[119] The local south winds which help the voyage upward below Rosario may be due to the high temperature of the water of the river; this also gives rise on the lower Parana to thick fog of which warning is given. In 1841 Rosas forbade navigation on the river. There was then a double blockade checking the trade of Argentina. The Franco-British fleet closed the Rio de la Plata and blockaded Buenos Aires, where the Government of Rosas was established. In addition, Rosas's troops on the _barranca_ of the right bank prevented any from going up the Parana, and cut off the interior provinces from the rest of the world. The injury then done to interests which were already fully self-conscious may be gathered from the agitation provoked by the decision of France and England in 1845 to break the blockade of the river. A convoy was at once organized at Montevideo, consisting of no less than ninety-eight ships, of 6,900 tons in all (MacKann). It went up the Parana under the protection of war-ships, which removed the chains slung across it by Rosas. The convoy dispersed up river as soon as it was out of range of Rosas. But it had needed so great an effort that the attempt could not be made again before the fall of Rosas. The closing of the Parana compelled a diversion of the trade of Paraguay toward the south-east. It crossed the isthmus of Misiones, between the Parana and the Uruguay, and passed down the Uruguay. At this time the whole commercial activity of Paraguay was concentrated at Itapua, on the upper Parana. The prosperity of the Uruguay was some compensation for the misery that reigned on the Parana. The populations of Paysandu and Montevideo greatly increased.