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.
Fig. 19.
The form of hydrophore represented in [Fig. 19] is used in deep water, to which the small one just described is inapplicable. It consists of an egg-shaped vessel a, made of thick lead to give the apparatus weight, having two valves, b and c, one in the top and another in the bottom, both opening upwards; these valves (which are represented as open in the diagram) are, to insure more perfect fitting, fixed on separate spindles, which work in guides, in the same manner as in the instrument shown in [Fig. 18]. The valves, however, in this instrument are not opened by means of a cord, but by the impact of the projecting part, d, of the lower spindle on the bottom, when the hydrophore is sunk to that depth. By this means the lower valve is forced upwards, and the upper spindle (the lower extremity of which is made nearly to touch the upper extremity of the lower one, when the valves are shut) is at the same time forced up, carrying along with it the upper valve, which allows the air to escape, and the water rushing in fills the vessel. On raising the instrument from the bottom, both valves again shut by their own weight, and that of the mass of lead, d, which forms part of the lower spindle. The mode of using this hydrophore is sufficiently obvious; it is lowered by means of a rope, made fast to a ring at the top, as shown in [Fig. 19], until it strikes on the bottom, when the valves are opened in the manner described, and the vessel is filled; on raising it the valves close, and the vessel can be drawn to the surface without its contents being mixed with the superincumbent water through which it has to pass. This instrument, shown on a scale of one twentieth of full size, weighs about half a hundredweight, and has been easily used in from thirty to forty fathoms water.
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Mr. Stevenson subsequently extended his experiments on the density of salt and fresh water to several firths and tidal rivers, and gave the results in a paper communicated to the Royal Society of Edinburgh in May 1817, of which the following digest is given in Thomson’s Annals of Philosophy:[14]—
“The waters of the Thames opposite the London Dock gates were found to be perfectly fresh throughout; at Blackwall, even in spring tides, the water was found to be only slightly saline; at Woolwich the proportion of salt water increases, and so on to Gravesend. But the strata of salt and fresh water are less distinctly marked in the Thames than in any of those rivers on which Mr. Stevenson has hitherto had an opportunity of making his observations. But these inquiries are meant to be extended to most of the principal rivers in the kingdom, when an account of the whole will be given.
“From the series of observations made at and below London Bridge, compared with the river as far up as Kew and Oxford, Mr. Stevenson is of opinion that the waters of the Thames seldom change, but are probably carried up and down with the turn of the alternate tides for an indefinite period, which, he is of opinion, may be one, if not the principal cause of what is termed the extreme softness of the waters of the Thames.
“Mr. Stevenson has made similar experiments on the rivers Forth and Tay, and at Loch Eil, where the Caledonian Canal joins the Western Sea. The aperture at Corran Ferry, for the tidal waters of that Loch, being small compared with the surface of Loch Eil, which forms the drainage of a great extent of country, it occurred to him that the waters of the surface must have less saline particles than the waters of the bottom. He accordingly lifted water from the surface at the anchorage off Fort William, and found it to be 1008·2; at the depth of 9 fathoms 1025·5; at the depth of 30 fathoms, in the central parts of the Loch, it was 1027·2; being the specific gravity of sea water.”
The hydrophore, which was originally devised and used by Mr. Stevenson, in 1812, at Aberdeen, has now reached its height of excellence of construction and scientific importance in the famous ‘Challenger’ Expedition.
CHAPTER XVII.
EXTRACTS FROM EARLY REPORTS.
Wide range of subjects on which Mr. Stevenson gave advice—Reports on ruins of Aberbrothock Abbey—St. Magnus Cathedral, and Earl’s Palace, Kirkwall—St. Andrews Cathedral—Montrose Church Spire—Melville Monument, Edinburgh—Lipping of joints of masonry with cement—Provision for flood waters in bridges—Hydraulic mortar—Protection of foreshores—Cycloidal sea wall—Checking drift sand—Night signal lamps—Cause of heavy seas in Irish Channel—Sea routes across Irish Channel—Build of ships—Prospective increase of population—Tidal scour—Unscrewing of bolts by the waves—Cement Rubble cofferdams—Buoyage system—Observations on fog signals—Regulations for steam vessels—Notes on shipwrecks.
Judging from Smeaton’s well known “Reports,” to which all have access, we may conclude that the “professional advice” given by early Engineers was very generally accompanied by a fuller and less reserved discussion of opinion than is to be met with in the brief and technical Engineering reports of the present day. In early times, Engineers did not hesitate to express themselves freely on physics, æsthetics, or commerce, provided their views had a collateral bearing on the subject under discussion, and this often added to the interest of their reports.
These early Engineers were also consulted on a much wider range of subjects than the Engineers of modern times. We know that the larger requirements of modern Engineering demand that its practice should be classified under distinct branches, such as harbours, navigations, water works, gas works, lighthouses, or railways, not to mention electrical and sanitary engineering, and other branches of modern growth, all of which cannot possibly be advantageously practised by any one member of the profession; for no one mind can grasp the theoretical knowledge, and no one life can compass the practical experience, to enable a man to attain eminence in all these departments of modern Engineering.
A biographical sketch of Mr. Stevenson’s professional life would, it seems to me, be incomplete if it did not convey to the reader some notion, however general, of the wide range of subjects brought under his notice, in these early times, and of his comprehensive and suggestive mode of treating every case on which he was professionally consulted. This object would be only imperfectly attained were I to restrict my reference to his reports to the examples given in the preceding chapters; for I have found in his numerous writings casual notices of a miscellaneous and fragmentary character, many of which seem to me to be interesting to the profession, and worthy of preservation, and I propose, in this chapter, to give a few of these extracts, without order of subject or date; and I think they will justify my remark as to the great variety and fulness of treatment to be found in the reports of early Engineers.
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It appears, for example, that Mr. Stevenson was often called to advise on matters which were more related to architecture than engineering. Of this nature was his tour of inspection to the jails of England, in company with Sir William Rae, the Sheriff of Edinburgh, in 1813, referred to in a former chapter.