Book III. THE ATMOSPHERE. 359 



2360. The cirrus appears to have the least density, the greatest elevation, the greatest variety of extent 

 and direction, and to appear earliest in serene weather, being indicatetl by a few threads pencilled on the 

 sky. Before storms they appear lower and denser, and usually in the quarter opposite to that from which 

 the storm arises. Steady high winds are also preceded and attended by cirrous streaks, running quite 

 across the sky in the direction they blow in. 



2361. The cumulus has the densest structure, is formed in the lower atmosphere, and moves along with 

 the current next the earth. A small irregular spot first appears, and is, as it were, the nucleus on which 

 they increase. The lower surface continues irregularly plane, while the upper rises into conical or hemi- 

 spherical heaps ; which may afterwards continue long nearly of the same bulk, or rapidly rise into moun- 

 tains. They will begin, in fair weather, to form some hours after sunrise, arrive at their maximum in 

 the hottest part of the afternoon, then go on diminishing, and totally disperse about sunset. Previously 

 to rain the cumulus increases rapidly, appears lower in the atmosphere, and with its surface full of loose 

 fleeces or protuberances. The formation of large cumuli to leeward in a strong wind, indicates the ap- 

 proach of a calm with rain. When they do not disappear or subside about sunset, but continue to rise, 

 thunder is to be expected in the night. 



2362. The stratus has a mean degree of density, and is the lowest of clouds, its inferior surface commonly 

 resting on the earth in water. This is properly the cloud of night, appearing about sunset. It compre- 

 hends all those creeping mists which in calm weather ascend in spreading sheets (like an inundation of 

 water) from the bottoms of valleys, and the surfaces of lakes and rivers. On the return of the sun, the 

 level surface of this cloud begins to put on the appearance of cumulus, the whole at the same time separat- 

 ing from the ground. The continuity is next destroyed, and the cloud ascends and evaporates, or passes 

 oft" with the appearance of the nascent cumulus. This has long been experienced as a prognostic of fair 

 weather. 



2363. Transition of forms. The cirrus having continued for some time increasing or stationary, usually 

 passes either to the cirro-cumulus or the cirro-stratus, at the same time descending to a lower station in 

 the atmosphere. This modification forms a very beautiful sky, and is frequently in summer an attendant 

 on warm and dry weather. The cirro-stratus, when seen in the distance, frequently gives the idea of 

 shoals of fish. It precedes wind and rain ; is seen in the intervals of storms ; and sometimes alternates 

 with the cirro-cumulus in the same cloud, when the different evolutions form a curious spectacle. A 

 judgment may be formed of the weather likely to ensue by observing which modification prevails at last. 

 The solar and lunar haloes, as well as the parhelion and paraselene (mock sun and mock moon), prognostics 

 of foul weather, are occasioned by this cloud. The cumulo-stratus precedes, and the nimbus accom. 

 l)anies rain. 



2364. Dew is the moisture insensibly deposited from the atmosphere on the surface of 

 the earth. This moisture is precipitated by the cold of the body on which it appears, and 

 will be more or less abundant, not in proportion to the coldness of that body, but in pro- 

 portion to the existing state of the air in regard to moisture. It is commonly supposed 

 that the formation of dew produces cold, but like every other precipitation of water from 

 the atmosphere, it must eventually produce heat. 



2365. Phenomena of dew. Aristotle justly remarked, that dew appears only on calm and clear nights. 

 Dr. Wells shows, that very little is ever deposited in opposite circumstances ; and that little only when the 

 clouds are very high. It is never seen on nights both cloudy and windy ; and if in the course of the night 

 the weather, from being serene, should become dark and stormy, dew which has been deposited will disap- 

 pear. In calm weather, if the sky be partially covered with clouds, more dew will appear than if it were 

 entirely uncovered. Dew probably begins in the country to appear upon grass in places shaded from the 

 sun, during clear and calm weather, soon after the heat of the atmosphere has declined, and continues to 

 be deposited through the whole night, and for a little after sunrise. Its quantity will depend in some 

 measure on the proportion of moisture in the atmosphere, and is consequently greater after rain than after 

 a long tract of dry weather ; and in Europe, with southerly and westerly winds, than with those which 

 blow from the north and the east. The direction of the sea determines this relation of the winds to dew; 

 for in Egypt, dew is scarcely ever observed except while the northerly or Etesian winds prevail. Hence 

 also dew is generally more abundant in spring and autumn than in summer. It is always very copious 

 on those clear nights which are followed by misty mornings, which show the air to be loaded with 

 moisture ; and a clear morning following a cloudy night determines a plentiful deposition of the retained 

 vapour. When warmth of atmosphere is compatible with clearness, as is the case in southern latitudes, 

 though seldom in our country, the dew becomes much more copious, because the air then contains more 

 moisture. Dew continues to form with increased copiousness as the night advances, from the increased 

 refrigeration of the ground. 



2366. Cause of dew. Dew, according to Aristotle, is a species of rain, formed in the lower atmosphere, 

 in consequence of its moisture being condensed by the cold of the night into minute drops. Opinions of 

 this kind, says Dr. Wells, are still entertained by many persons, among whom is the very ingenious Pro- 

 fessor Leslie. {Relat. of Heat and Moisture, p. 37. and 132.) A fact, however, first taken notice of by 

 Carstin, who published his Treatise on Dew in 1773, proves them to be erroneous ; for he found that bodies 

 alittle elevated in the air often become moist with dew, while similar bodies, lying on the ground, remain 



dry, though necessarily, from their position, as liable to be wetted, by whatever falls from the heavens, as ' 

 the former. The above notion is perfectly refuted by the fact, that metallic surfaces exposed to the air in 

 a horizontal position remain dry, while every thing around them is covered with dew. After a long 

 period of drought, when the air was very still and the sky serene. Dr. Wells exposed to the sky, 

 28 minutes before sunset, previously weighed parcels of wool and swandown, upon a smooth, unpainted, 

 and perfectly dry fir table, 5 feet long, 3 broad, and nearly 3 in height, which had been placed, an hour 

 before, in the sunshine, in a large level grassfield. The wool, 12 minutes after sunset, was found to be 

 14 colder than the air, and to have acquired no weight. The swandown, the quantity of which was much 

 greater than that of the wool, was at the same time 13 colder than the air, and was also without any ad- 

 ditional weight. In 20 minutes more the swandown was 14^ colder than the neighbouring air, and was 

 still without any increase of its weight. At the same time the grass was 15 colder than the air four feet 

 above the ground. Dr. Wells, by a copious induction of facts derived from observation and experiment, 

 establishes the proposition, that bodies become colder than the neighbouring air before they are dewed. 

 The cold therefore, which Dr. Wilson and M. Six conjectured to be the effect of dew, now ajjpears to be 

 its cause. But what makes the terrestrial surface colder than the atmosphere ? The radiation or 

 projection of heat into free space. Now the researches of Professor Leslie and Count Rumford have de- 

 monstrated that different bodies project heat with very different degrees of force. In the operation of 

 this principle therefore, conjoined with the power of a concave mirror of cloud, or any other awning, to 

 reflect or throw down again those caloric emanations which would be dissipated in a clear sky, we shall 

 find a solution of the most mysterious phenomena of dew. 



2367. Rain. Luke Howard, who may be considered as our most accurate scientific 

 meteorologist, is inclined to think that rain is in almost every instance the result of the 

 electrical action of clouds upon each other. 



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