CLIMATOLOGY.] 



METEOROLOGY. 



1155 



sist. The Will-o'-the-wisp is not a very frequent pheno- 

 menon anywhere ; but it is chiefly seen in marshes and 

 churchyards the latter locality apparently adding to 

 the hypothesis that it is nothing more than ignited phos- 

 phuretted hydrogen gag. (See Fig. G6). 



Scarcely less accurately demonstrated than the locality 

 of ague, is the locality of yellow fever, the focus of which 



Fig. C6. 



may be considered to be Vera Cruz. Strange to fay, 

 the yellow fever is totally unknown on the Pacific coast 

 it extends north as far as Now Orleans, but 

 rarely further. This condensation, so to speak, of febrile 

 energy, points to some local cause most probably of 

 Atmospheric origin ; but chemistry has been unable to 

 determine its nature. 



Reflections similar to the above are suggested by the 

 contemplation of several contagious diseases ; of which 

 the plague is a notable example. This scourge, although 

 capable of spreading, from its normal focus, into regions 

 wide apart, is in certain spots determined conditionally 

 by the existence of some unknown circumstances ; and 

 these are most probably atmospheric. The plague never 

 originates in very hot or very cold regions. Its focus of 

 development is Egypt, Turkey, and the Levant, from 

 which spots it can never be said to be absent altogether ; 

 but it only appears with violence at intervals of eight or 

 ten years. Still more extraordinary than any of the 

 preceding, are the varied conditions which give rise to 

 Asiatic cholera. Unlike the plague and the yellow fever, 

 and intermittent*, this fell destroyer seems independent 

 of region, recognisable limits, or other conditions of 

 di-marcation. From east to west it has extended its 

 ravages, under every vicissitude of season and of clime. 

 Much as there is mysterious in this absolutely ignorant 

 though we be of the proximate influences by the operation 

 of which cholera and other epidemics are generated, im- 

 portant facts have, nevertheless, been made out ; and 

 tln:ir consideration tends to allay the extreme fear 

 wherewith epidemics were formerly regarded. The most 

 important fact in connection with this subject is, that epi- 

 demic intlm-ncva, whatever their nature maybe, only as a 

 rule prevail over the weak, exhausted, ill-fed, or mentally 

 liri'k>-n-do'.vii individuals of a community; whence it 

 ns, that in our metropolis the medical statician is 

 led to lay his finger upon the regions of epidemic 

 virulence, as he would on the locality of a mountain or 

 ft coal-field ; and with equal satisfaction can he point to 

 localities where epidemics formerly raged, but whence 

 ivo been banished by the art of man. 



CLIMATOLOGY. Many of the effects of heat, in its me- 

 teorological relations, have already been incidentally con- 

 sidered; but reference has not yet been made to the 



sources of heat, and to the means of its. distribution over 

 the surface of our planet. The term heat, as applied to 

 the matter now under investigation, may be regarded as 

 synonymous with elevation of temperature ; and inas- 

 much as such elevation necessarily pre-supposes a con- 

 dition of antecedent depression, we may, without impro- 

 priety, comprehend the meteorological effects of high 

 and low temperatures (heat and cold) 

 under one and the same generalisa- 

 \ ^ A. ) tion. 



Central Heat of the Earth. Tho 

 hypothesis was first propounded by 

 t Leibnitz, that the whole of our planet 

 was once a molten mass, which, by 

 the operation of cooling, uninterrupt- 

 edly going on in successive ages, has 

 become superficially encrusted over, 

 the crust having become adapted to 

 the necessities of animal and vegetable 

 life. Various circumstances may be 

 adduced in favour of this notion, 

 more particularly the gradual increase 

 of terrestrial heat downwards,* tho 

 heat of deep springs, and the evi- 

 dences of fusion, in what geologists 

 term the igneous rocks. Whether the 

 idea of Leibnitz hold good in its 

 entire acceptation that is to say, 

 whether the centre of our planet 

 be one molten mass or not there 

 can be little doubt that all positions 

 of the globe, at a sufficient distance! 

 below the surface, have at somo 

 period been submitted to fusion. 

 Nevertheless, at this time, tho 

 earth's central heat may be altogether ignored as 

 tending to influence, in any manner, the climatic tem- 

 ' pentturo of our globe. Primarily, tho sun's direct rays 

 determine the climatic temperature ; those portions of 

 the world's surface being moat strongly heated on which 

 the sun shines vertically ft remark which of course 

 applies to the tropics ; while those are least heated on 

 which the direction of solar rays is most oblique a re- 

 mark which of course applies to the arctic and antarctic 

 regions. But the latitude of a region has less connection 

 with its climatology than might at first seem probable. 

 The varying conditions of insular and continental S<M- 

 level, or elevated table-land, valley, or mountain, and 

 still more the influence of thermal oceanic currents, have 

 much to do with the climatic result. The high table- 

 land of Mexico is strongly illustrative of the effect of 

 mere elevation. The traveller who disembarks first on 

 the Atlantic coast, and wanders inland, soon finds him- 

 self amidst all the luxuriance of a tropical forest, and 

 surrounded by all the dangers of tropical existence. 

 Still wending his way inland, bo ascends a mountain 

 elevation, well adapted to the growth of wheat, which 

 refuses to grow anywhere in tropical lowlands. 



Of all the causes which influence the climate of a 

 region, that attributable to oceanic currents lias been 

 hitherto least studied ; yet there is none which deserve? 

 to be scrutinised more narrowly. Looking at the enor- 

 mous amount of oceanic surface in comparison with tint 

 of the laud taking into consideration the mobility of 

 water, its susceptibility of thermal impressions, and tho 

 effect of the configuration of capes, headlands, and lines 

 of coasts 'he contemplative observer soon arrives at the 

 deduction, that the ocean presents to him, at least, as 

 wide a field for investigation as the atmosphere, and ouo 

 scarcely less interesting. " The fauna and the flora," says 

 Maury, "of the sea are as much the creatures of clim.itu, 

 and are as dependent for their well being upon trm 

 ture, as are the fauna and the flora of dry laud. Worn it 

 not so, we should find the fish and algae, the marino 

 insect and the coral, distributed equally and alike in all 



Careful observations have not confirmed this idea, as it has been 

 :"u..iri, m mines not at work, that there is no indication of an in. 

 temperature as a descent is made. It is probable that the increase ob- 

 served, in mines at work, may be due to the animal heat, lamp*, &c., of 

 the miners. to. 



