness to, a magnet. 

 Inductance coil: A device in a radio sending set for regulating wave 



length. 

 Inertia (in-ur'shi-d) : A property of matter by which it tends, when at 



rest, to remain at rest, and when in motion, to continue in motion. 

 Infection: The taking of a disease from germs introduced'into the body 



either by direct contact with a diseased person or through the air or 



other germ-bearing medium. 

 Inflammable (in-flam'd-b'l) : Easily set on fire. 

 Infra-red rays (Tn'frd) : Light rays having a longer wave length than 



red light ; they are invisible to the human eye. 

 Inhale: To breathe in. 



