90 Dr. G-. J. Stone j on the Cause 



vection-currents will suffice. Another, and probably the most 

 frequent, cause for little breezes in the neighbourhood of the 

 cloudlets is, that when the cloudlets are formed they imme- 

 diately absorb the heat of the sun in a way that the previously 

 clear air had not done. If they absorb enough they will rise 

 like feeble balloons, and slight return currents will travel 

 downwards round their margins, throwing all crystals in that 

 situation into disorder. 



I do not include among the causes which may agitate the 

 crystals another cause which must produce excessively slight 

 currents of air, namely that arising from the subsidence of the 

 cloudlets owing to their weight. The crystals will fall faster 

 where in cloud masses than in the intervening portions where 

 the cloud is thinner. But the subsidence itself is so slow, 

 that any relative motions to which differences in the rate of 

 subsidence can give rise are probably too feeble to produce an 

 appreciable effect. Of course, in general, more than one of 

 the above courses will concur ; and it is the resultant of the 

 effects which they would have separately produced that will 

 be felt by the crystals. 



If the precipitation had taken place so very evenly over the 

 sky that there were no cloudlets formed, but only one uniform 

 veil of haze, then the currents which would flutter the crystals 

 may be so entirely absent that the little plates of crystals can 

 fixedly assume the horizontal position which is natural to 

 them. In this event the cloud will exhibit no iridescence, 

 but, instead of it, a vertical circle through the sun will present 

 itself. This on some rare occasions is a feature of the phe- 

 nomenon of parhelia. 



It thus appears that the occasional iridescence of cirrus 

 clouds is satisfactorily accounted for by the concurrence of 

 conditions, each of which is known to have a real existence in 

 Nature. We may, in fact, recapitulate our knowledge on the 

 subject as follows : — Captain Scoresby's observations show 

 that the crystals of ice formed in the atmosphere do not ag- 

 glutinate into snow-flakes, except at temperatures bordering 

 on the freezing-point. At temperatures even a few degrees 

 lower, the crystals remain distinct from one another, and at 

 low temperatures are for the most part unmutilated and perfect 

 geometrical figures. He has also shown that all the forms 

 which crystals of ice can assume do not present themselves 

 together ; but that some one or two forms generally prepon- 

 derate over the others, the preponderating form varying 

 according to atmospheric conditions, which he does not seem 

 to have fully traced out. Thin tabular crystals are frequently 

 the preponderating form, and become more delicate and thin 



