HkUehim — Had mitt Energy of Standard Candle, 393 



mg the interval that the Cube and its contents were falling 5° 



■ mi a temperature about 65° above that of the air; time in 

 ttupied by the cube in falling the 5° as above. 



Knowing these quantities we can evidently compute in ergs 

 per second the radiant energy passing through the sq. cm. of 

 surface containing the thermal junction, and such that it will 

 produce a deflection of one division of the galvanometer soale. 

 A number of trials showed that 16*9 ergs per second was the 

 quantity required 



and method, — The constant was found bypassing the rays 

 of the sun through openings of 0*394 and 0*23 cm. diameter 

 and observing the galvanometer deflections when the thermo- 

 ph was exposed in the divergent beam at a point where the 

 diameter of the beam was 4:2 cm. and then computing the de- 

 flection for the undiminished sunlight. Simultaneously with 

 the above measures the radiation of the sun was observed with 

 Pouillet's pyrheliometer. 



The mean of several sets of measures by this method gave 

 the constant 17*02; agreeing better than could have been ex- 

 pected with the results of the first method. The candle em- 

 ployed was the ordinary sperm candle, six to the pound. It 

 burned in still air. without snuffing, 7*37 gm. per hour. The 

 radiation of the candle was measured by placing it behind the 

 screen in place of the cube employed in finding the instrumental 

 constant, the exposures being made in the same manner as for 

 the cube. The deflection given varied very much with the 

 length of the wick of the candle, constantly increasing for a 

 half hour or more after lighting. It therefore was necessary 

 to observe the deflection at what was considered to be an 

 average condition of the candle flame, that is, about 15 minutes 

 after lighting, when a deflection of 75 scale divisions was ob- 

 tained. 



This number multiplied by the constant previously found 

 es the radiant energy which from the candle passes through 

 each sq. cm. of a surface everywhere one meter from the can- 

 dle, provided we assume that the candle radiates equally in 

 every direction. To find the total radiant energy, we must, 

 as a first step, know the area of cross section of the candle flame 

 in a plane perpendicular to the direction of the flame to the 

 opening of the thermograph. To learn this, an image of the 

 flame was projected upon 400 sq. cm. of paper, taking care to 

 have the projecting lens midway between the candle and paper. 

 It was then easy to trace about the image of the flame with a 

 pen, and this having been done ten times upon the same sheet, 

 the whole sheet was weighed and then the tracings cut from it 

 and also weighed. In this manner the section of the candle- 

 flame was found equal to 1*303 sq. cm. 



