92 



THE NATURE BOOK 



"AT FIRST THE SWINGS ARE BUT LITTLE RE- 

 MOVED FROM A STRAIGHT LINE " (p. 90). 



when this occurs a tributary always 

 enters on the outer bend of the curve. 



There are two ways in which we can 

 approach the study of a stream ; one is 

 the statical method, the other is the dy- 

 namical method. 

 The first would 

 be content with 

 an answer to 

 the question 

 " What is it ? " 

 The second 

 seeks rather to 

 find an answer 

 to the query 

 " What does it 

 do ? " It is the 

 aspect of doing, 

 which we shall 

 keep in mind in 

 our further in- 

 vestigations. 



Now let us 

 question the 

 river closely as 

 to its activities, 

 and particularly 

 as to the charac- 

 teristics of its 

 flow. It is not 



dii^cult to get exact knowledge on this 

 point if we conduct a few simple experi- 

 ments. Choosing a suitable place in a 

 straight length we throw light objects 

 on the surface. Pieces of paper or 

 twigs will serve, but if the wind is blow- 

 ing it is best to use a substance like saw- 

 dust, which will offer the smallest possible 

 surface to the currents of the air. If a 

 series of floating objects start at the 

 same time from a line drawn across the 

 stream, we soon observe that those near 

 the middle outstrip those at the sides ; 

 in other words, on the surface the line of 

 maximum velocity is near the centre. 

 We can compare the rate of flow of sur- 

 face and bottom water by putting a 

 coloured substance soluble in water on 

 the bed of the stream. A coloured 

 liquid such as ink would do, but it is 

 not easy to get the liquid to the 

 bottom without colouring the water 

 above. A very convenient substance 

 is lump sugar which has been soaked 

 in an alcoholic solution of magenta. If 

 a cube of sugar so prepared is dropped in 

 the stream, it sinks to the bottom, slowly 

 dissolves, and sends out a thin, red line 

 in the direction of the flow. In this way 

 it may be demonstrated that the surface 

 water moves faster than that at the 



"BUT AS WE PROCEED FURTHER DOWN-STREAM THEY BECOME MORE 

 AND MORE ACUTE" (p. 90). 



