1885.] NEW YORK ACADEMY OF SCIENCES. 99 



The efficiency of these arc-light machines was not obtained, 

 but we are justified in assuming that 70 per cent of the absorbed 

 power will reappear as electrical power in the circuit, neglecting 

 its losses. 



That is, 



3 75 



— — -^ — - =6.3 lbs. of ordinary anthracite coal per electri- 



.80 X .70 -^ ^ 



cal horse-power per hour. 



994 



-—— = 158 candles, 

 o.d 



If we divide the candles per El. H. P. by the weight of coal 

 required to produce them, we find in the arc system that Ave ob- 

 tain 158 candles per lb. of coal for the naked light, and some- 

 thing less than 75 candles if ground glass or opal globes are 

 used, and the light seen from the most favorable position. 



Very different from the arc light is the incandescent. Its 

 light is so soft that we do not realize its brilliancy until we sub- 

 ject it to measurement. It gives out no products of combustion 

 to poison our air; it shows colors truly. A delicate hair of car- 

 bon sealed within a vacuum by walls of glass, glitters and glows 

 until at almost limpid incandescence it gives us a steady, clear 

 light, colorless as daylight. 



If you will take a book and hold it from one to two yards away 

 from a 16 candle light, you will find it sufficiently diffused to 

 read with comfort. 



Now all -know that the intensity of illumination varies in- 

 versely as the square of the distance. Therefore, roughly esti- 

 mating a shaded arc light at 500 candles, the same book would 

 have to be held somewhere between 5^ and 11 yards away from 

 it to be read with equal comfort, assuming tlie light to be steady. 

 We can then say that a 16 candle incandescent light will illumi- 

 nate a circle of 12|- sq. yards area, and that a shaded arc light 

 giving 500 candles out of 99-4 will illuminate a circle of 400 sq. 

 yards area, or 32 times as great. That is to say, about thirty- 

 two 16 candle lamps would supply an equal illumination with a 

 vastly better distribution of light for the use of the eyes. 



We can, therefore, say that 500 candle power from incandes- 

 cent lamps will far more than replace 1,000 candle power from 

 the arc light, under the conditions of actual usage. 



We can safely say that, for all purposes save that of obtaining 

 light to dispel darkness, the incandescent light is twice as valu- 

 able, light for light, as the arc light, and, therefore, should be 

 multiplied by two when compared with it. 



The objections most vehemently urged against incandescent 



