DENSITY OF SALT AND FRESH WATER.
Mr. Stevenson’s discovery that the salt water of the ocean flows up the beds of rivers in a stream quite distinct from the outflowing fresh water, was made in 1812, when investigating a question regarding salmon fishings on the Dee. It is described in the following extract from his Report:—
“The reporter observed in the course of his survey that the current of the river continued to flow towards the sea with as much apparent velocity during flood as during ebb tide, while the surface of the river rose and fell in a regular manner with the waters of the ocean. He was led from these observations to inquire more particularly into this phenomenon, and he accordingly had an apparatus prepared under his directions at Aberdeen, which, in the most satisfactory manner, showed the existence of two distinct layers or strata of water; the lower stratum consisting of salt or sea water, and the upper one of the fresh water of the river, which, from its specific gravity being less, floated on the top during the whole of flood as well as ebb tide. This apparatus consisted of a bottle or glass jar, the mouth of which measured about two and a half inches in diameter, and was carefully stopped with a wooden plug, and luted with wax; a hole about half an inch in diameter was then bored in the plug, and to this an iron peg was fitted. To prevent accident in the event of the jar touching the bottom, it was coated with flannel. The jar so prepared was fixed to a spar of timber, which was graduated to feet and inches, for the convenience of readily ascertaining the depth to which the instrument was plunged, and from which the water was brought up. A small cord was attached to the iron pin for the purpose of drawing it, at pleasure, for the admission of the water. When an experiment was made the bottle was plunged into the water; by drawing the cord at any depth within the range of the rod to which it was attached, the iron peg was lifted or drawn, and the bottle was by this means filled with water. The peg was again dropped into its place, and the apparatus raised to the surface, containing a specimen of water of the quality at the depth to which it was plunged. In this manner the reporter ascertained that the salt or tidal water of the ocean flowed up the channel of the river Dee, and also up Footdee and Torryburn, in a distinct stratum next the bottom and under the fresh water of the river, which, owing to the specific gravity being less, floated upon it, continuing perfectly fresh, and flowing in its usual course towards the sea, the only change discoverable being in its level, which was raised by the salt water forcing its way under it. The tidal water so forced up continued salt; and when the specimens from the bottom, obtained in the manner described, were compared with those taken at the surface by means of the common hydrometer of the brewer (the only instrument to which the reporter had access at the time), the lower stratum was always found to possess the greater specific gravity due to salt over fresh water.”