FOGS CLOCDS.] 



METEOROLOGY. 



1141 



smoke. As regards the production of foar, it is usually 

 referable to one of two circumstances : either non-visible 

 aqueous vapour may be converted into the visible or 

 vesicular form, by decrease of atmospheric temperature 

 immediately, or by the cooling agency of the earth's 

 surface, depressing the temperature of the layer of air 

 next to it, below the dew point. In this countiy, and in 

 Europe generally, fogs are of most frequent occurrence in 

 spring and autumn. There are regions, however, in 

 which fogs prevail throughout the year. The coasts of 

 California are almost constantly veiled in fog ; and the 

 same remark applies, in a minor degree, to the western 

 coast of the American continent, even so far south as 

 Peru. Newfoundland, Nova Scotia, and Hudson's Bay, 

 .ire all subject to frequent and dense fogs, attributable, 

 in these localities, to the condensation of vapour, which 

 arises from the hot gulf-stream, by contact with neigh- 

 bouring and colder air. Fogs do not occur so frequently 

 on level plains as in mountainous regions. In Arabia 

 and the arid table-land of Persia they are almost alto- 

 gether wanting. London and Amsterdam have acquired 

 a somewhat evil character for fogs ; but this meteoric 

 condition applies to many other European localities with 

 an almost equal amount of propriety. At Antwerp, 

 fogs are very prevalent ; and the navigation of the lower 

 ;iud middle Khine is sometimes impeded for weeks 

 together by the occurrence of this pest of the sailor. 

 Neither can Paris boast of much immunity from fogs ; 

 they are somewhat less dense and less frequent than our 

 own London fogs, it is true, but are, nevertheless, far 

 from contemptible. Amongst the regions which are 

 likely in future to be celebrated for the prevalence of 

 fogs, the Black Sea may be enumerated. Until recent 

 events, that locality was comparatively unknown to us ; 

 but during the time of the Crimean war, which neces- 

 sitated a constant navigation of the Black Sea, the 

 embarrassment of fogs became but too apparent. 



Dry Fogs. Under this name has been described a dull 

 opaque appearance which the atmosphere of certain 

 regions occasionally assumes, deaden- 

 ing the fiery beams of the sun, and 

 dulling that luminary so that he may 

 be looked at without pain by the naked 

 eye, and embarrassing respiration. 



The dry fog is most common in 

 certain parts of North America, during ,9i 

 the period known as the Iiidian 



ner. It also occurs in Germany, 

 and more rarely in England. There 

 CUM bo no doubt that many atmo- 

 spheric opacities, different in character 

 as well as in cause, have been sum- 

 marily classed under the. 1. -nomination 

 of dry f<>t. \Vlieii tin; phenomenon 

 occur* loc;illy, it can generally be 

 traced to such c uises as the burning of 

 I ixtricU of turf or of forests. 

 Wlieu ioiice is more general, 



the most rational explanation would 

 seem to lie that which attributes it 

 to volcanic eruptions. Some mete- 

 orologists have invoked electricity as 

 the cause of tliis phenomenon ; Imt 

 it is not easy to sue in what way 

 the assumed cause could produce the 

 effect in (ju.^tioii. Electricity has 

 long been to meteorologists what the i 

 class radinta was to Cuvier ; namely, * 

 tip; receptacle for things unknown or un 



CLOUDS. Clouds are perhaps the most beautiful of all 

 il phenomena. All tho charms of changeful variety 

 of colour, of form, and of motion are theirs ; nor is their 

 utility inferior to their beauty. Without clouds there 

 would be neither rain, nor snow, nor hail ; the conse- 

 quences of this deprivation may be anticipated, or they 

 may bo readily learned, by turning to the geography of 

 countries where rain, and snow, and hail are unknown. 



Som<* hare broached th* run m theory that *uch fogs are caused 

 by the iromcriion of the earth in the tail of a comet. Eu. 



All regions thus circumstanced, provided irrigation be 

 impossible, and the altogether exceptional condition of 

 copious dews be absent, are, despite the most favourable 

 conditions of climate and of soil, barren wastes. 



Notwithstanding the thousandfold varieties of clouds 

 their Protean shapes, their manifold colours, and other 

 distinctions when the observer comes to regard them 

 with a scrutinising eye, he will not fail to recognise broad 

 distinctions between them, admitting of being made the 

 basis of a philosophic classification. Thus, some clouds 

 are devoid of outline, their edges merging away into cir- 

 cumambient air ; some are black and massive, almost 

 conveying the idea of a hard substance ; some are white 

 and fleecy ; others extended like a pennon. All these 

 are forms of cloud which present manifest distinctions 

 amongst themselves. Mr. Howard was the first who 

 effected a regular classification of clouds. This classifica- 

 tion is now generally adopted. We shall, therefore, lire- 

 sent the reader with an outline of his system. Accord- 

 ing to this meteorologist, there are three elementary and 

 four secondary forms of cloud. 



Primary Forms. The first primary form is the cirrus, 

 consisting of feathery expansions, and which ia only seen 

 in clear weather. 



The second primary form of cloud is the cumulus, com- 

 posed of large hemispheroidal masses, superiorly, and ap- 

 parently resting below on a horizontal base. This form 

 of cloud chiefly occurs in summer. 



The third primary form of cloud is stratus, composed 

 of horizontal layers, tho smaller layers being underneath. 

 It is this form of cloud, more than any other, which pre- 

 sents itself under a variety of beautiful colours. It 

 chiefly appears at sunset. 



Secondary Forma The secondary forms of clouds are 

 (1) Cirro cumulus. It is a mixture, as its name indi- 

 cates, of cirrus with cumulus, ami is made of an aggre- 

 gation of small white clouds, which liavo been compared 

 to a Hock of sheep. (2) Cirro stratus. A compound cloud, 

 which is formed of the two primary clouds embodied in 



Fig. 13. 



I.ex] 



itsname. ( Tin's compound cloud chielly 



appears towards night, in dry windy weather, and is of a 

 leaden colour. (4) Nimbus, or rain cloiul. This cloud is 

 seen in greatest perfection during a thunder-storm. All 

 the varieties of clouds described are represented in the 

 above diagram (Fig. 48). 



It will be readily anticipated that clouds are frequently 

 so mingled and confounded, that they are not always 

 susceptible of the precise classification just announced ; 

 nevertheless, a prevalence of one type of cloud over 

 another will be generally seen to prevail. 



