977 



CLOCK AND WATCH-MAKING. 



CLOUD. 



978 



the houses. Until the formation of the sewers of London, there existed 

 no cloaca: or drains for a city which could be compared with the cloacae 

 of Rome. [SEWERS.] 



The maintenance of the Roman cloaca; was originally the business of 

 the censors, but afterwards belonged to the Eediles ; Agrippa, during 

 his fcdileship, made numerous large cloacae, of which Pliny (xxxvi. 15) 

 has spoken in terms of unbounded admiration. The emperors created 

 officers called curatores cloacarurn. The city of Pompeii had cloaca) 

 on a smaller scale, and from the mention made by Diodorus of the 

 cloaca; (tnrfoo/ioi) of Agrigentum there can be little doubt that other 

 cities were provided with cloaca;, though of course on a much smaller 

 and less complete scale than Rome. 



CLOCK AND WATCH-MAKING. There is a close connecting-link, 

 or rather series of links, between the mechanism of a pocket-watch, a 

 chronometer, a spring-clock, and a pendulum-clock, rendering it con- 

 venient to treat of them all under one heading. We shall therefore at 

 once refer the reader to the article HOROLOGY, for the requisite infor- 

 mation on these matters. 



CLOISTER, from the Anglo-Saxon claurtjx or clurten, adopted 

 from the Latin claustrum, a place inclosed or shut up. The term is 

 more particularly applied to the covered walk or inclosure which was 

 the usual appendage of our ancient monasteries and collegiate esta- 

 blishments, where the inmates met to converse and take exercise, or 

 frequented for the purpose of study. It was also very commonly used 

 as a place of sepulture for the members of the establishment. The 

 cloister was always attached to the church, and was usually carried 

 round the sides of a quadrangle ; see the plan of Durham Cathedral 

 in the article CHURCH. In a more general sense, cloister is used for a 

 monastery at large, whether of monks or nuns, the inmates being 

 inclosed or shut up from the world. The German word " kloster " 

 retains this original signification. 



CLOUD, in meteorology, a mass of aqueous vapour condensed 

 into the visible form, in which it consists of globular particles of 

 water, excessively minute, and probably not vesicular or hollow, 

 the entire congeries of which, when of a certain magnitude and 

 density of aggregation, normally reflects white light, but is translucent 

 and colourless for very small thicknesses, and from its powerful 

 absorptive action upon light in greater but still moderate thicknesses 

 is absolutely opaque. It may float in the atmosphere, or rest on the 

 surface of the earth ; but in all cases it is a portion of the aqueous 

 vapour in the atmosphere, separated by reduction of temperature, 

 however effected. When resting on the earth's surface, or produced 

 or existing very near it, that which, when seen above, is termed 

 cloud, is familiarly known as mist or fo;i ; the latter term being also 

 applied to the same object when modified by certain circumstances 

 and additions. The consideration of the mode of production, appear- 

 ance, disappearance, and results of the clouds, is an important branch of 

 meteorological knowledge, which we shall introduce to our readers by 

 a review of their more obvious phenomena, as made subjects of scien- 

 tific investigation. 



Science is indebted to the veteran meteorologist, Mr. Luke Howard, 

 F.R.S., the author of ' The Climate of London,' and other works, and 

 now (1859) vice-president of the British Meteorological Society, for a 

 classification and nomenclature enabling observers to fix their ideas of 

 the proverbially ever- varying and fleeting forms presented by the con- 

 densed vapours of the atmosphere, which, after the experience of more 

 than half a century, in the hands of men of science, meteorologists and 

 physicists, of every country and of every grade, has been found 

 universally applicable, in all climates, and in all general or local con- 

 ditions of the atmosphere, and has scarcely been found susceptible 

 either of change or of addition. This classification and nomenclature 

 was first enunciated in an essay read by Mr. Howard before the 

 Askesian Society (already noticed in this Cyclopa;dia, Bioo. Div., PEPYS, 

 WILLIAM HASLEDIJJE; PHILLIPS, RICHARD; and PHILLIPS, WILLIAM), in 

 the session 1802-1803, and originally published in the ' Philosophical 



Magazine,' first series, vols. xvi. xvii., 1803, 1804, in the form of a 

 paper, containing the substance of that essay, ' On the Modifications 



which Mr. Howard was the author ; and this was reprinted by William 

 Nicholson, in his well-known ' Journal of Natural Philosophy,' &c., 8vo 

 Series, for September, 1811, vol. xxx., p 35-62. 



At the end of the second volume of his work on the Climate of 

 London,' published in 1820, Mr. Howard gave the ' Terminology of 

 Clouds ' &c., as an extract from his paper ; and m the second edition of 

 that work published in 1833, he republished the paper itself, which, 

 with some notes, forms a portion of the introductory matter to the 

 first volume having also been separately printed and published in 

 the preceding year. From this is derived the following view of the 



term 



Howard applies " to the various forms of suspended water " the 

 er '" modifications of cloud ;" and after stating that " there are three 

 simple and distinct modifications, in any one of which the aggregate of 

 minute drops, called a cloud, may le formed, increase to its yrea/est 

 , ,-toit and Anall'/ decrease and disappear," he proceeds to describe 

 'ntially in the following terms, first the simple, and then the 

 intermediate and compound modifications, prefacing the description 



ABTS AND SCI. DIV. VOL. II. 



of each by its definition. To the characters of the first three modi- 

 fications, some particulars derived from Sir John F. W. Herschel, 

 are here added. The primary modifications are thus defined and 

 described : 



Cirrus. Parallel, flexuotis, or diverging fibres, extensible by increase 

 in any or all directions. They are the earliest appearance after serene 

 weather, being first indicated by a few threads pencilled as it were on 

 the sky. These increase in length, and new ones are at the same time 

 added to them. The first formed threads serve as stems to support 

 numerous branches, which in their turn give rise to others. The 

 upward direction of the fibres or tufts of this cloud has been found to 

 lie a decided indication of the condensation of the aqueous vapour in 

 the air preceding rain ; their downward direction indicates evaporation 

 and fair weather. The cirrus, of all the modifications of cloud, has 

 the least density of aggregation, the greatest elevation, and the greatest 

 variety of extent and direction. 



The name of mare's-tails, by which cirri are commonly known, 

 describes their aspect well. Their filamentous structure clearly indi- 

 cates them as in the act of originating from the production of visible 

 vapour from the union of aerial currents of different temperatures 

 running parallel to each other. From their great elevation it is more 

 than probable that their particles are frozen, and of course crystalline, 

 and that to this constitution it is owing that halos, corona;, and other 

 optical appearances, referable (as originally inferred, we believe, by 

 Dr. T. Young) to reflections and refractions in ice-crystals, appeal- 

 almost invariably in cirrus and in its derivative forms, especially the 

 cirro-stratus. Cirrus is said to be often a precursor of windy weather. 

 Some interesting, but at present obscure, relations between this modi- 

 fication of cloud and the POLAR LIGHTS will be alluded to in the article 

 on that subject. 



Cumulus. Convex or conical heaps, increasing upward from a hori- 

 zontal base. Clouds in this modification are commonly of the most 

 dense texture; they are generally formed in the lower nubiferous 

 atmosphere, and move along with the current which is next the earth. 

 A small irregular spot first appears, and is, as it were, the nucleus on 

 which the increase takes place. The lower surface continues irregularly 

 plane, while the upper rises into conical or hemispherical heaps, which 

 may afterwards continue long nearly of the same bulk, or rapidly grow to 

 the size of mountains. [Prof. James Forbes observed from the summit 

 of the Jungfrau, in the Bernese Alps, a single " magnificent cumulus- 

 headed cloud," reaching apparently from the valley below to at least 

 2000 feet above the mountain, so that its vertical thickness or height 

 must have been at least 10,000 feet, or about two miles.] In the for- 

 mer case cumuli are usually numerous and near together, in the latter 

 comparatively few and distant ; but whether there are few or many, 

 their bases lie nearly in one horizontal plane, and their increase upward 

 is somewhat proportionate to the extent of base. The cumulus of 

 fair weather has a moderate elevation and extent, and a well-defined 

 rounded surface. Previous to rain it increases more rapidly, appears 

 lower in the atmosphere, and with its surface full of loose fleeces 

 or protuberances, it being now efflorescent, as it were, on the great 

 scale. 



A particular account of the production of this modification, by Sir 

 Tohn Herschel, will be found in the general views of the formation and 

 nature of clouds given in the sequel of this article. 



Stratum. A widely extended, continuous, horizontal sheet, increasing 

 from below upward. It is of intermediate density of aggregation, 

 but is the lowest of clouds, its inferior surface commonly resting on 

 the earth or on water. Contrary to the cumulus, which may be con- 

 sidered as belonging to the day, this is properly the cloud of niyht ; 

 the time of its first appearance being about sunset. It comprehends 

 all those creeping mists which in calm evenings asceud in spreading 

 sheets (like an inundation) from the bottom of valleys and the surface 

 of lakes, rivers, and other pieces of water, to cover the surrounding 

 country. Its duration is frequently through the night. On the return 

 of the sun the level surface of this cloud begins to put on the appear- 

 ance of cumulus, the whole at the same time separating from the ground. 

 The continuity is next destroyed, and the cloud ascends and evaporates, 

 or passes off with the morning breeze. This change has been long 

 experienced as a prognostic of fair weather, and indeed there is none 

 more serene than that which is ushered in by it. 



When the dew-point is attained at some definite altitude above the 

 ground, stratus is also formed there ; of which Sir J. Herschel records 

 an instance in which the cloud was extended in the air at the rate of 

 300 miles in an hour, in the direction of^the depression of temperature 

 caused by the setting of the sun. 



The two intermediate modifications are t 



Cirro-cumulus. Small, well-defined, roundish masses, in close hori- 

 zontal arrangement or contact. The cirrus having continued for 

 some time increasing or stationary, usually passes either into the 

 cirro-cumulus or the cirro-stratus, at the same time descending to 

 a lower station in the atmosphere. It is formed from a cirrus, or 

 from a number of small separate cirri, by the fibres collapsing as it 

 were, and passing into small roundish masses, in which the texture of 

 the cirrus is no longer discernible, although they still retain somewhat 

 of the same relative arrangement. This modification forms a very 

 beautiful sky, sometimes exhibiting numerous distinct beds of these 

 small connected clouds, floating at different altitudes. It is frequent 



SB 



