644 Our Surroundings 



Turbine: An encased water wheel having a series of curved blades or 



buckets against which the water strikes, causing rapid motion. A 



powerful form of steam engine used in steamships. 

 Tympanic membrane (tim-pan'ik) : The ear drum. 

 Typhoid fever (ti'foid) : A dangerous and infectious disease. Spread 



by flies, by improper sewage disposal, and by food, water and milk 



containing typhoid germs. 

 Typhoon (ti-fodn') : A hurricane when it occurs over the Pacific ocean. 



U 



Ultra-violet rays: Light rays having a shorter wave length than violet 

 light ; they are invisible to the human eye. 



Umbra (utn'brd) : The darkest part of a shadow. 



Undershot wheel: A water wheel operated by the force of water flow- 

 ing against its blades from underneath. 



Universe: Everything that exists. This includes the entire solar and 

 sidereal systems. 



Urine (u'rin) : A fluid waste of the body given off by the kidneys. 



Vaccination (vak-si-na'shwn) : Inoculation with a vaccine, usually cow- 

 pox, to produce a mild form of smallpox, resulting in subsequent 



immunity. 

 Vaccine (vak'sin) : A substance employed in vaccination to combat 



smallpox. 

 Vacuole (vak'u-61) : A small area in a protozoan containing food, air 



or waste. 



Vacuum (vak'u-wm) : A space from which all air has been exhausted. 

 Vacuum bottle: A glass bottle within a larger bottle, sealed to it at the 



neck. The space between is a vacuum. Used to keep hot liquids hot 



or cold liquids cold. 

 Vacuum tube: A glass bulb used in radio to change the frequency of 



sound and radio waves. 

 Valve: A device for regulating the flow of a gas or liquid, as the valve 



in a water faucet or the valve by which water is admitted to the tanks 



of a submarine. Any of the thin, strong flaps of tissue in the heart 



which allow the blood to flow in only one direction. 

 Vapor: The gaseous form of a liquid or solid. 

 Vaporize: To convert into vapor by means of heat. 

 Variable condenser: A device in a radio receiving set which permits 



only radio waves of a certain length to pass, thereby allowing but one 



station at a time to be heard. 

 Vein: A tube which conveys blood to the heart. A channel through 



which fluid circulates through a leaf. 

 Velocity (ve-16s'i-ti) : Rate of speed or movement. 

 Ventilation: The act of changing or renewing the air supply. 

 Ventilation systems 



Gravity system: A system of ventilation by which air flows nat- 

 urally into a building through windows and doors, or through air- 

 shafts. 



Plenum system (ple'nwm) : A forced system of ventilation by which 

 fresh air is blown into a building by fans, displacing and forcing out 

 the bad air. 



