931 



CLOUD. 



the reverse of these conditions obtain, namely, a copious and continual 

 supply from below, and a state of vaporous tension already existing 

 aloft approaching saturation, a portion will be precipitated in visible 

 cloud, on ariving at a certain level. When this process takes place in a 

 calm state of the air, and the evaporating surface is limited in extent, 

 or irregularly distributed in patches (as over marshy ground, rivers, 

 lakes, &c.), or if any other cause dispose the vapour to rise in columnar 

 bodies of greater or less extent, the summits of these are marked 

 by protuberant masses, or piles of cloud, with generally rounded 

 outlines, which appear to repose on flat bases, indicating the " vapour 

 plane," [VAPOUR-PLANE] or that level where hygrometric saturation 

 commences. These are cumuli. So produced, " they abound," Sir J. 

 Herechel observes, " in the calm latitudes of the equatorial seas, and 

 form a distinguishing feature in the meteorology of that region." 



" That the mere self-expansion of the ascending air," he con- 

 tinues, " is sufficient to cause precipitation of some of its vapour, 

 when abundant, is rendered matter of ocular demonstration in that 

 very striking phenomenon so common at the Cape of Good Hope, 

 where the south or south-westerly wind which sweeps over the 

 Southern Ocean, impinging on the long range of rocks which termi- 

 nates in the Table Mountain, is thrown up by them, makes a clear 

 sweep over the flat table-land which forms the summit of that moun- 

 tain (about 3850 feet high), and thence plunges down with the violence 

 of a cataract, clinging close to the mural precipices that form a kind of 

 background to Cape Town, which it fills with dust and uproar. A 

 perfectly cloudless sky meanwhile prevails over the town, the sea, and 

 the level country ; but the mountain is covered with a dense white 

 cloud, reaching to no great height above its summit, and quite level, 

 which, though evidently swept along by the wind, and hurried furiously 

 over the edge of the precipice, dissolves and completely disappears on a 

 definite level, suggesting the idea (whence it derives its name) of a 

 ' Table-cloth." Occasionally, when the wind is very violent, a ripple is 

 formed in the aerial current, which, by a sort of rebound in the 

 hollow of the amphitheatre in which Cape Town stands, is again 

 thrown up, just over the edge of the sea, vertically over the jetty, 

 where we have stood for hours watching a small white patch of 

 cloud in the zenith, a few acres in extent, in violent internal agitation 

 (from the hurricane of wind blowing through it), yet immovable, as 

 if filed by some spell, the material ever changing, the form and 

 aspect unvarying." 



Of the suspension of the clouds in the air, a subject apparently very 

 simple, but which really involves a variety of physical principles, the 

 most recent view is that of Sir J. Herschel also, stated in the following 

 terms, in the article already cited : 



" When the sun shines on a cloud which absorbs its heat, the cloud 

 itself is necessarily partially evaporated, and the vapour, by its levity, 

 tends to produce an upward current, and thus tojcounteract the effect 

 of gravity on the globules of which it consists. A globule of water 

 ^55 in. in diameter, in air of five-sixths of the density on the surface, 

 or at the height of about 5000 feet, would have its gravity counter- 

 acted by resistance, with a velocity of descent of one foot per second 

 (supposing no friction and no drag) ; and even if the terminal velocity 

 were reduced to half that quantity by these causes, would still require 

 some such upward action to enable it to maintain its level a 

 circumstance which sufficiently accounts for the lower level gene- 

 rally observed of cloud during the night. It is more than probable, 

 however, that when not actually raining, cloud is always in process 

 of generation from below and dissolution from above, and that the 

 moment this process ceases, rain in the form of ' mizzle ' commences. 

 In a word, a cloud, in general, would seem to be merely the visible form 

 of an aerial space in which certain processes are at the moment in 

 >irin, and all the particles in a state of upward movement." 



Just pausing to call attention to the important inference with re- 

 spect to the abstract nature of a cloud, first drawn, we believe, by the 

 accomplished philosopher we are quoting, and contained in the last 

 period of this extract, we may observe that, to complete the view he 

 takes of the causes of the suspension of the clouds, nothing seems to be 

 required but an adequate estimate of the effects of the 'friction' 

 and the 'drag,' which are supposed not to exist. But it appears to 

 have escaped the attention of Sir J. Herschel, that Professor G. G. 

 Stokea had already shown that the internal friction of the ah-, 

 together, of course, by implication, with the drag which it occasions, 

 in itself one of the causes in his opinion, indeed, the main cause 

 of the suspension of the clouds. As this particular subject is new, 

 and of great importance in meteorology, .we shall treat it at some 

 comparative length. 



Clouds consist of an aggregation of separate minute globules of 

 water ; and the resistance of such a globule falling through the air 

 with its terminal velocity depends almost wholly on the cause just 

 Htated. " Since the index of friction of air is known from pendulum 

 experiments, we may," Professor Stokes observes, " easily calculate the 

 terminal velocity of a globule of given size, neglecting the part of the 

 resistance which depends upon the square of the velocity. The ter- 

 minal velocity thus obtained is so small in the case of small globules, 

 such as those of which we may conceive a cloud to be composed, that 

 the apparent suspension of the clouds does not seem to present any 



difficulty Since in the case of minute globules falling with 



their terminal velocity the part of the resistance depending upon the 



CLOUD. 983 



square of the velocity is quite insignificant (as will presently be shown) 

 compared with the part which depends on the external friction of the 

 aii-, it follows that were the pressure equal in all directions in air in 

 the state of motion (which according to the common theory of the 

 fundamental assumption in hydrodynamics, it would be), the quantity 

 of water which would remain suspended in the state of cloud would be 

 enormously diminished. 



To render this view of the subject complete, and to explain the 

 value of the last observation, it must here be stated that Professor 

 Stokes had before shown that the fundamental assumption of hydro- 

 statics and hydrodynamics, that the pressure of a fluid is equal 

 in all directions, though fully justified by experiment in the case 

 of a fluid at rest, is not true in the case of a fluid in motion. The 

 viscosity attributed to water by Dubuat, and the inherent property, 

 " analogous to that of viscidity in liquids," ascribed to elastic fluids by 

 Captain (now Major-General) Sabine, from their respective pendulum 

 experiments, were generalised by Professor Stokes as consequences of 

 the internal friction of fluids in general, all fluids exerting a resistance 

 to bodies passing through them, independently of their density ; and 

 when this is taken into account, it is evident that the pressure cannot 

 be equal in all directions. 



The suspension of the globules of water forming the clouds is only 

 a particular case of the more general fact, that fine powders remain 

 nearly suspended in a fluid of widely different specific gravity. Pro- 

 fessor Stokes has demonstrated that the resistance of the fluid, whether 

 liquid or gaseous, is proportional, not to the surface, but to the radius 

 of the spherule, and consequently the quotient of the resistance 

 divided by the mass, in other words, the accelerating force of the 

 resistance, increases much more rapidly as the radius diminishes, than 

 if the resistance varied as the surface : on which principle the suspen- 

 sion, or proximate suspension, of the particles or globules depends. 

 When the downward motion of a globule is so slow that the part of 

 the resistance which depends on the square of the velocity may bo 

 neglected, the terminal velocity of a globule of water forming part of 

 a cloud may be determined. For a globule the one-thousandth of an 

 inch in diameter, we have the velocity V593 inch per second. For a 

 globule the one ten-thousandth of an inch in diameter, the terminal 

 velocity would be a hundred times smaller-, so as not to amount to the 

 one sixtieth part of an inch per second. 



The amount of that part of the resistance which varies as the 

 square of the velocity, is the only kind of resistance that could exist if 

 the pressure were equal in all directions, for the velocity 1'593 inch 

 per second is not quite the one four-hundredth part of the weight ; and 

 for a sphere only the one ten-thousandth of an inch in diameter, the 

 ratio of the resistance to the weight would be ten times as small. Both 

 these proportions, it is manifest, are quite insignificant. 



The conclusion thus arrived at by Professor Stokes as to the cause 

 of the suspension of the clouds, illustrates in a remarkable manner 

 the connection of different branches of science. It is an application by 

 him of the theory of internal friction, as applied to the ball pendulum, 

 and verified by recorded experiments on that instrument, and is con- 

 tained in his memoir ' On the Effect of the Internal Friction of Fluids 

 on the Motion of Pendulums,' published in the ' Transactions of the 

 Cambridge Philosophical Society,' vol. ix. The erroneous extension of 

 the fundamental law of hydrostatics had been discussed by him in a 

 paper ' On some Cases of Fluid Motion,' inserted in the preceding 

 volume of that work. 



The height of the clouds in the atmosphere at any particular locality 

 necessarily varies, like their production itself, with the quantity of 

 vapour in the air, the climate, the season, the temperature at the time 

 of observation, the local decrement of temperature for ascent, the 

 elevation of the superimposed currents of air, the extent of the land, 

 the proximity of the sea, the existence and altitude of mountains, 

 and a variety of other circumstances relating both to the earth and 

 the atmosphere. It would appear that the altitude of the highest 

 mountains gives the upper limit of the region of the clouds, which 

 is thus seen to comprise those denser strata of the atmosphere 

 resting immediately on the earth's surface, which include considerably 

 more than two-thirds of the ponderable body of air incumbent on 

 the globe, the highest mountains known not exceeding five miles 

 and a-half in height ; while the principal mass of the clouds probably 

 belongs to a region of which the superior limit is about a mile 

 below this. Mr. Dalton correctly asserted, that in England the height 

 of small fleecy patches of cloud, meaning rim, was frequently from 

 three to five miles. The following observations, relating to the 

 elevation of the clouds in the interesting district of the English lake 

 mountains, are given by Mr. Dalton in the first edition of his 

 ' Meteorological Essays ' (1793), the observer being Mr. Crosthwaite of 

 Keswick. By accurate measurement of Skiddaw, and fixing marks 

 on the side of the mountain, which is 1050 yards high [correctly 1007 

 yards], he was able to ascertain by inspection the height of any cloud, 

 when it did not exceed that of the mountain. 



This he did three times each day, morning, noon, and evening, for 

 five years, missing only as many observations as amounted to omitting 

 less than a week per year. The result is as follows, the last column 

 but one giving the number of times in which either the clouds \vc iv 

 above the mountain, or there were no clouds at all, the last circum- 

 stance occurring about once out of thirty times. 



