The Aurora Borealis, or northern lights,[162] appear with great brilliancy in the clear Canadian sky, especially during the winter nights. Starting from behind the distant horizon, they race up through the vault of heaven, spreading over all space one moment, shrinking to a quivering streak the next, shooting out again where least expected, then vanishing into darkness deeper than before; now they seem like vast floating banners of variegated flame, then as crescents, again as majestic columns of light, ever changing in form and color. It is said that a rustling sound like that of silk accompanies this beautiful appearance. The climate of Canada has undergone a slight change since the discovery of the country; especially from the year 1818, an amelioration has been perceptible, partly owing to the motion of the magnetic poles, and partly to the gradual cultivation and clearing of the country. The winters are somewhat shorter and milder, and less snow falls than of old; the summers are also hotter.[163] The felling of the forests, the draining of the morasses, partial though it may still be, together with the increasing population, have naturally some effect. The thick foliage, which before interposed its shade between the sun and the earth, intercepting the genial warmth from the lower atmosphere, has now been removed in many extensive tracts of country: the cultivated soil imbibes the heat, and returns it to the surrounding air in warm and humid vapors. The exhalations arising from a much increased amount of animal life, together with the burning of so many combustibles, are not altogether without their influence in softening the severity of the climate.[164]