“This coast, with its sandbanks in the offing, its submarine forest, and the evidence of living witnesses as to the encroachment of the sea upon the firm ground, is altogether highly interesting to the geological and scientific inquirer. The remains of forests in the bed of the ocean occur in several parts of the British coast, particularly off Lincoln, on the banks of the Tay near Flisk, at Skail in the mainland of Orkney, and in other places noticed in the Transactions of this Society, and are strong proofs of the encroachments of the sea upon the land. However difficult, therefore, it may be to reconcile the varied appearances in nature regarding the sea having at one time occupied a higher level than at present, yet its encroachment as a general and almost universal principle seems to be beyond doubt in the present day.
“Since I had last the honour of addressing the Society on this subject, opportunities have been afforded me of making many additional observations on the British shores, and of personally extending these to almost every port on the Continent between the Texel and the Garonne. I have also, through the obliging communications of friends, been enabled to extend my inquiries to other quarters of the globe, and I am now prepared to state that, with a few comparatively trifling exceptions, the sea appears to be universally gaining upon the land, tending to confirm the theory that débris arising from the general degradation of the land, being deposited in the bed of the minor seas, is the cause of their present tendency to overflow their banks.”
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.”
THE HYDROPHORE.
The instrument Mr. Stevenson then invented and used was that to which the term hydrophore has been applied. [Figs. 18] and [19] show two forms of hydrophores made under his directions.
Fig. 18.
[Fig. 18] is used for procuring specimens of water from moderate depths, drawn on a scale of one-tenth of the full size. It consists of a tight tin cylinder, a, having a conical valve in its top, b, which is represented in the diagram as being raised for the admission of water. The valve is fixed dead, or immoveable, on a rod working in guides, the one resting between two uprights of brass above the cylinder, and the other in its interior, as shown in faintly dotted lines. The valve rod is by this means caused to move in a truly vertical line, and the valve attached to it consequently fills or closes the hole in the top of the cylinder with greater accuracy than if its motion was undirected. A graduated pole or rod of iron, c, which in the diagram is shown broken off, is attached to the instrument, its end being inserted into the small tin cylinder at the side of the large water cylinder, and there fixed by the clamp screws shown in the diagram; the bottom of the water cylinder may be loaded with lead to any extent required, for the purpose of causing the apparatus to sink; but this, when an iron rod is used for lowering it, is hardly necessary. The spindle carrying the valve has an eye in its upper extremity, to which a cord is attached for the purpose of opening the valve when the water is to be admitted, and on releasing the cord, it again closes by its own weight. When the hydrophore is to be used, it is lowered to the required depth by the pole which is fixed to its side, or, if the depth be greater than the range of the pole, it is loaded with weights, and let down by means of a rope so attached as to keep it in a vertical position. When the apparatus has been lowered as far as is required, the small cord is pulled, and the vessel is immediately filled with the water which is to be found at that depth. The cord being then thrown slack, the valve descends and closes the opening, and the instrument is slowly raised to the surface by means of the rod or rope, as the case may be, care being taken to preserve it in a vertical position.