EfcQs of Wattr to tqualize ike Heat of difertnt Climate/, 567 



Though fummer and winter, fpring and autumn, and all the variety of the feafons, are 

 produced by the (Imple and admirable contrivance of the inclination of the axis of the earth 

 to the plane of the ecliptic ; yet this mechanical difpofition would not have been alone fuf- 

 ficient to produce that gradual change of temperature which the neceflitics of animal and 

 vegetable life appear to require. It feems necelTary not only that the changes {hould be 

 gradual, but that the extremes of heat and cold fhould be mitigated by fome equalizing 

 power. 



The cold air from the northern and fouthern regions of the earth ruflies towards the 

 heated parts in the regular winds which are known to prevail on its furface; and the return 

 of thefe mafles of air, which, when they become heated, rife and flow back in the upper de- 

 partments of the atmofphere towards their former ftation, muft greatly tend to preferve a 

 more equal temperature of climates than would elfe have prevailed. But the agency of 

 water in producing the fame effeft has been overlooked, though the remarkable law of its 

 condenfation by cold renders it wonderfully fuited to that purpofe. 



M. de Luc has fhewn, that by cooling water from the boiling point 80° to the freezing 

 point o, if the whole contra£lion be divided into 80 equal | ortions, the condenfation or 

 lofs of bulk for each ic° willbe refpeftively, beginning at 80°, as follows : 18.0 ; — 16.2 ; — 

 13.8; — ii-s; — 9.3; — 7-'>~3-9> — and 0.2. Whence it is feen, thatthe increafe of fpecific 

 gravity, by cooling when near the mean temperature in England, is at lead ninety times 

 lefs than when the water is near boiling hot. 



The vaft extent of the ocean, and its great depth, but ftill more its numerous currents, 

 and the power of water to abforb a vaft quantity of heat, render it peculiarly well adapted 

 to ferve as an equalizer of temperature. 



On the retreat of the fun after the folftice, it is clofely followed by the cold winds from 

 the regions of eternal froft, which are continually endeavouring to prefs on towards the 

 equator. As the power of the fun to warm the furface of the earth and the air diminitlies 

 very faft in high latitudes on the days growing fliorter, it foon becomes too weak to keep 

 back the denfe atmofphere which prefles in from the polar regions, and the cold increafes 

 very faft. 



There is however a circumftance by which thefe rapid advances of winter are in feme 

 meafure moderated. The earth, but more efpecially the water, having imbibed a vaft 

 quantity of heat during the long fummer day;, while they received the influence of the fun's 

 vivifying beams ; this heat being given off to the cold air which ruflies in from the polar 

 region, ferves to warm it and foften it i and confequently to diminifli the impetuofity of its 

 motion, and take off the keennefs of its blaft. But as the cold air Hill continues to flow in 

 as the fun retires, the accumulated heat of fummer is foon exhaufted, and all folld and 

 fluid bodies aie reduced to the temperature of freezing water. In this ftnge the cold in the 

 atmofphere increafes very fall, and would probably increafe ftill fafter, were it not for the 

 vaft quantity of heat which is communiciited to the air by the watery vapours, which are 

 firft condcnfed, and then congealed in the atmofphere, and which afterwards fall upon 

 the earth in the form of fnow ; and by that ftill larger quantity which is given ofl" by the 

 water in the rivers and lakes and in the ground, upon its being frozen. 



But in very cold countries the ground is frozen and covered with fnow, and all the lakes 

 and rivers arc frozen over in the very beginning of winter. The cold llicn firil begins to be 



extreme. 



