376 GENERAL SCIENCE 



sixteen times as much light as a standard candle. It is usually 

 customary to say that if a candle is placed one foot from a surface 

 the amount of illumination received by it is called one foot candle. 

 If the distance is increased, the illumination decreases in proportion 

 to the square of the distance. Hence, at two feet the one candle 

 would give only one-fourth (}) foot candle power, and a sixteen (16) 

 candle power lamp would give at two feet one-fourth of sixteen 

 (JX16) or four foot candles. 



The reason for this is very plain. It is simply that the same 

 amount of light is spread over a greater and greater surface. As 

 the distance is increased, the light which falls on one square foot at 



Kerosene ft 

 Kerosene 

 GoaOpen Flame* 

 Gas Mantle 

 Carbon f/ec/nc" 

 Cem' Electric 

 Tungsten i 



S 10 IS to 25 90 

 Coatc/tOOO candle-hours in cents 



FIG. 260. How many times more expensive would it be to light a house with 

 candles than with electricity? Compare the cost of lighting with different 

 kinds of electric lights and gas light. What would be the difference between 

 gas and electricity? What method of lighting by gas is the cheapest? What 

 method of lighting by electricity is the cheapest? 



one foot distance must, at two feet, illuminate a surface which is 

 two feet on a side; that is, four square feet. At three feet distance, 

 the light is spread over nine square feet, and so on. Therefore, the 

 amount of illumination that each square foot will receive will be 

 the square of the distance inverted. 



Measuring Candle Power of a Lamp. The candle power of a 

 lamp may be roughly measured by placing a white piece of card- 

 board upright on the table and sticking a pencil in a piece of cork 

 so that it will stand upright about 2 inches from the paper. Place 

 a candle about 1 foot distant from the pencil. Place the lamp far 

 enough back so that the shadow caused by the candle will be of the 

 same density as the shadow caused by the light. If the distance 



