OF ARTS AND SCIENCES. 137 



Had the moon's rays, uncondensed, acted upon the thermograph, 

 the deflection would have been : 



1S4 8 



.„, ' = 0.5003 scale divisions. 



o09.1 



On the following morning, the altitude of the sun being 42°, its 

 rays were reflected through a small opening, and being received by 

 the thermograph, placed at a distance, a deflection of 147.16 scale 

 divisions was found to be the mean of 30 readings. The opening 

 through which the sun's rays were reflected was a carefully reamed 

 hole in a metal plate. Its diameter, being measured under the micro- 

 scope, with the aid of a stage micrometer which purported to be 

 divided into tenths and hundredths of a millimeter, was found to be 

 1.67 mm. A paper screen received the spot of light, formed by the 

 rays from this opening. The diameter of the spot was 40.6 mm. 

 The thermograph was thrust through an opening in the screen until 

 its strip lay in the plane of the paper. The spot of light could then 

 be thrown into it by turning one of the screws of the heliostat. 



We have for the reduction of the sun's rays: 



M, = 615.3. 

 1.67 2 



Had the thermograph received the undiminished sunbeam, the de- 

 flection would have been : 



615.3 x 147.16 = 90596. 



The ratio of the sun's radiation to that of the moon is, therefore, 



90596 



0.5008 



= 180,900. 



Second Method. 



In the second method, the sunbeam is employed without diminution, 

 but its effect is reduced by interposing a resistance in the galvanom- 

 eter circuit. This method is free from all objections, as far as can be 

 seen, for its accuracy depends upon the measurement of a resistance, 

 an operation which can be accomplished with great accuracy. More- 

 over, it removes any effect due to minor disturbing causes, such as the 

 heating of the metal plate in the first method, since, when a large 

 resistance is interposed, the apparatus becomes insensible to such 

 influences. 



The resistance of the thermograph, galvanometer, and the connect- 

 ing wires, is 0.2095 ohm. On October 21, 1888, the sun's altitude 



