Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.2): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

LEVEL OF IllVEB AND OCEAN. IW add, that the reverse takes place under opposite circumstances. Some persons attribute this change of level to the horizontal action of wind ; but I am inclined to think it occasioned chiefly by vertical pressure of the atmosphere, increased, doubtless, during strong winds by their driving force. Before a pampero, the barometer continues to fall during several days, and invariably the water then rises. The gale commences, the barometer ceases falling and begins to rise, and very soon afterwards the level of the river is found to be sink- ing. For many following days the glass remains high, but the water continues to fall, and, generally speaking, the river is low while the mercury is steady and above the average height, which I should consider to be 29'9 inches. In the Plata I never saw the barometer higher than 30'3, nor lower than 29'4<.* I will not delay here to speak of corresponding ele- vations or depressions of the ocean at other parts of the South American coast, and attempt to trace out the effects of gales in high latitudes, the space through which those winds extend, and whether they reach or affect places in a low or even middle latitude ; but leaving such inquiries for another place, take a few more glances at the vicinity of the Plata, and then con- tinue the narrative. Having already noticed the width and average shallowness of this immense river, and the lowness of its adjacent shores, I need only add, that on the northern side there is a sprinkling of hills, of a granitic structure, scattered amidst extensive plains, while on the south, or right bank, there is neither a hill, a rock, or even a stone. -|- So low is the land between Point Piedras and Cape San Antonio, that around the great bay, called Sanborombon, it is extremely difficult to say where the water ends, or the coast line begins. Each difference, of • In estimating weather, or force of wind, by the height of the mer- curial column, due regard should be had to the goodness of the instrument, as some barometers, used in ships, differ from others even tenths of an inch. + Which has not been carried there by man, or by running water.