A Bathymetrical Survey of the Lakes of New Zealand Author(s): Keith Lucas Source: The Geographical Journal, Vol. 23, No. 5 (May, 1904), pp. 645-660 Published by: geographicalj Stable URL: http://www.jstor.org/stable/1776009 Accessed: 21-06-2016 04:12 UTC Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://about.jstor.org/terms JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The Royal Geographical Society (with the Institute of British Geographers), Wiley are collaborating with JSTOR to digitize, preserve and extend access to The Geographical Journal This content downloaded from 137.99.31.134 on Tue, 21 Jun 2016 04:12:15 UTC All use subject to http://about.jstor.org/terms A BATHYMETRICAL SURVEY OF THE LAKES OF NEW ZEALAND. 645 will probably have to be by taking numerous measurements with record- ing instruments (of which several have been invented), which will enable the complex wave disturbance to be analyzed into its component simple waves. I trust that the Admiralty may see their way to have this done.* I have to record my indebtedness first to the late Sir G. G. Stokes, who kindly explained to me points of mathematical theory; secondly, to Captain D. Wilson-Barker, who allowed me to examine some of his original notes of observations at sea; and, lastly, to the Editors of IKnowledge, who have lent the blocks for the figures, which were originally drawn by me for a series of articles in that journal (January to July, 1901). It may not be amiss to mention that the series of articles was drawn up in connection with the proposal to observe waves from the Discovery, and that they, together with additional notes, were sup- plied by request to Captain Scott, R.N., before sailing, these together forming almost a rough draft for much of the present paper. The chapter on " Deep-sea Waves " in Sir W. White's 'Naval Architecture' (which is in the Library of the Discovery) contain3 the best general review of the subject with which I am acquainted, and my treatment takes the matter up from the point where Sir W. White has left it. Some practical directions for estimating the heights of waves are also given in the 'Antarctic Manual,' having been drawn up by Captain Wilson- Barker for the use of the Discovery. A BATHYMETRICAL SURVEY OF THE LAKES OF NEW ZEALAND.t By KEITH LUCAS. THE expedition with whose results this paper deals was, in its original intention, biological. It had as its object the collection of the faura from a typical series of the lakes of New Zealand in the hope that some light might thereby be thrown upon the distribution of animals within that country. It was, however, obvious from the first that no systematic collection could be made until some knowledge had been obtained of the forms of the lake-basins. At the time when the work was first proposed, in the early part of 1902, there was only one of the New Zealand lakes, Lake Taupo, in which a survey of any degree of completeness had been made. In the remainder there had been made, in a few cases, a small number of soundings, in the majority no observations of depth whatever. The problem, therefore, presented itself of deciding to what degree of detail it was desirable that the lakes should be mapped, in order * " The heights of waves and their velocity are subjects on which observations are never amiss " (vide ' Admiralty Manual of Scientific Enquiry'). t Map, p. 704. No. V.-May, 1904.] 2 u This content downloaded from 137.99.31.134 on Tue, 21 Jun 2016 04:12:15 UTC All use subject to http://about.jstor.org/terms 616 A BATHYMETRICAL SURVEY OF THE LAKES OF NEW ZEALAND. that the biological work might be most efficiently carried out. On the one hand, a survey involving the study of small details of configuration would demand so great an expenditure of time, that the biological work would be undesirably curtailed, yet too hasty a survey might overlook features of biological importance. And a third point also demanded consideration: the opportunity of mapping the lakes was one which did not seem likely to occur with any frequency, for which reason it was desirable that the survey should be sufficiently detailed to show in outline all the gross configuration of each lake which was visited. On these grounds the decision was finally made, that the survey should be detailed enough to allow of the drawing of rough isobaths in all parts of the lakes, but that minor details of form should be neglected. In the maps which accompany this paper are shown all the soundings which were made. Compared with the detailed work which has been done in the lakes of Great Britain or of the continent, these soundings must appear sparse, but in New Zealand the time is not yet ready for a bathymetrical survey of such a degree of detail as would far surpass the survey of the surrounding land. The methods adopted for fixing the positions of soundings were, with small modifications, those made use of in previous limnological work. The essential method was the steering of a straight course between fixed points on opposite shores of a lake, and the making of soundings at known intervals along such a course. This method must depend for its accuracy primarily upon the accuracy with which those points are fixed, which serve as the extremes of lines of soundings. In this particular there was a wide difference between the lakes which were visited. In a few cases, as in Taupo and Rotoiti, for example, actual traverses had been made of the lake-shores, so that it was possible to use as the determining points any prominent objects, such as headlands or the mouths of rivers. The same method was adopted in cases where, in the absence of an actual traverse, the trigonometrical stations situated on the shores were fairly numerous, and at the same time the natural outline of the lake was characterized by such pro minent features as could be identified without fear of mistake. Such cases were Waikaremoana and Rotorua. Wakatipu demanded different treatment. There numerous trigonometrical stations are placed along the shore, but the outline affords scarcely any prominent features by which the positions of the stations can be unmistakably recognized. It was in this case necessary to put flags upon the trigonometrical stations, and fix the soundings by direct reference to them. Finally, Manapouri needed even more preliminary work, for there were pre- viously only two stations placed upon its shores, the outline being, except in the immediate neighbourhood of the surveyed township, merely a sketch. In this case it was necessary to carry out a triaDgu- This content downloaded from 137.99.31.134 on Tue, 21 Jun 2016 04:12:15 UTC All use subject to http://about.jstor.org/terms A BATHYMETRICAL SURVEY OF THE LAKES OF NEW ZEALAND. 647 lation, by which stations were fixed which served as the end points of lines of soundings. The degree of accuracy being known with which the end points of each line of soundings were fixed, there remains the consideration of errors which might arise in locating the soundings between those points. The first of these errors arises when the boat deviates from the D FIG. 1.-INSTRUMENT USED TO CHECK DEVIATION OF LINES OF SOUNDINGS. straight line which lies between the two determining points. To guard against such deviation an instrument was used (Fig. 1) having an arrangement of prisms similar to that of a sextant, with the exception that the prisms, when once adjusted, were immovable. Any two objects so situated that the straight line joining them passes through this instrument appear in the field of view vertically one above the other. 2 u 2 This content downloaded from 137.99.31.134 on Tue, 21 Jun 2016 04:12:15 UTC All use subject to http://about.jstor.org/terms 648 A BATHYMETRICAL SURVEY OF THE LAKES OF NEW ZEALAND. Derangement of the prisms is readily detected by making two obser- vations of the same objects before and after a rotation of the whole instrument through 180? in either a vertical or a horizontal plane; if the same objects do not coincide before and after such rotation, correction is applied by moving one of the prisms, until the apparent difference between the two observations is halved. Frequent use was made of this instrument while lines of soundings were being carried out, and any deviation from the straight line was quickly seen and corrected. Though a line of soundings lies evenly between its extreme points, error may still arise in the distribution of the soundings along the line. The instrument used to prevent such errors was a modification of the D F A B E FIG. 2.-RECORDER 'USED FOR SUBDIVIDING LINES OF SOUNDINGS. ship's log. The rotator of a "Cherub" log was towed astern of the boat, connected by a short flexible line to the end of a brass screw 2 feet long, which was allowed to run out freely through a nut (Fig. 2). The nut was mounted on trunnions, so that when the screw had run out to its limit, screw and nut could be turned on the trunnions through 180', and the log-line could be made fast to the opposite end of the screw.