532 EEPORT— 1886. 



differential motion pre^-ious to its becoming visible, but in this climate it soon 

 rises to the small height at which it is condensed. 



In summer, when the air is in sufficient motion, and especially in tropical 

 climates, it may travel long distances in its invisible or gaseous state, so that it 

 has time to become elongated by differential action, and when condensed it at once 

 assumes the form of the true stratus, in which case it is not seen as a cumulus or 

 cumulo-stratus at all. When it has risen to condensation height its lines more 

 approach the horizontal, are clearly defined, and the irregularities that would be 

 curves under more rapid decrease of temperature at an earlier stage are seen as 

 elongated lines extending to points as the pure stratus.^ 



We will now leave the lower and denser atmosphere which contains the 

 cumulus, cumulo-stratus, and stratus, and proceed to examine the next stratum 

 above. We here find that a rather rapid change talies place to a drier atmosphere, 

 as was proved by Mr. Glaisher's investigations regarding the dew-point in his 

 balloon ascents. It is almost like starting from a new globe, and the vapour of this 

 drier air does not become visible until a higher altitude is attained, where this 

 diminished quantity arrives at its level of visibility. Here the vapour is less dis- 

 turbed and carried for great distances, so that at the region of this attenuated 

 vapour the cloud is generally level. The cumulus and cumulo-stratus have never 

 reached this elevation, and we have instead the cirro-cumulus and cirro-stratus. 

 The differential movement of the atmosphere, however, though much diminished, is 

 still an important agent in its results, and effects are produced that are not possible 

 in the more bulky and dense clouds of the lowest range. The clouds are, at this 

 higher level, in a highly attenuated condition, and if the globules of vapour are too 

 small in the lower clouds to break through the air that separates them, or even to 

 descend, they are still less capable of doing so at a greater altitude, where they must 

 be still more minute ; but though they can neither coalesce nor descend, they can be 

 spread out gently by the differential action, or if rapidly expanded from beneath, 

 broken up into groups, larger or smaller according to the depth and density of the 

 stratum. 



Now, suppose the heat of the sun to have caused the lower atmosphere to 

 expand considerably, the effect of it will be to lift up and spread the whole of the 

 superincumbent air, and with it the thin upper stratum of cloud, further from the 

 earth. The result of the vertical elevation or secondary action will be to rend it 

 asurider— -the uplifted air becomes more attenuated as it rises, and will spread out in 

 all directions, and the flat cloud we are supposing to exist will of course do the 

 same; but in so doing will be rent into fragments or small groups, and thus produce 

 what is called a ' mackerel sky,' just as a similar result is produced, but by the 

 reversed action, in mud that has dried up and shrunk into small patches, while the 

 damp earth beneath remains expanded by the moisture it stiU contains. Should, 

 however, the expansion in the lower atmosphere take place very sloivly, it is possible 

 that the stratum of cloud may remain intact, and not be actually ruptured ; the 

 result would then be to spread out and attenuate the cloud as a whole ; all these 

 effects are supposed to take place in a still or very slowly moving atmosphere, but 

 supposing the same conditions take place in an atmosphere in rapid motion or 

 current of air, the tendency will be to elongate the cloud or group of clouds in the 

 direction in which the current is moving, and for this reason: the air is of course 

 going to where it is in demand, it is finding its equilibrium somewhere ; that which is 

 nearest the region requiring it will move forward fastest, that which is immediately 

 behind wiU follow with diminishing velocity, as it is further from the demand, the 

 result being to elongate the stream. Let it be supposed that a thin cloud is in a 

 current, it will have the same elongating influence communicated to it—ihia,ifaccom- 

 panied icith expansion from beloiv, will rupture the cloud into ribs or bars at right 

 angles to the current ; if, however, it is continuous but without expansion from 

 below, the reverse action will take place, and longitudinal bars will be formed in the 

 direction of the current. When, however, the main portion of a cloud is in a 



' In tropical or semi-tropical climates where the invisible vaporised moisture 

 travels furthest the stratus is a most striking and important cloud, and is often 

 very conspicuous in the evenings towards sunset as the air becomes cooler. 



