BISHOP'S RING AROUND THE SUN. 473 



and it is this office that the finest and highest of the Krakatoa dust is 

 thought to have performed. And here a peculiar cycle of operations, 

 first suggested by Wollaston years ago, and generally neglected since 

 then, may be reconsidered. So long as the water-substance is in the 

 vaporous condition, it acts as a gas, and tends to expand upward ; 

 part of it would thereby be condensed, generally in the solid state, and 

 on losing the gaseous condition the frozen particles would at once 

 tend to fall toward the earth, impeded only by the presence of the thin 

 air ; but, after a certain length of falling, they would reach air warm 

 and dry enough to allow them to re-evaporate, whereupon their vapor 

 would again expand upward, and the cycle of operations begins anew. 

 Wollaston suggested that the gases of the air might be thus affected 

 by the extreme cold of upper space, and that a limit of the atmos- 

 phere might so be determined. There is, however, no experimental 

 evidence yet adduced to prove that oxygen and nitrogen would behave 

 in such a way, and the limitation of the atmosphere must be due to 

 other causes ; but the upward extension of water-vapor might be thus 

 controlled. May we not, therefore, imagine that the vapor of the 

 upper atmosphere, re-enforced liberally by steam from Krakatoa and 

 other volcanoes in eruption at the same time, found its opportunity for 

 condensation much improved for several months by the lava-dust from 

 the same sources ; and thus explain the brilliant sunsets and the 

 strength of color in Bishop's ring during the winter of 1883-'84 ? But 

 gradually the dust settles down, very slowly on account of its large 

 ratio of surface to weight ; and the vapor also decreases by slow down- 

 ward diffusion ; then the brilliancy of the display is lost, and the 

 moderate residual of vapor, condensing as well as it can alone, pro- 

 duces only a fainter-colored ring and sunset glows that are visible only 

 under especially favorable circumstances. 



Be all this as it may, it is well to bear in mind that some such ex- 

 planation must be found and accepted, for the facts of diffracting 

 particles and their relation to Krakatoa are too well proved to be 

 doubted, unless evidence not yet forthcoming shall appear in great 

 strength. 



The ring is doomed to disappear, and hence deserves a close watch- 

 ing. For, as Forel has pointed out, the outburst of Krakatoa must 

 have had its rivals in ancient if not in modern times, and rings like 

 Bishop's must in all probability have resulted from former dusty ex- 

 plosions. But these had all faded long before Bishop's ring appeared, 

 and we must, therefore, conclude that it will fade away also. It should 

 be carefully watched, especially from high-level stations, and those 

 who make a persevering record of it should not fail to inform Pro- 

 fessor Kiessling, of Hamburg, about what they see. 



Cambridge, Massachusetts, December, 18S5. 



