1868.] 



Constitution of the Sun and Stars, 



39 



Section V. — Of Clouds in the Outer Atmosphere. 



66. But to return to what is more to be relied on, we may be sure that some 

 small part of the carbon, or whatever else the luminous clouds mainly con- 

 sist of, and similar traces of any other ingredients that enter in less quan- 

 tities into their composition, must escape precipitation, and will diffuse 

 themselves upwards, and the more freely as they come first to a region 

 where they are raised to a higher temperature as well as subjected to less 

 pressure. Through this hot stratum they will continue gaseous, but a short 

 distance above it they will meet with a temperature low enough to condense 

 them. Here, then, separated from the photosphere by the whole depth, of 

 the hot stratum, they will form a second film of luminous clouds, one, how- 

 ever, which is so attenuated as to be visible only during an eclipse, when it 

 constitutes the lowest of the clouds that then present themselves. They 

 may be traced in Dr. De La Rue's photographs of the eclipse of July I860* 

 as continuous arcs of cloud extending about 35° on either side of the points 

 of first and last contact. Hence, and from the apparent magnitudes of the 

 sun and moon on that occasion, we may conclude that this upper shell of 

 clouds was at an apparent distance of about 11" of space from the edge of 

 the sun's disk, which corresponds to an absolute height above the photo- 

 sphere of 8 metre-sixes, or 1^ time the earth's radius t« And as the 

 clouds of which we are now speaking are a little outside the hot stratum 

 that lies immediately over the photosphere, we shall not be far wrong in 

 concluding this stratum to be about as thick as the earth's radius is long. 

 The clouds outside it probably form a nearly continuous shell round the 

 sun. They are everywhere of extreme tenuity, but may nevertheless be 

 very variable in density ; and it is probably owing to this that the concave 



* Philosophical Transactions for 1862, p. 333. 

 f A metre-six means a metre multiplied by 10 6 . 



Dr. De La Rue took two eye-sketches also of the eclipse, commencing the first about 

 thirty seconds, according to his estimate, after the eclipse began. Now the arc of cloud 

 about the point of first contact is represented on the first, and indeed on both drawings, 

 and must have been at a greater height in the sun's atmosphere than I have assigned to 

 it, to have been seen by Dr. De La Rue, unless we may suppose that he overestimated 

 the interval of time which had elapsed by a few seconds. This, however, on an occasion 

 of so much hurry, may perhaps have happened, and it seems difficult otherwise to re- 

 concile the eye-draughts with the photographs. The data made use of to get out the 

 result in the text are : — 



o / // 



Length of arc of cloud visible five seconds \ _ *q 



after the moment of contact J* ~ 



Sun's apparent semidiameter = 15 45 



Moon's apparent semidiameter = 16 33 



Sun's diameter =13 7 metre-eights. 

 Earth's diameter =12*7 metre-sixes. 



Approach of the sun and moon's centres per minute of time =25" of space. 



The allowance of 5 seconds from the moment of contact has been made, because the 

 cloud seems to have taken about that time to impress itself upon the photographic plate. 



