444 METEOROLOGY. 



rise to others. The increase is sometimes perfectly indeterminate ; at others, it has a 

 very decided direction. Thus the first few threads l)eing once formed, the remain dei 

 "will be propagated in one or more directions laterally, or obliquely upward or down- 

 ward, the dii-ection being often the same in a great number of clouds visible at the 

 same time ; for the oblique, descending tufts appear to converge toward a point in the 

 horizon, and the long, straight streaks. to meet in opposite iJoints therein; which is 

 the optical effect of ]iarallel extension. The upward direction of the fibers or tufts of 

 this cloud is found to bo a decided indication of rain; the downward as decidedly indi- 

 cates fair weather. Their duration is uncertain, varying from a few minutes after the 

 first appearance to an extent of many hours, and even days. It is long when they 

 appear alone and at great heights, and shorter when they are formed lower and in the 

 vicinity of other clouds. This modification, although in appearance almost motionless, 

 is intimately connected with the variable motions of the atmo.sphere. Considering 

 that clouds of this kind have long been deemed a prognostic of wind, it is extraordi- 

 nary that the nature of this connection should not have been more studied, as the 

 knowledge of it might have been x)roductive of useful results. In fair weather, with 

 light, variable breezes, the sky is seldom quite clear of small groups of the oblique 

 cirrus, which frequently come on from the leeward, and the direction of their increase 

 is to windward. Continued ivet weather is attended with horizontal sheets of this 

 cloud, which subside quickly and pass into the cii'ro-strafus. Before storms they ap- 

 pear lower and denser, and usually in the quarter opposite to that from which the 

 storm arises. Steady, high winds are also xireceded and attended by streaks running 

 across the sky in the direction they blow in. 



II. — Cirro-stratus, (Howard.) 



Thread-cloud, (illustrations jSTos. Y and VI.) — Howard's cirro-sfrafus is 

 distiug-uislied from tlie pure cirrus by its filaments being- smaller, more 

 compact, more ramified, and, so to say, completely stratified. They are 

 lower and more dense, for the sun's rays often pierce them with difficulty. 

 Their whitish tint is clearer, and it becomes rose-color in similar circum- 

 stances. Their motion is a little m ore rai:)id. When at the horizon, we only 

 seeing the vertical projection, they take the appearance of a long and very 

 narrow band. According to Howard, " this cloud appears to result from 

 the subsidence of the fibers of the cirrus to a horizontal position, at the 

 same time that they approach each other laterally. The form and rela- 

 tive position, when seen in the distance, frequently give the idea of 

 shoals of fish. Yet in this, as in other instances, the structtire must be 

 attended to rather than the form, which varies mucb, presenting at 

 times the appearance of parallel bars, or interwoven streaks like the 

 grain of polished wood. It is thick in the middle, and attenuated 

 toward the edge. The distinct appearance of a cirnts, however, does 

 not always i>reccde the i^roduction of this and the last modification. 

 The cirro-stratus precedes loind and rain, the near or distant apj)roach 

 of which may sometimes be estimated from its greater or less abund- 

 ance and permanence. It is almost always to be seen in the inter- 

 vals of storms. Sometimes this and the cirro-cumulus appear together 

 in the sky, and even alternate with each other in the same cloud, when 

 the different evolutions which ensue are a curious spectacle ; and a judg- 

 ment may be formed of the weather likely to ensue by observing which 

 modification finally prevails. ,^Th.e cirro-stratus is the modification which 



