Intelligence and Miscellaneous Articles. 397 



of x calculated by means of the preceding formula, adopting 

 a* =143400. 









metres. 



H. 

 mil- 

 lims. 



/- 

 rail- 

 lims. 



II 



H+V 





X. 











cos z 



cosz 





Calc. 



Obs. 



h ra 



















July 4. 



12 



Seyssinet ... 



(213) 



748-8 



19-07 



808 



969 



49140 



1408 



1407 







Moucherotte. 



(1906) 



613-5 



13-82 



662 



779 



60535 



1435 



1435 



July 5. 





Seyssinet ... 



(213) 



749-0 



1769 



811 



960 



49620 



1410 1410 



Sept. 3. 



11 





, 



7460 



1719 



969 



11429 



40540 



138311383 







Moucherotte. (1906) 



6110 



6-53 



793 



858-5 



55460 



14241424 



May 2. 



4 43Galibier |(2653) 



5531 



330 



1340 



1402 



30550 



13461347 



From the value adopted for a x we deduce 

 X=1550°. 



Such is therefore the effective temperature of the sun, correction 

 being made for the influence of the atmosphere. 



III. I essayed, finally, to determine the true mean temperature 

 of the solar surface, defined by aid of the following considerations : — 



In general, when a body emits calorific or luminous radiations, 

 these do not emanate merely from points belonging to the external 

 surface of the body, but also from points situated at certain depths 

 beneath the surface, so that a radiating stratum of a certain thick- 

 ness has always to be considered. We can, then, legitimately ex- 

 tend to the sun, whatever may be its external constitutiou, the 

 ordinary definition of a radiating surface. The thickness of the 

 stratum at each point will be defined, as usual, by the distance 

 from the outer surface of the last point whose radiation is sensible 

 beyond that surface. The mean temperature of the radiating stra- 

 tum (whatever its thickness) at a point will then be named the 

 temperature of the surface at that point ; and the true mean tem- 

 perature of the sun will be the mean of the temperatures of the 

 various points of the surface. We see also what we must under- 

 stand by the emissive power of the sun at a given point of the sur- 

 face : it will be the ratio between the intensity of the radiation 

 emitted at that point and the intensity of that which would be 

 emitted by a body possessing an emissive power equal to unity, and 

 raised to the temperature of the sun's surface at the point under 

 consideration ; so that the true temperature of tlie sun can also be 

 defined as " the temperature which must be possessed by a body of 

 the same apparent diameter as the sun, in order that, endowed with 

 an emissive power equal to the mean emissive power of the solar 

 surface, it may emit in the same time the same quantity of heat as 

 the sun." 



Some experiments made at the Allevard forges with my actino- 

 meter, but by the dynamic method, have permitted me to determine 

 the emissive power of steel in fusion, just as it issues, possessing a 



