Januaky 1, 1897.] 



KNOWLEDGE 



19 



leaving the dust - particles in invisible suspension. Of I 

 course, the water vapour was there ; otherwise I could not 

 have seen in the sky the varied shades of blue, from the i 

 horizon to the zenith, deepening as one looked upwards. 

 Though this was a striking illustration on account of the 

 particular temperature and drought, still it was only a 

 practical instance of the theory of cloud decay thus indi- 

 cated. 



Clouds decay in another way. Whenever a cloud is 

 formed it begins to rain, and the drops shower down in 

 immense numbers, though most minute in size. No doubt 

 it is only in certain circumstances that these drops are 

 attracted together so as to form large drops, which fall to 

 the earth in genial showers to refresh the thirsty soil, or 

 in a terrible deluge to cause great destruction. But when 

 the temperature and pressure are not suitable for the for- 

 mation of what we commonly know as rain, the line drops 

 fall into the air under the cloud, where they immediately 

 evaporate from their dust-free surfaces, if the air is dry and 

 warm. AVhenever a cloud overhangs, fine rain is falling, 

 so that the cloud is in the process of decay ; but this rain 

 may not reach the earth on account of the dryness of the 

 stratum of air beneath the cloud, and the heat of the air 

 over the earth. So that on a summer day, with the gold- 

 fringed fleecy clouds sailing overhead, it is really raining, 

 but the drops, being very small, evaporate long before 

 reaching the earth. It rains, but much of this melting of 

 the clouds is reproduced by a wonderful circularity, the 

 moisture evaporating, seizing other dust-particles in a 

 cooler stratum, forming cloud-particles, falling again, and 

 so on '"/ iiiiiiiitiiiii during the existing circumstances. 

 That is the reason why surfaces may be exposed to a cloud 

 on a mountain-side without being wetted. Radiant heat is 

 the cause of the remarkable result. The rays of the sun, 

 which strike the upper part of the cloud, not only heat that 

 surface, but also penetrate the cloud and fall on the surface 

 of bodies, generating heat there. These heated surfaces, 

 again, radiate heat into the superincumbent air. This warm 

 air receives the fine raindrops from the decaying cloud, 

 and dissolves the moisture from the dust-particles before 

 the moisture can reach the surfaces exposed. That a vast 

 amount of radiant heat rushes through a cloud is clearly 

 shown by exposing a thermometer with black bulb //( vm-wi. 

 On some occasions a thermometer would indicate from 

 forty degrees to fifty degrees above the temperature of the 

 air, thus rendering the surface quite dry. Hence seats, 

 walls, posts, etc., may be dry, though in the middle of a 

 cloud which is gradually decaying. 



To the decay of clouds Mr. Aitken has lately been 

 directing his attention. His observations, made in the 

 clouds themselves, have shown that there is a difference of 

 structure in clouds which are in the process of formation 

 and those which are in the process of decay. In clouds in 

 formation the water-particles are much smaller and far 

 more numerous than in clouds in decay ; and, while the 

 particles in clouds in decay are large enough to be seen 

 with the unaided eye, when they fall on a properly lighted 

 micrometer, they are so small in clouds in formation that, 

 if the condensation is taking place rapidly, the particles 

 cannot be seen without the aid of a lens of considerable 

 magnifying power. The form of some clouds cannot be 

 explained by the processes going on in the ordinary 

 formation of clouds. We must look on the processes 

 going on in decay for an explanation of these forms. 

 Most authorities have made a wrong assumption as to the 

 difi'erent shapes of clouds, as classified by Mr. Luke 

 Howard in 1802, viz., the primary forms — cirrus, cumulus, 

 and stratus; the intermediate — cirro-cumulus, cirro- 

 atratus, and oumulo-atratus ; and the compound form, 



nimbus. They have assumed that these different shapes 

 are in the process of formation, and that the whole 

 explanation of the shapes taken by the clouds is founded 

 on this supposition. Now, Mr. Aitken has pointed out 

 that ripple-marked clouds, for instance, have been clouds 

 of decay. It was generally understood that these ripple 

 markings are due to the general movements of the air 

 giving rise to a series of eddies, the axes of the eddies 

 being horizontal and roughly parallel to each other. 

 Now, according to Mr. Aitken, " it is very evident that the 

 air revolving round these horizontal axes — that is, in a 

 vertical plane — will at the lower part of its path be sub- 

 jected to compression, and at the upper part to expansion. 

 The natural result of this will evidently be, suppos-ing the 

 air to be nearly saturated with moisture, a tendency for 

 cloudy condensation to take place in the air at the upper 

 part of its path, and it is this cloudy condensation in the 

 upper part of the eddies that is supposed to produce the 

 ripple-like cirrus, each ripple mark indicating the upper 

 part of an eddy." Now, it is diflioult to imagine that the 

 small amount of elevation and consequent expansion and 

 cooling could give rise to so dense an amount of clouding 

 as is generally observed, for any clouding produced in this 

 way would likely be very thin and filmy. Accordingly 

 another explanation must be given for the production of 

 ripple -marked cirrus clouds, and that is in the decay of 

 clouds. These are generally formed out of some cirro- 

 stratus or similar cloud. When these are observed in fine 

 weather it will be found that they frequently change to 

 ripple-marked cii-rus in the process of decay to vanishing. 

 The cirro-stratus gradually thins away till it attains such 

 a depth that if there are any eddies at their level, the 

 eddies break the cloud up into nearly parallel masses, 

 the clear air being drawn in between the eddies. The 

 eddies here do not produce the clouding, but break 

 up the uniform cirro-stratus cloud into the ripple- 

 marked cirrus. Mr. Aitken points out, in support 

 of this theory, that lenticular cirrus clouds are fre- 

 quently observed with ripple markings in one or more 

 sides of them, just where the cloud is thin enough to be 

 broken through by the eddies. The ripple marking? get 

 nearer and nearer the centre as the cloud decays ; and at 

 last, when nearly dissolved, these markings are extended 

 quite across the cloud, in the peculiar " mackerel " appear- 

 ance so gloriously seen on the 1st of November. Of 

 course ripple clouds may be producad by formation, 

 but this seems exceptional : and the decay theory is the 

 more probable. Whether, then, we consider the cases 

 of clouds gradually melting away back into their primil 

 state of blue water vapour, or the constant fine raining 

 from clouds and re-formation by evaporation without 

 wetting any extended bodies on the surface of the earth, 

 or the transformation of such clouds as the cirro-stratu? 

 into the ripple cirrus, we are forced to the inevitable 

 conclusion that in clouds there is not always develop- 

 ment but sometimes degeneration ; not always formation, 

 but sometimes decay. To this subject meteorologists must 

 direct more attention. 



PROTECTED AND UNPROTECTED WARSHIPS. 



By Nautilus. 



THE comparative numbers and eSiciency of the pro- 

 tected and unprotected battleships of the British 

 Navy, and those of her rivals, have of late inspired 

 much discussion among experts. The difference 

 between a protected and an unprotected battleship 

 consists in the first-named being fitted with a narrow belt 



