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HARDWICKE'S SCIENCE-GOSSIP. 



the results, even then, would be far from satisfactory. 

 The same may be said, with double force, of the 

 extraordinary varieties of form presented by cirrus ; 

 lines crossing and curving in almost every conceiv- 

 able direction. But what we aim at is this — to obtain 

 such general laws as may agree with the analogy of 

 the subject and be of some use, as prognostics, and 

 also to depict and to classify those varieties which 

 are decidedly significant of weather change. 



It may be thought, indeed, that classification is 

 very much a matter of theory : accurate delineation 

 of separate phenomena has, however, a very practical 

 bearing ; and the readers of Science-Gossip will, 

 perhaps, endorse the expression recently used by a 

 writer in the " Examiner," who speaks of our present 

 subject as "highly interesting, intimately connected 



may be well observed on the coast of East Kent 

 when a north-east wind is setting in. It is curious 

 to notice how these vapour masses advance in regular 

 line to some far distant point with a steady onward 

 march, like vultures scenting their prey afar off. 



Some writers seem to regard the clouds themselves 

 as the origin of the aerial currents, and not mere 

 attendants upon them. One fact, however, ap- 

 pears evident, viz., that masses of cumulus moving 

 in this way, are generally accompanied by subor- 

 dinate and complicated eddies and currents — rotary, 

 spiral, &c, which whirl about them without altering 

 the direct line of advance. 



Thus two masses of cloud moving with but a 

 slight interval between them will be attended by 

 their own peculiar, and sometimes opposite, currents, 



Fig. 177. — Electrical stratus, with capped cumulus in distance. 



with, and absolutely necessary to, the science of 

 weather forecasts." 



We now draw our readers' attention to the manner 

 in which banks and masses of grey and highly con- 

 densed cumulus drift over the sky, chiefly in the 

 spring and winter months, in breezy weather, especially 

 when this is accompanied by heavy rain. This ap- 

 pearance may be contrasted with the phenomenon of 

 the " calling of the sea," the noise caused by the swell 

 of the waves reaching the shore before the wind 

 comes up. In the latter case we have undulations 

 radiating from a centre of disturbance ; in the 

 former we have vapour masses advancing to a centre 

 of attraction. 



Fracto-cumulus,* as Prof. Poey has termed it, has 

 often a resemblance to an undulating sea of vapour, 

 in the manner in which the masses arrange themselves 

 in banks above each other — an appearance which 



* In the case of this cloud, the arrangement in banks and 

 strata is mainly the effect of perspective. 



while they are both moving on with the general drift, 

 in a steady and regular line. 



We will not at present enter upon the inquiry here 

 suggested, as to the extent to which a sudden change 

 in the molecular arrangement may originate move- 

 ments of the surrounding air ; we desire, now, to 

 draw the readers' attention to the manner in which 

 the eddies and spiral currents above mentioned may 

 affect the form of the mass. To take one instance, 

 such a spiral current, varying in its radius would be 

 a very probable cause of the lateral compression or 

 characteristic " anvil " form.* 



In confirmation of this view, it may be remarked 

 that previous to the gale which these clouds so often 

 portend, we may sometimes observe small cyclones 

 whirlwinds moving upon the surface of the earth, 

 where this spiral motion is more evident from the 



* There are several varieties, however, of this cloud, about 

 which we hope to say more at a future time. See a paper of 

 mine in "Popular Science Review," October 1873. 



