1885.] NEAV YORK ACADEMY OF SCIENCES. 97 



These facts, taken from the labors of many impartial and skil- 

 ful workers in scientific research, do not correspond with the 

 alluring statements frequently set before us, but I believe them 

 reliable and practical. 



The broad lesson to be drawn from them is that we do not ob- 

 tain 4j of the power in coal in the form of electricity; and that 

 11 remain to be obtained by the discoverer of an economical 

 method of direct conversion of heat into electricity. 



When the direct method of conversion of heat into electrical 

 energy yields a larger percentage of the power in coal than the 

 indirect method which I have Just described, at the same cost, 

 then will the dynamo supplant the steam engine. Until then it 

 must remain what it is — a distributor of power for the steam 

 engine, or other mechanical motor. 



The Conversion of Electrical Power into Light. 



There are at present in use two methods of converting electri- 

 cal power into light. The first and apparently the most economi- 

 cal is by means of the voltaic arc between carbon points^ the 

 second, by means of the incandescence of a carbon filament in a 

 vacuum. 



The first method is open to severe criticism, save on the point 

 of economy and for lighting large spaces. 



The briefest look at the intense ^^ot of light formed by the arc 

 between the points of carbon causes a painful and persistent 

 image on the eye. The light has a vicious way of hissing, which 

 become unendurable to sens ive nerves, and it varies the monot- 

 ony of this noise by sudden jum])s and flickers. Its ghastly 

 effects are due to its bluish color and the deep, sharply-defined 

 shadows. 



In some cases the arc has a way of rotating around the axis of 

 the carbons, whicli also causes variations of the intensity of the 

 light in different directions. 



Opal glass-globes, which cut off something more than one-half 

 the light, are required to make the light tolerable; and as for the 

 lamp itself, I do not think the greatest skill and taste of design- 

 ers have yet rendered it ornamental when not lighted. 



As a rule, the arc light is most intense when viewed at an 

 angle of 45 degrees from the vertical, and for this reason it is 

 usually used for lighting open spaces from aconsiderable height. 

 I will assume its power as an average of the illumination at 30, 

 45, and 60 degrees from the vertical. 



From the report on Electric Lamps of the Franklin Institute, 

 .June, 1885, I take the following data: 



