122 Prof. R. Clausius on a Mechanical Theorem 



be the extent of the illuminating surface of the photosphere, the 

 exterior parts of the corona will only receive an excess of light 

 over that received by the interior part equal to the amount of 

 photospheric light received by those parts during totality, or, as 

 in the case above taken, the excess will be equal to that given by 

 a ring of light from the photosphere 0"*75 wide (or GE in the 

 figure) , so that, when a few seconds of photosphere are visible to 

 the observer, the difference between the spectra of the exterior 

 and interior parts of the corona would be inappreciable. 



5. What spectrum ought the corona to give before totality on 

 the following side of the moon ? In this case, when the angular 

 distance of the limits of the sun and moon is some seconds, the 

 difference between the spectra of the exterior and interior parts 

 of the corona is small, since no part of the atmosphere in this 

 case will be illuminated by the photosphere ; so we ought to ob- 

 tain a chromospheric spectrum, together with a faint photospheric 

 one caused by a small amount of photospheric light reflected 

 from the photosphere by the chromosphere. 



6. On the foregoing hypothesis, during totality the parts of the 

 corona nearest the centre should give a differen t spectrum from 

 the more distant portions, since the portions nearer the centre 

 receive less photospheric light than the more distant parts, and 

 the same amount of light from the chromosphere. 



In order to test the correctness of this theory, advantage may 

 be taken of the following facts : — 1st. At that period of the 

 eclipse when the limb of the sun and moon are in line with the 

 observer, there will be a difference between the central and dis- 

 tant parts of the corona ; and this difference will decrease as the 

 moon passes on, whereas, by the other theory, there should be 

 the same difference as long as the corona is visible. 2nd. If the 

 corona be terrestrial, the spectrum of any portion of it ought to 

 be continually changing during the passage of the moon ; but if 

 solar, the spectrum should remain unchanged. 



XVI. On a Mechanical Theorem applicable to Heat. 

 By R. Clausius*. 



IN a treatise which appeared in 1862, on the mechanical 

 theory of heatf, I advanced a theorem which, in its sim- 

 plest form, may be thus expressed : — The effective force of heat is 



* Translated from a separate impression communicated by the Author, 

 having been read before the Niederrheinischen Gesellschaft fur Natur- und 

 Heilkunde, on June 13, 1870. 



t Phil. Mag. S. 4. vol. xxiv. pp. 81, 201 ; The Mechanical Theory of 

 Heat, p. 215. 



