202 Management of Light in Illumination. 



Paris, we can estimate the cost of the Hght which is 

 produced by each of them. We will begin by deter- 

 mining the cost of 1 00° of light furnished during one 

 hour by a good wax candle. 



A bundle of wax candles called a pound, but which 

 weigh only 450 grammes {^= 6954 grains Troy, or 

 53 grains less than a pound avoirdupois), is now sold 

 at Paris for three francs, or two shillings and sixpence 

 sterling, if we take the exchange at what it was for- 

 merly in time of peace. 



One of these candles furnishes just 100° of light, 

 and consumes just seven grammes of wax per hour. 

 The five candles will burn 64 hours, 17 minutes, and 

 8 seconds; or 64? hours ; and, as the five candles cost 

 2S. 6d. = 120 farthings, the 100° of light furnished by 

 one of them costs ^ = 1.8666 of a farthing per hour. 



Six tallow candles of the best quality usually sold in 

 the shops of Paris, weighing together 476.4 grammes 

 (= 7358.8 grains Troy, or iSji^ ounces avoirdupois), 

 are now sold for sixteen sous, or eightpence ster- 

 ling. And I find that one of these candles consumes 

 10.35 grammes (= 166 grains Troy) of tallow per hour, 

 when the most scrupulous attention is paid in burning 

 this candle to keep it constantly well snuffed. 



Now as six of these candles weighing 476.4 grammes 

 cost eightpence, or 32 farthings, the quantity of tallow 

 consumed in one hour = 10.35 grammes must cost 

 I o- 3 5x32 __ Q 5^22 1 of a farthing. 



If this tallow candle had furnished the same quan- 

 tity of light as was furnished by the standard wax 

 candle, viz. 100°, the cost of its light would have been 

 to the cost of that furnished by the wax candle as 



