The Survey of British Lakes

Scottish Geographical Magazine

ISSN: 0036-9225 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/rsgj19

The Survey of British lakes

James Chumley

To cite this article: James Chumley (1902) The Survey of British lakes, Scottish Geographical Magazine, 18:8, 413-422, DOI: 10.1080/00369220208733376

To link to this article: http://dx.doi.org/10.1080/00369220208733376

Published online: 27 Feb 2008.

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Download by: [University of California, San Diego] Date: 28 June 2016, At: 16:53 A NOTE UPON THE METHODS OF BOTANICAL GEOGRAPHY. 413

(6) J. F. Schouv, Grundziiye einer allgemeinen Pflanzengeographie. Berlin, 1823. (7) .Dr. Eugen Warming, Lehrluch der Ohologischen Pflanzengeographie. Copenhagen, 1896. (8) H. 0. Watson, The Geographical Distribution of British Plants. Third edition, 1843. (9) Moritz Willkomm, Grundzitge d. Pflanzenverhreitung auf der iberischen Halbinsel. Leipzig, 1896.

THE SUEVEY OF BRITISH LAKES.

By JAMES CHUMLEY, of the Challenger Expedition Office, Edinburgh. As indicated in a previous number of the Scottish Geographical Magazine, the systematic work of the Lake Survey was started early in April last, and has since been carried on uninterruptedly under the personal super- intendence of Sir John Murray, K.C.B. From Eannoch Lodge, where Sir John Murray and his family took up their residence towards the end of March, excursions were made to the lakes in the neighbourhood, and we are enabled to report that most satisfactory progress has been made. At the time of writing twenty-two lakes had been completely surveyed, viz., Lochs Earn, Tay, Tummel, Eannoch, Lochan Sr6n Smeur, Eigheach, Laidon, Ba, Lochan na h-Achlaise, Lochan na Stainge, Buidhe, Lyon, Iubhair, and Dochart, in the basin of the Tay; Lochs Laggan, Lochan na h-Earba, Treig, Ossian, and Ghuilbinn, in the basin of the Spean; Loch Tulla flowing into Loch Awe, Loch Arkaig flowing into Loch Lochy, and Loch Crunachan flowing into the Spey. Lochs Morar, Shiel, and Eilt have since been surveyed, and the staff are now engaged on the surveys of Loch Maree and neighbouring lochs. In carrying out this work Sir John Murray has had the continuous assistance of Mr. T. N. Johnston, M.B., CM., and Mr. James Parsons, B.Sc, and for several weeks he was assisted by Mr. E. M. Clark, B.Sc. In the surveys of Lochs Ossian and Ghuilbinn he had also the assistance of Dr. J. Sutherland Black, M.A.; in the survey of Loch Laggan he was assisted by Sir John Jackson, and in the surveys of Lochs Tulla and Tummel he was assisted by Mr. D. C. M'Intosh, M.A. Mr. James Murray joined the survey in the end of April, and Mr. J. Hewitt and Mr. T. E. H. Garrett, of Jesus College, Cambridge, in the middle of June,

Downloaded by [University of California, San Diego] at 16:53 28 June 2016 while Mr. D. J. Scourfield has been studying the Entomostraca of the lakes during the past few weeks. In order to facilitate the progress of the survey in every way, Sir John Murray has added to his resources a motor car, specially built for him by the Albion Motor Car Co., Glasgow, and an experienced driver, Mr. Thomas Hardie, was engaged some weeks . ago; the surveying staff will thus be enabled to proceed from one lake to another with the least possible delay. The circumstances which led up to the present work of the Lake . Survey are well known to the readers of the Scottish Geographical Magazine. A few years ago Sir John Murray and the late Mr. Fred. P. Pullar 414 SCOTTISH GEOGRAPHICAL MAGAZINE.

undertook jointly to carry out a systematic bathymetrical and physical survey of the fresh-water lochs of Scotland, and they commenced -with those lying in the basin of the Forth. The first part of their report, dealing with Lochs Katrine, Arklet, Achray, Vennacher, Drunkie, Lubnaig, Voil, and Doine, was published in vol. xvi. p. 193 ; the second part, dealing with Lochs Chon, Ard, Menteith, and Leven, in vol.- xvii. p. 113; and the third part, dealing with Lochs Ericht and Garry in the basin of the Tay, in vol. xvii. p. 169. Just after this third part had been prepared for publication the work was unfortunately brought to a standstill by the accidental death of the late Mr. F. P. Pullar, who sacrificed his life in a gallant attempt to save others (see obituary notice, vol. xvii. p. 148). As a memorial to this talented young author, his father, Mr. Laurence Pullar, and his collaborator, Sir John Murray, decided that the work in which he took such interest should be completed at their joint expense. Only four of the lakes recently surveyed by the Lake Survey had, as far as we are aware, been previously systematically sounded, viz., Lochs Tay, Earn, Eannoch, and Tummel, which were surveyed by Mr. Grant Wilson of the Geological Survey of Scotland, in 1888 (see vol. iv. p. 251); Loch Earn was again sounded by Sir John Murray and the late Mr. Pullar in 1900, though the results were not published. Mr. "Wilson's soundings agree, on the whole, with those of the Lake Survey, the differences observed being probably due to a difference in the level of the surface waters at the time the surveys were made. For instance, the maximum depth observed by the Lake Survey in Loch Tay was 508 feet, whereas Mr. Wilson records a depth of 85 fathoms, or 510 feet; in Loch Earn the maximum depth observed by the Lake Survey was 287 feet, while Mr. Wilson records 48 fathoms, or 288 feet; in Loch Tummel the maximum depth observed by the Lake Survey was 128 feet, while Mr. "Wilson records 20-f fathoms, or 124 feet. The methods and apparatus employed by the Lake Survey are the same as those used by Sir John Murray and the late Mr. F. P. Pullar, which were fully described in the first part of their report, and need not be repeated here, but special attention is now paid to the deter- mination of the level of the surface of each lake above the level of the sea at the time the survey is being carried out, so that the true relation between the contour-lines above and below the water-level may.be accurately known. Before sounding out a lake a horizontal line is painted, if possible on some convenient rock-surface,

Downloaded by [University of California, San Diego] at 16:53 28 June 2016 one, two, or three feet above the surface of the water, thus: L j —S the figure above the horizontal line indicating the height in feet of that line above the level of the water. The height above sea-level is then care- fully determined by levelling with a dumpy level to the nearest Ordnance . Survey bench-mark. Each day the level of the water is compared with the Lake Survey level-mark, and any change noted, for sometimes the water of the lake may rise or fall while the survey is in progress. In Loch Treig, for instance, the water rose to the extent of a foot during THE SURVEY OF BRITISH LAKES. 415

the time devoted to the survey, and the earlier soundings had to be corrected accordingly. An interesting point in connection with the work of the Lake Survey is the record, probably for the first time in Scotland, of " seiches " in some of the larger lakes. A short note on one of these examples of rhythmic pulsation in the waters of a lake has been contributed by Mr. Parsons in a recent issue of Nature (see Nature, No. 1702, June 12, 1902, p. 162). Professor Forel believes that these pulsations are due to variations in barometric pressure, while they have been attributed by others to wind-velocity or to earth-movements. No continuous observa- tions on this novel phenomenon have yet been made by the Lake Survey, but in the course of a few weeks a " limnograph," which has been con- structed in Geneva under the supervision of Dr. Ed. Sarasin, will be set up on the margin of one of the lakes, for the purpose of giving a continuous record of the variations in the level of the surface waters of the lake. The thanks of the staff of the Lake Survey are due to Sir Eobert Menzies, Sir John Stirling Maxwell, Lochiel of Achnacarry Castle, Sir John Eamsden, and other Highland proprietors, for valuable assistance rendered during the recent surveys. Some of the maps of the lakes recently surveyed are now sufficiently far advanced to enable us to offer a few preliminary remarks regarding their depth and conformation, which may be interesting to readers of the Scottish Geographical Magazine. The large figures giving the cubic mass of water in each lake are difficult to grasp, arid, with the view of rendering them more intelligible, we have in each case indicated the dimensions of an imaginary cubical reservoir, into which the waters from each loch may be supposed to be run off.

LOCH TAY. Loch Tay may fairly claim to be the largest lake in the basin of the Tay, though Loch Ericht is quite comparable as regards length, maximum breadth, and maximum depth, but the average breadth of Loch Tay is greater than that of Loch Ericht. Its superficial area is therefore larger, its mean depth is greater, and it holds a larger cubic mass of water. Loch Tay is over fourteen and a half miles in length, over one mile in maximum width, the mean breadth being nearly three-quarters of a mile, and its waters cover an area of over ten square miles ; it drains an area of 187^ square miles. Its maximum depth is 508 feet, its mean depth is 199 feet, and the bulk of water contained in the lake is estimated at

Downloaded by [University of California, San Diego] at 16:53 28 June 2016 about 56,550,000,000 cubic feet. That is to say, if the water in the lake could be placed in a cubical receptacle, it would require to be 1279 yards (or nearly three-quarters of a mile) in length, breadth, and height. Loch Tay forms a simple basin, the deepest portion (over 500 feet in depth) occupying a central position, about nine miles from the south-west end and five and a half miles from the north-east end, lying in the centre of the depression with depths exceeding 400 feet, which is over three and a half miles in length. About three miles from the south-western end there is a small depression, over half a mile in length, with depths exceeding 300 feet, separated by an interval of about one mile from the main 300-feet depression, which is seven and a half 416 SCOTTISH GEOGRAPHICAL MAGAZINE.

miles in length. The 200-feet depression extends from about two miles above the south-west end to within one and a half miles from the north- east end, and is eleven miles in length. The 100-feet depression approaches to within a quarter of a mile from the south-west end and one-third of a mile from the north-east end. Of the entire lake-floor, 30 per cent, is covered by less than 100 feet of water, 24 per cent, is covered by water, with depths between 100 and 200 feet, 21 per cent, by,water with depths between 200 and 300 feet, and 25 per.cent, by water exceeding 300 feet in depth.

LOCH EARN. Loch Earn is nearly six and a half miles in length, over three-quarters of a mile in maximum breadth, and over half a mile in mean breadth ; its superficial area is nearly four square miles, while it drains an area of nearly 55 square miles. The maximum depth is 287 feet, and the mean depth 138 feet, the volume of water being estimated at 14,421,000,000 cubic feet, sufficient to fill a cubical reservoir 811 yards (or nearly half a mile) in length, breadth, and depth. Loch Earn is'of simple conformation, the deepest part being found approximately in the centre of the loch, though nearer the western than the eastern end. The depression, with depths exceeding 250 feet, is nearly two miles in length, extending from one and a half miles from the west end, to three miles from the east end, the maximum depth of 287 feet having been found about two and three-quarter miles from the west end, and three and three-quarter miles from the east end. The 200-feet depression is four and a half miles in length, stretching from three-quarters of a mile from the west end to a mile and a quarter from the east end. The 100-feet depression approaches to within less than a quarter of a mile from the west end, and less than half a mile from the east end, and is five and three-quarter miles in length. Of the entire lake-floor, 39 per cent, is covered by less than 100 feet of water, 31 per cent, is covered by water with depths between 100 and 200 feet, and 30 per cent, by water exceeding 200 feet in depth.

LOCH TUMMEL. Loch Tummel is two and three-quarter miles in length, the maximum breadth being half a mile, and the mean breadth over one-third of a mile, its superficial area being slightly less than one square mile; it drains an area of 62J square miles. The maximum depth is 128

Downloaded by [University of California, San Diego] at 16:53 28 June 2016 feet, the mean depth is 48 feet, and the bulk of water is estimated at about 1,317,000,000 cubic feet, i.e. a cubical reservoir 365 yards (or over one-fifth of a mile) in length, breadth, and depth would be required to contain the water in the loch. The floor of Loch Tummel is irregular, falling into three deep basins, separated by two shallow ridges, as already observed by Mr. Wilson. The deepest basin is situated near the west end of the loch, the maximum depth of 128 feet occupying a central position in this basin, but a short distarice to the south a sounding of 74 feet is recorded in close proximity to a sounding of 127 feet. The central basin, separated from the western basin by a ridge with a maximum depth of 52 feet nn it, is nearly half a mile in length, and has a THE SURVEY OF BRITISH LAKES. 417

maximum depth of 119 feet. The eastern basin has a maximum depth of 99 feet, and is separated from the central basin by a rise with only 32 feet on it. The 50-feet depression is a continuous area stretching from within a quarter of a mile from the west end to less than 100 yards from the east end of the loch. Of the entire lake-floor, 37 per cent, is covered by less than 25 feet of water, 19 per cent, by water between 25 and 50 feet in depth, 34 per cent, by water between 50 and 100 feet in depth, and 10 per cent, by water exceeding 100 feet in depth. LOCH EIGHEACII. Loch Eigheach is over three-quarters of a mile in length, the maximum width being less than a quarter of a mile, and the mean width one-tenth of a mile. It covers an area of less than one-tenth of a square mile, and receives the drainage from an area of nearly 14 square miles. The maximum depth is 28 feet, and the mean depth 6 feet, the bulk of water being estimated at 16,000,000 cubic feet, i.e. it would require a cubical reservoir 84 yards in length, breadth, and depth to hold the water in the loch. Loch Eigheach may be looked upon as three expansions of the River Gaoire, the south-western expansion being the shallowest (maximum 3' feet), the north-western expansion a little deeper (maximum 7 feet), and the eastern expansion the deepest, the maximum depth of 28 feet having been observed approximately in the centre, but nearer the east end than the west end. of this portion of the loch. Of the entire lake-floor, 91 per cent, is covered by less than 10 feet of water, and less than 2 per cent, exceeds 20 feet in depth. LOCHAN-SR6N SMEUR. This loch is over half a mile in length, nearly a quarter of a mile in maximum breadth, and one-seventh of a mile in mean breadth. It covers an area of less than one-tenth of a square mile, and drains an area of nearly two square miles. The maximum depth is 33 feet, and the mean depth over 10 feet, the cubic contents being estimated at 23,000,000 cubic feet, i.e. it would require a cubical reservoir 95 yards in length, breadth, and depth to contain the water of the loch. The loch is of simple conformation, but the deep water occurs in the eastern half, the maximum depth of 33 feet having been observed about one- eighth of a mile from the east end, while the western half is compara- tively shallow. Of the entire lake-floor, 61 per cent, is covered by less than 10 feet of water, 27 per cent, by water between 10 and 20 feet in Downloaded by [University of California, San Diego] at 16:53 28 June 2016 •depth, and 12 per cent, by water exceeding 20 feet in depth. LOCH LAIDON. This looh is about five and one-third miles in length from north- east to south-west, but it sends out an arm towards the west, which is over one and a half miles in length, and a line following the axis of the loch from the north-east end to the extremity of the western arm would be over six miles in length. Its maximum breadth is nearly three- quarters of a mile, the mean breadth being about one-third of a mile, and its waters cover an area of over one and three-quarter square 418 SCOTTISH GEOGRAPHICAL MAGAZINE.

miles; it drains an area of 30J square miles. The maximum deptb is 128 feet, and the mean depth 35 feet, the cubic mass of water being estimated at about 1,762,000,000 cubic feet, i.e. it would require a cubical reservoir 403 yards (or nearly a quarter of a mile) in length, breadth,, and depth to hold the water of the loch. Loch Laidon is of compara- tively simple form, though with minor undulations of the lake-floor, the deepest water occupying the centre of the loch, where there is a depression three-quarters of a mile in length and over 100 feet in depth, the maximum depth of 128 feet having been observed about two and three- quarter miles from the south-west end and two and a half miles from the north-east end. Separated from this main 100-feet depression by shallower water, there is a sounding of 104 feet a short distance to the south-west, and half a mile further south there is an isolated sounding of 100 feet; there is also an isolated sounding of 100 feet a quarter of a mile to the north-east of the main depression. The principal 50-feet depression extends from less than a mile from the south-west end to less than one and a half miles from the north-east end, and is nearly three miles in length. Separated from this larger depression by an interval of a quarter of a mile is a smaller one, about one-third of a mile in length, situated in the north-eastern part of the loch, and nearly midway between them is an isolated sounding of 50 feet. The western arm of Loch Laidon is shallow and filled with rocks and boulders, the greatest depth observed being 17 feet in three different places. Of the entire lake-floor, 53 per cent, is covered by less than 25 feet of water, 21 per cent, is covered by water between 25 and 50 feet in depth, 22 per cent, by water between 50 and 100 feet in depth, and 4 per cent, by water exceeding 100 feet in depth.

Loch Ba, Lochan na h-Achlaise, Lochan na Stainge, and Loch Buidhe- are shallow lakes, lying among glacial moraines in the Black Forest, covering together an area of over one and one-third square miles, and draining an area of 17h square miles. They all flow into Loch Laidon,. and form the true head-waters of the River Tay.

LOCH BA. Loch Ba is over two miles in length, with a maximum breadth of over one mile and a mean breadth of less than half a mile, its superficial area being rather less than one square mile. The maximum depth is 30 feet, and the mean depth 8 feet, the cubic mass of water being

Downloaded by [University of California, San Diego] at 16:53 28 June 2016 estimated at 206,000,000 cubic feet, i.e. it would require a cubical reservoir 197 yards (or one-ninth of a mile) in length, breadth, and depth to contain the water in the loch. It is extremely irregular in form, studded with larger and smaller islands, and filled with rocks and boulders; the bottom contours are in consequence also very irregular. The deepest water occurs in the northern portion between the islands Eilean Molach and Eilean na h-lolaire, where there is a small depression less than a quarter of a mile in length and over 20 feet in depth, the maximum depth of 30 feet having been observed about one-sixth of a, mile to the north of the northern point of Eilean Molach. An isolated THE SURVEY OF BRITISH LAKES. 419

sounding of 20 feet was taken close to the western shore of the southern portion of Eilean Molach. There are three irregular depressions with depths exceeding 10 feet, the central one enclosing the 20-feet depression and extending on both sides of Eilean Molach and to the west and north of Eilean na h-Iolaire, the southern one occupying the wide south-eastern portion of the loch, and the third situated in the north- eastern extension of the loch. Of the entire lake-floor, no less than 71 per cent, is covered by less than 10 feet of water, and only li per cent, exceeds 20 feet in depth.

LOCHAN NA H-ACHLAISE. This loch is over four-fifths of a mile in length, the maximum width being three-quarters of a mile, and the mean width over one- third of a mile. The maximum depth is 28 feet, the mean depth itj feet, and the bulk of water about 76,000,000 cubic feet, i.e. it would require a cubical reservoir 141 yards (or less than one-twelfth of a mile) in length, breadth, and depth to contain the water in the loch. This loch is also irregular in outline and studded with islands. The deep water occurs in the eastern larger half of the loch, the western portion being all less than 10 feet in depth, except for one isolated sounding of 11 feet. The 10-feet depression approaches close to the southern shore and to the shore of the large island in the north-eastern part of the loch, and is over half a» mile in length. It encloses two 20-feet depressions, the larger one irregular in shape and separated from the smaller one by a sounding of 18 feet. The maximum depth of 28 feet was observed about one-eighth of a mile from the eastern shore. Of the entire lake-floor, 63 per cent, is covered by less than 10 feet of water, and only 9 per cent, exceeds 20 feet in depth. LOCHAN NA STAINGE. This loch is over half a mile in length, the maximum breadth being two-fifths of a mile and the mean breadth over one-seventh of a mile. The maximum depth is 14 feet, and the mean depth 5 feet, the cubic contents being about 11,000,000 cubic feet, i.e. it would require a cubical reservoir 74 yards in length, breadth, and depth to contain the water in the loch. The loch is shallow (all under 8 feet in depth), except for two soundings of 12 feet and 14 feet near the central constriction in the outline of the loch.

Downloaded by [University of California, San Diego] at 16:53 28 June 2016 LOCH BUIDHE. Loch Buidhe is about one-third of a mile in length, nearly a quarter of a mile in maximum width, and nearly one-sixth of a mile in mean width. It is very shallow, the deepest water found by the Lake Survey being 3 feet, and the bulk of water is estimated at 2,000,000 • cubic feet, i.e. it would require a cubical reservoir 42 yards in length, breadth, and depth to contain the water in the loch.

LOCH LYON. Loch Lyon is about one and three-quarter miles in length, the 420 SCOTTISH GEOGRAPHICAL MAGAZINE.

maximum breadth being a quarter of a mile, and the mean breadth one- fifth of a mile, the superficial area being over one-third of a square mile ; it drains an area of 10h square miles. The maximum depth is 100 feet, the mean depth 45 feet, and the cubic mass of water about 460,000,000 cubic feet, i.e. it would require a cubical reservoir 258 yards (or one- seventh of a mile) in length, breadth, and depth to contain the water in the lake. The loch forms a simple basin, the deep water approaching much closer to the south-western than to the north-eastern end. The maximum depth of 100 feet was observed in the centre of the loch; the 75-feet depression is over one mile, and the 50-feet depression over one and a quarter miles, in length. Of the entire lake-floor, 39 per cent, is covered by less than 25 feet of water, 15 per cent, by water between 25 and 50 feet in depth, 24 per cent, by water between 50 and 75 feet in depth, and 22 per cent, by water over 75 feet in depth.

LOCH IUBHAIR. Lochs Iubhair and Dochart may almost be looked upon as one lake, Loch Iubhair receiving the outflow from the smaller loch by a river less than half a mile in length; together their waters cover an area of over a quarter of a square mile, and they drain an area of 44J square miles. Loch Iubhair is over one and one-third miles in length, one- third of a mile in maximum breadth, and nearly one-sixth of a mile in mean breadth. The maximum depth is 65 feet, and the mean depth 25 feet, the bulk of water being estimated at 147,000,000 cubic feet, i.e. it would require a cubical reservoir 176 yards (or one-tenth of a mile) in length, breadth, and depth to hold the water in the loch. It is peculiar in form, somewhat like the italic letter /, with a constriction in the central portion. The deepest water occurs in the north-eastern part, where the Joch is widest, the maximum depth of 65 feet having been observed approximately in the centre of this portion of the loch. The bottom rises on approaching the central constriction, the greatest depth in which is 36 feet, and falls again as the loch widens out towards the south-west, the maximum depth in this part of the loch being 49 feet. The 25-feet depression is over a mile in length, approaching close to the northern shore, but distant about a quarter of a mile from the southern end of the loch. • Of the entire lake-floor, 53 per cent, is covered by less than 25 feet of water, and only 3 per cent, exceeds 50 feet in depth.

LOCH DOCHART.

Downloaded by [University of California, San Diego] at 16:53 28 June 2016 Loch Dochart is over half a mile in length, the maximum breadth being one-sixth of a mile, and the mean width over one-tenth of a mile. It is quite shallow, the maximum depth being 8 feet, and the mass of water is estimated at 8,000,000 cubic feet, i.e. it would require a cubical reservoir 67 yards in length, breadth, and depth to contain the water of the loch. The deepest water occurs near the south-west end.

LOCH TREIG. Many speculations have been indulged in regarding the depth of Loch Treig, some people maintaining that its depth exceeded 1000 feet, THE SURVEY OF BRITISH LAKES. 421

others declaring that it had no bottom. Loch Treig is also specially interesting because of its connection with the origin of the parallel roads of Glen Roy. The ice which came down the valley now occupied by the waters of the loch into the valley of the Spean is supposed to have dammed up the mouth of Glen Eoy, and to have been the cause of the parallel roads. Loch Treig, which trends in a NNE. and SSW. direction, is over five miles in length, and three-quarters of a mile in maximum breadth towards the southern end, whence it narrows gradually, though irregularly, towards the northern extremity, its mean breadth being nearly half a mile. Its waters cover an area of nearly two and a half square miles, and it drains an area of nearly 43 square miles. The cubic mass of water is estimated at 13,907,000,000 cubic feet, i.e. it would require a cubical reservoir 802 yards (or nearly half a mile) in length, breadth, and depth to contain the water of the loch. It forms a simple basin, the deepest water being found in the centre of the loch, where there is a long narrow depression nearly two miles in length, in which the depth exceeds 400 feet, the maximum depth observed by the Lake Survey being 43G feet, towards the southern end of this depression and nearly two miles from the southern end of the loch. The 300-feet depression is nearly three miles in length, stretching from within a mile and a half from the northern end of the loch to less than a mile from the southern end. The 200-feet depression is nearly four and a half miles in length, approaching to within less than half a mile of the northern end and a quarter of a mile of the southern end. Of the entire lake-floor, 27 per cent, is covered by less than 100 feet of water, 48 per cent, by water between 100 and 300 feet in depth, and 25 per cent, by water exceeding 300 feet in depth. These figures show that Loch Treig is a very deep lake considering its superficial area, the mean depth being computed at about 207 feet. This exceeds the mean depth of Loch Tay, Loch Ericht, and Loch Katrine, which have all a greater maximum depth than Loch Treig.

LOCH OSSIAN. Loch Ossian is three and one-fifth miles in length, and about half a mile in maximum breadth, the mean breadth being one-third of a mile. Its waters cover an area of over one square mile, and it receives the drainage from an area of 10J square miles. The maximum depth is 132 feet, and the mean depth 42f feet, the volume of water being estimated at 1,224,000,000 cubic feet, i.e. it would require a cubical reservoir 357

Downloaded by [University of California, San Diego] at 16:53 28 June 2016 yards (or over one-fifth of a mile) in length, breadth, and depth to contain the water in the lake. The floor of Loch Ossian is rather irregular, the deeper water being contained in the eastern half of the loch. About one and a quarter miles from the west end there is a rise in the bottom covered by less than 25 feet .of water, the shoalest water on which is 11 feet, and the contour-lines in the vicinity of this rise are very irregular. The depression with depths exceeding 100 feet is half a mile in length, and is situated about three-quarters of a mile from the east end, and nearly two miles from the west end. About a quarter of a mile from . the east end there is a depression with a maximum depth of 78 feet, 422 SCOTTISH GEOGRAPHICAL MAGAZINE.

separated from the main depression by a ridge on which the maximum depth is 41 feet. The main 5O-feet depression is two miles in length, extending from about one-third of a mile from the east end to less than one mile from the west end. Of the entire lake-floor, 32 per cent, is covered by less than 25 feet of water, 36 per cent, by water with depths between 25 and 50 feet, 27 per cent, between 50 and 100 feet, and 5 per cent, by water exceeding 100 feet in depth.

LOCH TULLA. Loch Tulla is two and a half miles in length, the maximum width being rather less than a mile, the mean width nearly half a mile, and it covers an area of over one square mile; it receives the drainage from an area of over 49J square miles. Its maximum depth is 84 feet, and its mean depth 38 feet, the cubic mass of water being estimated at 1,167,000,000 cubic feet, i.e. it would require a cubical reservoir 351 yards (or one-fifth of a mile) in length, breadth, and depth to contain the water in the lake. It is of simple conformation, except that a small depression with depths exceeding 50 feet occurs to the south- west of the island Eilean Stalchairichd, separated from the larger deep portion of the loch lying to the north-east of that island, where there is a depression three-quarters of a mile in length, and over 75 feet in depth, the maximum depth of 84 feet having been observed near the southern end of this depression, and a quarter of a mile from the island. Of the entire lake-floor, 37 per cent, is covered by less than 25 feet of water, 31 per cent, by water between 25 and 50 feet in depth, and 32 per cent, by water exceeding 50 feet in depth.

The number of soundings taken in a lake varies according to its size and conformation, relatively fewer soundings being necessary, as a rule, in a large lake than in a small one. For instance, in Loch Ba, with a superficial area of less than one square mile, 315 soundings were taken, and in Loch Tulla, with a superficial area of rather more than one square mile, 318 soundings were taken, whereas in Loch Earn, with a superficial area of nearly four square miles, 499 soundings were taken, and in Loch Tay, with a superficial area exceeding ten square miles, 924 soundings were taken. This gives an average of 91 soundings per square mile in Loch Tay, 128 soundings per square mile in Loch Earn, 289 soundings per square mile in Loch Tulla, and 350 soundings per square mile in Loch Ba. In the sixteen lakes dealt with in the preceding paragraphs,

Downloaded by [University of California, San Diego] at 16:53 28 June 2016 which cover an area of over twenty-three and a half square miles, nearly 4000 soundings were taken, or an average of 165-soundings per square mile. In making the planimeter measurements and calculations regarding these lakes, the writer of this article has been assisted by Mr. D. C. • M'lntosh, M.A., and Mr. Robert Dykes.