Dr Fleming directed the attention of the Society, in the first instance, to the characteristic features of sea and river proper; and then proceeded to consider the peculiarities of that common space, alternately sea and river, to which he restricted the term estuary. He then considered the nature of the space between high and low water, and pointed out the mean level, or mid-tide mark, as the only constant and universally applicable boundary plane. The influence of the tidal wave in reversing the current, checking the velocity, and increasing the depth of the river, was next brought under notice, and an experiment exhibited, illustrating the conservation of force, which causes the waters at the head of an estuary, and the connected river, in certain circumstances, to attain a higher level than the high-water mark of the neighbouring sea-shore.