6. _Hurricanes._—Sudden and tremendous bursts of storm are common in mountainous districts, and in the plains which lie at the base of those vast piles of nature’s building. Their peaks, exposed by elevation to intense cold, and covered with perpetual snow, cool and condense the warm air rising up from the regions below, which descends with an impetus proportioned to its own gravity and the lighter condition of the air over the regions below, and a tempest ensues upon considerable condensation and rarefaction in adjoining regions of the atmosphere. This is the origin of the _pamperos_, or south-west winds, which rush from the snows of the Andes, and sweeping over the level pampas with unchecked violence, become hurricanes before their arrival at Buenos Ayres, and carry to the city clouds of dust collected from the plains, occasioning almost total darkness in the streets. So sudden is the operation of the pampero, that persons bathing in the river Plate have been drowned by the agitation of its waters, through the tempest, before they could possibly reach the shore. Captain Fitzroy relates, when in his ship upon the river, that a small boat had been hauled ashore above high-water mark, and fastened with a strong rope to a large stone; but the pampero set in, and afterward the boat was found far from the beach, shattered to pieces, but still fast to the stone, which it had dragged along. [Illustration: =Hurricane in the Tropics.=] But this violent movement of the atmosphere is remarkably beneficial in its general effect to the inhabitants of the pampas of Buenos Ayres and on the banks of the Plata. The prevailing winds through a great part of the year are northerly; and these passing over extensive marshy tracts bring with them a degree of humidity, which renders the land rife with fever and pestilence, till the pampero rushes down from the Andes and clears the atmosphere. A somewhat similar wind is one of our own physical phenomena, hitherto unexplained, to the violence of which the tourist to the Cumberland lakes may occasionally be exposed in spring and autumn. This is the Helm-wind. Hutchinson, in the history of the county, and the Rev. J. Watson, in a report to the British Association, have given an account of its singular features. When not a breath of air is stirring, or a cloud is to be seen, a line of clouds will be suddenly formed over the summits of the lofty ridge of mountains at Hartside, extending several miles on the western side. To this collection of vapors the term Helm is applied from its shape. It exhibits an awful and solemn appearance, spreading a gloom over the regions below, like the shadows of night. Parallel to this, another line of clouds, called the Bar, begins to form. The two lines unite together at their extremities, and embrace between them an elliptical cloudless space, from half a mile to four or five miles in breadth, and from eight to thirty miles in