250 Mr. W. S. Jevons on the Forms of Clouds, 



Thus the injected liquidj instead of gradually subsiding into a 

 longitudinal stratum^ as its intermediate specific gravitj^ causes 

 it to do in the case of the cumulostratus^ becomes by degrees of 

 least specific gravity, and ascends through the superincumbent 

 stratum in the form of small streams or threads. In my pre- 

 vious paper I attempted to show that these liquid threads are 

 analogous to, and produced by the same simple cause as those 

 atmospheric threads which are termed cirri. In the present ex- 

 periment, then, we have succeeded in producing a cumulostratus 

 of which the higher parts acquire a tendency to expand them- 

 selves upward as a cirrose crest. 



The form of the miniature thunder-cloud thus obtained, under 

 favourable circumstances, is shown in fig. 5, but for obvious 

 reasons the cloud is always in a state of change, and does not 

 endure for many minutes. 



26. It will very probably be impossible for many of my readers 

 to recognize in the peculiar resiilt of the last experiment any 

 resemblance whatever to the ordinary conception of a thunder- 

 cloud. During the continuance of a storm, nothing of course is 

 visible to a spectator beneath except one black unbroken mass of 

 cloud. It is only to a very distant observer that the true shape 

 and elevation of this cloud would be presented, and he probably 

 sees nothing in it to merit his particular attention. Hence very 

 few persons will be able, I think, to describe what the form of a 

 thunder-cloud is. Moreover, under varying circumstances, a 

 hundred thunder-clouds may be produced which will appear to 

 a superficial observer to differ completely in form and nature, but 

 in which closer examination may detect, in greater or less degree, 

 all the essential characteristics of the perfect or typical thunder- 

 cloud. To obtain, indeed, the type or single clear conception of 

 a thunder-cloud, it is necessary for the mind to carry on a process 

 of abstraction upon all thunder-clouds which meet the eye, and 

 the result of such will be, I think, a form resembling in all 

 essential pai'ticulars that produced in the third experiment. 



The essential parts of a thunder-cloud appear to be three, viz. 

 1st, the cumulus, which is the centre and source of all ; 2nd, stra- 

 tus, or portions of cloud which extend themselves outwards and 

 lie in equilibrium with the surrounding air ; and 3rd, cirrus, 

 which consists only of small quantities of moist air and cloud 

 forcing their way upwards, by virtue of their buoyancy to higher 

 elevations. 



These, it will be seen, are exactly represented by the motions 



and forms of the injected liquid in the experiment just described. 



27. There is one thing that it is so essential to understand 



with regard to this miniature thunder -cloud, that I repeat here 



what I before stated (7). It is produced by the simplest dyna- 



