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lakes and in different districts, but may be taken to be at least forty 

 to fifty feet. It appears from this that we here in the south-eastern 

 section of England have no lakes in the modern scientific sense ; it 

 is indeed necessary to go to the Lake District, to North Wales, to 

 Scotland and Ireland to get real lakes with a characteristic lake 

 fauna and flora. 



Coming now to the actual work of surveying lakes, we can consider 

 the subject under two distinct heads, according to whether the 

 investigations are of a physical or a biological character. From the 

 physical side the first thing to be done is obviously to get a clear idea 

 of the shape of the lake-basin. This must, of course, be obtained 

 by sounding, and the method generally adopted for this purpose, 

 where approximately accurate results only are required, is to make a 

 series of traverses of the lake in a rowing boat, the depths being 

 recorded after each twenty or other definite number of strokes. For 

 comparatively shallow lakes a heavy plummet attached to a thin 

 cord, marked in feet or yards, is often sufficient to secure fairly 

 accurate readings of the depth, but it is better to use a special 

 sounding machine by means of which the depth is registered 

 automatically as the cord or wire runs out. For deep lakes some 

 such apparatus is absolutely essential. From the results obtained by 

 sounding the contour lines of the lake-basin may be put in on a map 

 just as in the case of the ordinary land surface. In connection 

 with the sounding it should be mentioned that every time a depth is 

 taken a sample of the bottom mud may be brought up, and in this 

 way a considerable amount of light may be thrown on the deposits 

 being formed in the lake, and indirectly on many other matters. 



Continuing the physical investigations, attention must now be 

 given to the water contained in the lake-basin. This is by no means 

 such a simple matter as might be imagined, for it is necessary to 

 make a large number of observations, extending over at least a year, 

 upon the chemical composition, temperature, movement, colour, 

 transparency, etc., of the water before a definite conception of its 

 peculiar character in any one lake can be ascertained. It is not 

 proposed to consider any of these questions in detail, but a few 

 remarks on temperature and movement may be interesting. In very 

 deep lakes the water near the bottom remains all the year round at 

 a temperature very close to that of the maximum density of water- 

 namely, 4° C. In summer the temperature falls as we go down, but 

 in winter, when the surface water is near the freezing point, the 

 temperature naturally rises from o° to 4° C. The stratification in 

 summer, however, is not uniform. In most lakes that have been 

 examined from this point of view it has been found that there is a 

 rather thick upper layer of water exhibiting very little difference of 

 temperature from top to bottom — possibly only 1° or 2°. Imme- 

 diately under this there is a thinner layer in which a very con- 

 siderable fall takes place — 10° or more, perhaps, — after which the 

 temperature decreases slowly and pretty uniformly to the bottom. 



