1104 



METEOROLOGY. 



[DISTBIBTTTIOX OF HBAT. 



frequently fall* no low as 24 Fah. in tho winter ; the 

 heat in dimmer being exceedingly intense, 



In our own country we sometime* experience severe 

 changes in the weather during the winter season ; and, 

 occasionally, the thermometer ranges very high in the 

 height of summer. An extraordinary instance of the 

 former kind occurred in 1860, when the mean daily 

 average, for some days, fell surprisingly below the 

 monthly average. In December of that year, the mean 

 temperature of the 24th was 22-4 ; on the 25th it full 

 so low as 90 '2. In many parts of the country the 

 mercury fell far below zero ; indeed, we noticed it, at 

 about four miles north of London, at 2 below zero, or 

 34 below freezing point, early in the morning of the 

 ; and at 4A.M. of that day, on leaving a hot 

 room and passing into the open air, the entire dress 

 was iiutantly coated with fine particles like snow, arising 

 from the congelation of the moisture which had per- 

 meated the clothing as perspiration. In some parts, a 

 temperature so low as 14, or 46 below freezing point, 

 was registered. Amongst other instances of the extreme 

 severity of the weather, the breaking of the tires of 

 railway carriage-wheels, owing to the contraction 

 produced by the cold, was one of the most noticeable 

 and common. 



DISTRIBUTION OF HEAT. Tho distribution of heat 

 over the globe, in respect to latitudes and climates, is a 

 subject of deep interest in many respects ; and it has 

 frequently, although perhaps incidentally, been noticed 

 in the preceding pages. Wo shall devote a short 

 space to its consideration, as being a question inti- 

 mately connected with that which we have just been 

 discussing. 



It has been already remarked, that the regions near 

 the equator may be considered as the chief heating surface 

 on the globe. To the absorption of an immense amount 

 of caloric by that portion of the continent of Africa, and 

 its subsequent radiation, the genial temperature of 

 Western Europe is due. On comparing the climate of 

 temperate climes, in the latter continent, with those of 

 others, we find fewer instances of excessive temperature ; 

 the heat and cold, at no season of the year, being intense, 

 except under extraordinary and very rare occasions. 

 Taking the area, including Britain, Denmark, Holland, 

 France, Spain, and Portugal, we find climates 

 in which every necessity of man, with respect to food, 

 clothing, comfort, and luxury, can be produced ; and to 

 this cause, and also to the absence of positive noxious 

 influences, Europe is chiefly indebted for its power as 

 the centre of commerce, arts, manufactures, &c. This 

 area is included between 35 and CO", north of the 

 equator ; and if we refer to a map on Mercator's projec- 

 tion, bearing in mind the remarks already made on the 

 climate of other countries, as Russia, Thibet, Tartary, 

 &c., which are also included within the same parallels, 

 we shall at once be impressed with a sense of the 

 advantages that such portions of our continent 



One of the most accurate indications of the climate of 

 any country is obtained from the monthly average of its 

 temperature ; and if this be noted, for each political or 

 natural division, a series of lines may be drawn, which 

 will intersect, on the map, places having an equal annual 

 temperature. Those have been called isothermal lines, 

 and are generally drawn for each month of the year, 

 besides that of tho year itself. From the immense 

 variety of local interfering causes, such lines are not 

 coincident with the latitude of any two or more places. 

 It is impossible, indeed, that they could bo, unless the 

 globe were entirely covered with water ; for, as we have 

 already seen, elevation, proximity to the sea, radiation, 

 &c., all tend to produce an unceasing diversity of 

 climatic characteristics, where land of even a moderate 

 extent of area is found. 



It was formerly considered that a mean between the 

 average yearly temperatures, for a number of years, 

 would indicate what we may term tho climatic average. 

 But that such would not be the true average will bo 

 plainly seen, if wo consider the variety of temperatures 



which are observed in our own country, for example, 

 monthly, in following years. Thus it is by no means a 

 rare occurrence for us to experience, in succession, cold 

 and damp springs and summers, which, for tho years in 

 which they occur, would seriously diminish tho annual 

 j average. If, however, the same months are compared 

 with each other, in successive years, and a moan struck 

 between; ITI I if the monthly average be taken, as indi- 

 cative of tho proper mean for our climate, wo shall 

 obtain a much more accurate idea of the mean which 

 governs the year. This, therefore, is the method which 

 is now universally adopted by meteorologists ; and, at 

 page 117'^, the monthly means for Greenwich is given, 

 which will illustrate these remarks. 



To give some idea of the inflection of an isothermal 

 line from any parallel of latitude, we may take that of 

 the freezing point, or 32 Fahr. In January, com- 

 mencing at Philadelphia in the United States, it crosses 

 the banks of Newfoundland, touches the south of 

 Iceland, and, descending to Holland, is extended by the 

 Balkan to the Corea in Tartary, passing as fur as tho 

 Aleutian Islands, in the North Pacilic Ocean ; the north 

 latitudes being respectively, in round numbers, and 

 commencing at Philadelphia 40, 48, 64, 52, 42, 40 J , 

 52, for each of the positions we have named. Con- 

 necting this range of temperature with the north-easterly 

 progress of the Gulf-stream,* we shall notice an immense 

 wave of heat proceeding so far north as 64, and pro- 

 ducing a temperature as high as that found in a latitude 

 of 40 (Philadelphia), which, although so much further 

 south, has not the advantage of the heating effect arising 

 from the Gulf-stream. The western shores of Europe, 

 therefore, are situated under isothermal lines whicli havo 

 a great northerly inflection ; and these contrast greatly, 

 in direction, with those passing over extensive ranges of 

 land. Thus we find, from the Balkan, where the Gulf- 

 stream has no influence, to tho Corea. that the isothermal 

 line is nearly parallel with the latitude (42 to 40 3 ) for a 

 distance of several thousand miles ; whilst between 

 Philadelphia and Holland, a distance of 75 of longitude, 

 there is a northerly inflection -of 24 a surprising 

 instance of the influence which extensive surfaces of 

 laud and water possess in modifying climatic indications ; 

 tho latter, in this instance, having an action in raising 

 the temperature to an extent which has no parallel in 

 any other portion of the globe. Tho warmer portion of 

 this heat-ocean lies within 45 west of the meridian of 

 Greenwich ; whilst for 120 to the east, the monthly 

 mean in the same latitude is much lower, ami tho 

 isothermal inflection curves proportionally nearer to the 

 equator. 



Hitherto we have confined our remarks to latitudes 

 north of the equator ; but on the south of that lino we 

 find an entirely different condition to subsist. To 40 

 south, the average temperature is colder than up to tho 

 same parallel north of the equator ; and South Shetland, 

 Georgia, &c., although situated in a latitude as far south, 

 as Scotland, in our island, is north of the equinoctial, 

 have a climate in which the snow-line is level with the sea 

 for tho whole year, and they are destitute of all signs of 

 vitality, except in the lowest forma of vegetable pro- 

 ductions, as mosses, lichens, &c. 



RAINFALL. We need here only glance at another 

 subject, which has already been repeatedly under review 

 in this section viz., tho annual fall of rain in different 

 regions. In our climate, where excessive ranges of 

 temperature are rare, the amount of rain, although 

 Creator in some mouths than others, is pretty equally 

 distributed throughout tho year ; whilst in tropical 

 climates, in which tho air is loaded with moisture at 

 certain seasons, the fall of rain is chiefly confined to de- 

 linito periods a circumstance which has given rise to tho 

 terms " dry" and "rainy" season. In some such districts 

 the amount of rainfall is prodigious. Thus, in French 

 Guiana, about 130 inches fall annually; in Seiiegaml i.i, 

 190 ; and at Paramaibo, 230 inches fall during the saino 

 period. At Cayenne, 21 inches have boeu inea- 

 in one day an amount about equal to the annual 

 See ante, p. 114G. 



