10 



SCIENTIFIC NEAVS. 



Qan. 6, 1888. 



be a little compressed, and therefore denser and heavier 

 than the surrounding air. Gay Lussac remarks that 

 a soap-bubble in a room descends, but if it is formed in 

 the open air above a heated soil, the rising current pushes 

 the bubble before it. This observation is no doubt 

 correct, but the idea that all clouds are supported by 

 conveniently arranged currents of heated air which 

 balance them as in the Japanese butterfly trick is 

 hardly worthy of further consideration. Gay Lussac, 

 whose classical researches on the expansion of gases 

 would alone place him in the first rank of the 

 philosophers of his day, knew well that air has very 

 appreciable weight, and that the problem was not to 

 divest thej water of which the cloud is composed, of 

 its weight, but to show how it could become lighter than 

 the surroimding air. 



It has been argued that if clouds are composed merely 

 of small drops, they ought always to show a rainbow 

 when the sun shines at a suitable angle on them. 

 There are, however, good reasons for supposing that 

 in general the drops are so small that their dimensions 

 may be compared with the length of a wave of light, 

 and the absence of a rainbow goes to prove that they 

 are of such minute size that the necessary reflection 

 and refraction cannot take place in the usual way. 



It must be remembered that clouds are not invariably 

 composed of water ; there are, unfortunately, such things 

 as clouds of smoke and dust. Smoke is little else than 

 fine particles of carbon, probably identical with soot. 

 The cloud of dust on the high road is almost entirely 

 mineral, simply of powdered flint or other stone. There 

 can be no vesicular structure here, and though such a 

 cloud does not float for long, it has a good deal more 

 buoyancy than could have been expected of the material 

 of which it is composed. 



Fresnel, the distinguished French optician, pointed 

 out that a cloud of minute globules of water or crystals 

 of snow is composed of a mixture of such particles 

 with air, and that the whole weight of water forms but 

 D small fraction of the weight of the air. There is 

 moreover what he called a multiplied contact with the 

 air, that is to say, the innumerable particles expose a 

 very considerable surface to the atmosphere. The air 

 is, as it were, entangled with the more sohd part of 

 the cloud, owing to the friction at each surface. For 

 no particle can move without carrying with it a coating 

 or skin of air, which in turn cannot move through the 

 surrounding air without considerable friction. The 

 thickness of such a coating is only a few thousands of 

 an inch, and its presence would not be felt unless the 

 "body it surrounds is both light and small compared 

 with its surface. 



It is somewhat difficult to estimate the proportion 

 between the quantities of air and of water in an 

 ordinary cloud. It must be remembered that cloud or 

 mist is not the same thing as steam. Steam is as trans- 

 parent and as invisible as air, it is only when it is chilled 

 that it becomes a visible cloud. It is then no longer a 

 vapour, but a mixture of minute particles of water with 

 air. It probably occupies about the same space, and 

 we know that such condensed steam by itself occupies 

 less than the sixteen-hundredth part of its original 

 volume. A cubic foot of air weighs about an ounce 

 and a quarter ; it is probable that the water in a cubic 

 foot of cloud would weigh about a quarter of a grain. 



Now pure air is as transparent to rays of heat as to 

 rays of light, and therefore, since the heat can pass 



through without being stopped, the air does not become 

 heated. Water absorbs a considerable quantity of heat 

 rays, and Professor Tyndall has found that where the 

 vapour of water is present in the atmosphere on a day 

 of average dampness, its absorption is upwards of sixty 

 times that of the air itself When the sun shines on ai 

 cloud, each drop of water, not being perfectly trans- 

 parent to the heat, becomes warmed, and in turn warms 

 the air surrounding it, thus the great mass of air in which 

 the drops are entangled, rises in temperature, and ex- 

 pands and becomes lighter than the surrounding atmo- 

 sphere, and, as we have seen, the weight of the water 

 being a mere trifle, it will rise until it reaches a level 

 where the air is of the same density. It will float at 

 this level in equilibrium. A cloud of smoke will behave 

 in exactly the same way, and the equilibrium is sc« 

 delicate that it has been noticed that the smoke of Vesu- 

 vius as it hangs over the mountain in still weather,, 

 rises and falls with every change of the barometer. 



The flying of the gossamer spider may be accounted 

 for in precisely the same way. The little creature ha& 

 been observed to place itself at the extremity of a leaf, 

 or some similar situation, and throw out a fine single 

 thread. It chooses its position so that the thread is. 

 gently blown away by the wind, and when the thread is 

 long enough it launches itself into the air at the end of 

 its rope, which is not even attached to a " sky-hook."" 

 It is clear that the wind alone is not sufficient tCN 

 account for the length of time during which it will 

 sail about, and there is every reason to believe 

 (especially as it may be noticed that it generally makes 

 its excursions when the sun is shining) that the gossamer 

 thread behaves like the drop of water, and becoming 

 heated, warms its sheath of air, which dilates and thus 

 bears up the thread and the passenger at its extremity. 



Although the buoyancy and the possibility of the 

 rising of a cloud from near the earth to comparatively 

 high regions is thus accounted for, we must remember 

 that a pound of little water drops weighs as much as a 

 pound of water, and is attracted to the earth •with no> 

 less certainty. Professor Stokes has, however, shown 

 how remarkably slowly a minute drop will fall through 

 the air. This is in consequence of the viscosity or internal 

 friction of the air. He tells us that a drop of water falling 

 through air one thousand times rarer than itself (which 

 we may suppose to be the case at the ordinary height 

 of a cloud) would fall about eight-tenths of an inch in a 

 second if its diameter were one-thousandth part of an 

 inch. If the diameter of the drop were only one ten- 

 thousandth of an inch the rate at which it would make 

 its way through the air would be a hundred times 

 smaller, or half an inch a minute. Some fogs are sO' 

 heavily charged with moisture that they lie like lakes 

 in hollows, and pour down valleys like strearnxs. 



There is, in conclusion, one more reason why clouds 

 should float, and that is, that the little drops do not 

 float in dry air, but in damp air. The vapour of water 

 is much lighter than air, and damp air is lighter than, 

 dry air of the same temperature and pressure. 



OwEKS College, Manchester. — Bishop Berkeley (Re- 

 search) Fellowships have been awarded to E. G. W. Hewlett, 

 B.A. (Trinity College, Cambridge), in classics and philology ; 

 William Bott, Ph.D., in chemistry ; and to O. H. Latter, B.A. 

 (Keble College, Oxford), in zoology ; and renewed for a 

 second period to Henry Holden, B.Sc, in physics, and to 

 William A. Shaw, M.A., in history. 



