The temperature of a particular place on the surface of the earth is determined by a variety of circumstances, some of which are regular in their operation, others ac cidental. Among the former of these, may be stated, the direct influence of the solar rays, and the latitude ; among the latter, the winds, evaporation, and perhaps the evolution or absorption of heat by operations going on in the central regions. Even the causes of tempera ture which we hays denominated regular, are not abso lutely so : the heat produced by the rays of the sun may he affected by spots upon his surface ; and the tempera ture resulting from geographical position may be modi fied by local peculiarities. Thus the sea limits the range of temperature, by moderating alike the extremes of heat and cold ; while large tracts of land are equally favourable to both. The winds have a very powerful in fluence on the temperature of a place. When the sur face of the earth is much heated by the influence of the solar rays, the air immediately above it is rarified, and becoming specifically lighter, ascends into the higher re gions of the atmosphere. Its place is quickly occupied by a fresh portion of air rushing in from every side, which, in its turn, being heated and rarified, also ascends. The warm air which has thus ascended, is gradually waft ed to colder regions, where it gives out its heat, and mo derates the rigour of the climate to which it has been transported.
Evaporation is one of the principal sources of natural cold, the conversion of water into vapour being neces sarily accompanied with the absorption of much caloric ; hence, the agricultural improvement of a country, or whatever tends to facilitate the escape of the water from its surface by any means but evaporation, has a remark able influence on its temperature. The gradual ameli oration of the climate of America, is undoubtedly to be ascribed to this cause, as well as that of Europe. Hume, in his E.esay on the Ponulousness of Ancient Afiations, has proved, by the most ample authorities, that a very great change has taken place on the general temperature of Europe since no try distant period " It is an observa tion of L'Abbe de Bas," says lie, " that Italy is warmer at present than it was in ancient times. The annals of Rome tell us, that in the year 480 ab U. C. the winter was so severe that it destroyed the trees ; the Tyber froze at Route, and the ground was covered with snow for forty days. When Juvenal describes a superstitious woman,l,e represents her as breaking the ice of the Tyber, that t',L might perform her ablutions : ilybernum fracta glade deseendet in amnem, Ter matutine Tyberi mergetur.
He speaks of that river's freezing as a common even. Many passages of Horace suppose the streets of Rom: full of snow and ice. We should have more certainty
with regard to this point, had the ancients known the use of thermometers ; but their writers, without intend ing, give us information sufficient to convince us, that the winters are now much more temperate at Rome than formerly. At present, the Tyber no more freezes za Rome than the Nile at Cairo. The Romans esteem the winter very rigorous, if the snow lie two days, and if one see, for eight-and-forty hours, a few icicles hang from a fountain that has a north exposure.
" The observation of this ingenious critic," continues Hume, " may be extended to other European climates. Who could discover the mild climate of France in Dio dorus Siculus's (lib. iv.) description of that of Gaul ? c As it is a northern climate,' says he, c it is infested with cold to an extreme degree. In cloudy weather, instead of rain there fall great snows ; and in clear weather, there freezes so excessive hard, that the rivers acquire bridges of their own substance ; over which, not only sin gle travellers may pass, hut large armies, accompanied with all their baggage and loaded waggons. And there being many rivers in Gaul, the Rhone, the Rhine, &c. almost all of them are frozen over ; and it is usual, in order to prevent falling, to cover the ice w ith chaff and straw at the places where the road passes.' Colder than a Gallic winter, is used by Pet•onius as a proverbial ex pression. Aristotle says, (De Generat. Anim. lib. ii.) that Gaul is so cold a climate that an ass could not live in it.
" North of the Cevennes," says Strabo, (lib. iv.) "Gaul produces not figs and olives ; and the vines which have been planted bear not grapes that will ripen.
" Ovid (Trist. lib. iii. eleg. 9.) positively maintains, with all the serious affirmation of prose, that the Euxine sea was frozen over every winter in his time ; and he ap peals to Roman governors, whom he names, for the truth of his assertion. This seldom or never happens at pre sent in the latitude of Tonti, whither Ovid was banished. All the complaints of the same poet seem to mark a ri gour of the seasons, which is scarcely experienced at pre sent in Petersburgh or Stockholm.
" Tournefort, a Provcnial, who had travelled into the same country, observes, that there is not a finer climate in the world ; and he asserts, that nothing but Ovid's melancholy could have given him such dismal ideas of it. But the facts mentioned by that poet, arc too circumstan tial to bear any such interpretation.
" Italy, says Varro (lib. i. cap. 2.) is the most tem perate climate in Europe. The inland parts, (Gaul, Ger many, and Pannonia, no doubt,) have almost perpetual winter.