Life in Lakes and Rivers. T. Macan T.

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СКАЧАТЬ gently to avoid disturbance. The glass tube is detached from the frame, and the sample of bottom deposit, complete with the water immediately above it, just as it was at the bottom of the lake, can be carried into a laboratory for chemical and other tests. The person who devised this most useful apparatus was a retired engineer, Mr B. M. Jenkin, and it is worthy of note that the first experimental model, which he made largely out of meccano, operated so well that it was still in frequent use at the laboratories of the Freshwater Biological Association at Windermere ten years later.

      Mr Jenkin was set another and much more complicated problem, namely to devise an instrument capable of extracting cores from the bottom deposits in lakes, if possible to a depth of twenty or thirty feet below the mud surface. A good deal of trial showed that an ordinary open tube or pipe was useless for this purpose because it compresses and disturbs the layers of deposit too much. After some thought, Mr Jenkin hit upon the idea of a sampler which could be thrust into the deposit first and then made to carve out a core by means of a curved cutting blade working on a long pivot. The business end of the instrument, which cuts out the core, is about four feet long, and consists of a tube cut in half lengthways and covered with a metal plate except for a slit down one side. A second half-tube lies within the first, attached on an axis in such a way that it can be rotated out through the slit. When the apparatus has been driven to the required depth, the inner half-tube is rotated, its sharp leading edge passes out of the slit, and, travelling through 180°, comes up against the far side of the plate. Between the inner half-tube and the plate there is now a sample of mud isolated from its surroundings with the minimum of disturbance. The rotation of the inner half-tube is effected by a system of cogs and a driving-wheel worked by a wire from the surface. The cutter can be attached to a series of tubes so that it can be driven to the desired depth in the mud before it is operated. The force required to press the whole instrument into the bottom is provided by a series of heavy lead weights, of which the number is adjusted according to the depth at which the particular sample is required. Thus a complete core, say twenty feet in length, is obtained in a series of overlapping cores each four feet in length.

      The method of using this instrument is briefly as follows. First a pontoon with a derrick is firmly anchored over the spot from which the core is to be taken. Next a flat weight on a thin wire is lowered to the bottom to serve as a guide and as an exact measure of the depth to which the main instrument is subsequently lowered. Then the coring machine itself is lowered from the derrick on a stout wire with a pair of arms clutching the aforementioned guiding wire. The machine is allowed to sink into the deposit to the required depth, when a sample is required from near the surface; for a deep core the machine is allowed to sink as far as it will go and driven the rest of the way. Then a messenger is despatched down the guiding wire in order to release the arms, and the guiding wire is hauled up, an action which also operates the machinery for cutting out the core. It remains for the whole machine to be hauled to the surface and laid flat before the half revolution of the cutting blade is reversed and the core is exposed ready for transfer to the laboratory. It will be appreciated that the successful handling of this apparatus is no mean task; in fact it requires a team of three or four operators well trained in the particular functions which each has to perform at the right moment. With its aid, however, a large number of cores, some of them covering twenty-one vertical feet of deposit, have been collected from many parts of Windermere, and these have provided valuable information about the history of lakes since the Ice Age.

      Fig. 5 Diagrammatic cross-section of the Mackereth core-sampler (not to scale). The letters are referred to in the text. (Limnol. Oceanogr. 1958)

      Mr Jenkins’ apparatus proved excellent for use on Windermere, where the necessary pontoons could be borrowed, but not elsewhere, and accordingly Mr F. J. H. Mackereth devised a portable model (Fig. 5). The problem was to ensure stability while the core was being obtained. This he solved by basing the corer on a large cylinder resembling a dustbin (G). This sinks some way into the mud when the apparatus has been lowered, and is then forced farther in by means of a pump (P) which removes the water from its upper portion. A secure base has now been secured for the rest of the operation. The core is obtained in a long tube (B) housed inside a second tube (A), which is attached to the centre of the top of the anchoring cylinder. The second problem was how to drive the corer into the mud from a small boat that could not easily be kept exactly above the apparatus. Compressed air passing down a flexible tube (O) was the solution. It involved a piston fitting inside the outer tube and closing the top of the inner one (C). Some means of evacuating the inner tube as it moved downwards was essential, for otherwise a solid cylinder rather than a tube was being forced into the mud. This is achieved by a fine central tube (D) which holds in position a piston (F) at the mouth of the inner tube when this is retracted, passes through the upper piston and out through the top of the outer tube (L) to which it is attached. When compressed air admitted to the top of the outer tube forces the inner tube down into the mud, the air in the inner tube escapes through the fine central tube and the corer passes into the mud, causing no more compression than is due to the friction of the walls. When the inner tube is nearly fully extended, the compressed air escapes into a side tube (I), which leads it into the anchoring cylinder. This is forced out of the mud and brings the whole apparatus to the surface. Compressed air passed into the fine inner tube brings the inner tube back into the outer and, at the same time, ejects the core. This apparatus has been carried to tarns in the mountains by a helicopter and successfully used there.

       CHAPTER 4

      DIFFERENT KINDS OF LAKES

      Lakes, geologically speaking, are transitory features of the landscape. The biologist who studies a lake is likely sooner or later to find that the answer to some problem he is seeking to solve is to be sought in past history, and particularly in the way the lake was formed. Lakes and ponds have originated in many different ways and much ingenuity has been devoted to fitting them into schemes of classification. We shall not dwell on the groupings and subgroupings which have been suggested, but, in the first part of this chapter, prefer to take the lakes as they come, starting in the north of Scotland and travelling southwards.

      The Great Glen is a tear in the earth’s surface, and Loch Ness, which lies in it, provides an example of a lake associated with faulting. Loch Ness is 21³/₄ miles (35 km.) long and a little under one mile (1.6 km.) in average breadth, so it is a long and narrow lake; its greatest depth is 754 feet (230 m.) and its mean depth 433 feet (130 m.), so it is deep and steep-sided. Its mean depth is greater than that of any other British lake by quite a big margin, though there is one, Loch Morar, which is deeper at the deepest part (1017 feet = 310 metres). All these features are characteristic of tectonic lakes, that is lakes formed originally by movements of the earth’s crust. Also in this class are some of the most striking lakes of the world, such as the Dead Sea and the lakes in the Great Rift Valley of Eastern Africa. These were caused partly by lateral tearing, as is the Great Glen, but there followed a lowering of a strip of the earth’s crust so that what is now the floor of the rift valley was once level with the high land on either side.

      Most of the other lochs in the Scottish Highlands owe their present form to the work of ice when the country was covered with it during the Ice Age and so were the llyns of the Welsh mountains and the lakes of the English Lake District. Indeed, nearly all the larger stretches of water in Britain were formed by glaciers, at least in part. In some cases their basins were gouged out by glaciers flowing down mountainsides, usually in valleys cut by a stream in an earlier, more clement period. When the mass of snow and ice and rubble reached the bottom of the slope it dug into the ground and excavated a great trench. This trench became the basin of the lake when the glacier retreated with the onset of warmer conditions. The mass of material which the glacier plucked from the land it passed over was deposited in mounds or moraines at the snout of СКАЧАТЬ