Intelligence and Miscellaneous Articles, 519 



sidered to be an average condition of the candle-flame, that is about 

 fifteen minutes after lighting, when a deflexion of 75 scale-divisions 

 was obtained. 



This number, multiplied by the constant previously found, gives 

 the radiant energy which from the candle passes through each 

 square centimetre of a surface everywhere one metre from the 

 caudle, 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 pro- 

 jected upon 400 square centim. 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*308 square centim. 



We now have the whole radiant energy of the candle, 



4tt x 100 2 x 75 x 17 -, OQ -, n8 -, 



e= — = 1*23 x 10 8 ergs per second. 



To find what portion of this total energy lies in the visible spec- 

 trum could be satisfactorily accomplished only by measures made 

 in every part of the spectrum of the candle. Such measures have 

 been made by Langley* in the spectrum of an argand gas-lamp 

 with a glass chimney. He finds 2*4 per cent, of the total radiant 

 energy to be visible. It is easy to compare the candle with such a 

 lamp. At a certain distance from the thermograph an argand 

 lamp, whose light was that of ten candles, gave a deflexion of 238 

 scale-divisions. When the lamp was replaced by the candle the 

 deflexion was 29. Hence we see that very nearly 2 per cent, of 

 the radiant energy of the candle is visible : or the visible part is 

 2-46 x 10 6 ergs per second ; about 10-9 ft.-lbs. per minute. 



We may now proceed to find the mass of a meteor, first upon 

 the supposition that its rays have the same ratio of visible to total 

 energy as have those of the candle, and later correct, if possible, 

 the value thus found. 



Let the meteor at a distance of 50 miles have a light equal to 

 that of Vega; let it continue for 2 seconds with a velocity of 

 25 miles per second. From the best data we find that if the 

 meteor were at 1 metre distance, the log of its candle-power would 

 be 3-9851. Hence to find the energy e, we have : — 



log candle-power 3*9851 



log energy of candle 8-0899 



log 2 0-3010 



'5 



loge 12-3760 



* Science, vol. i. p. 482. 



